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General Discussion

Chapter 10. General Discussion

Population density, size-frequency and reproductive data on an assemblage of shallow water, coral-reef starfish (Asteroidea) were gathered over five years at Heron Reef. Heron Reef, which is located near the southern end of the Great Barrier Reef, has not been known to carry an outbreak of the crown-of-thorns starfish (Acanthaster planci) and its coral cover is well developed. While there has been detailed study of the starfish assemblages on some reefs that have recently undergone Acanthaster planci population outbreaks (Yamaguchi, 1975 b; 1977 a), the composition of these assemblages may well be different from pre-outbreak assemblages.

Abundance, size-frequency and reproductive data were collected by means of intertidal traverses which ran between the cay and the reef crest (0.5 to 2 kilometres apart) and also between two points both on the reef crest (0.5 to 6 kilometres apart). Most traverses included both reef flat and reef crest zones, and all exposed starfish within a 4 meter width were collected. A selection of large and small, dead coral slabs occurring on these traverses were overturned and cryptic specimens located beneath these slabs were collected also. In total, 72 intertidal traverses were conducted covering an area of approximately 120 hectares (1.2 square kilometres). Cryptic species were also sampled using metre square quadrats in particular areas where previous traverse sampling had shown that starfish abundance was relatively high. Subtidal specimens of starfish were collected on the reef slope and off-reef floor by the use of SCUBA.

Of the 25 starfish species found on Heron Reef, Asteropsis carinifera, Dactylosaster cylindricus, Fromia milleporella, Linckia laevigata, Nardoa novaecaledoniae, N. pauciforis, Ophidiaster confertus, O. granifer, O. lioderma, O. robillardi, Asterina anomala, A. burtoni, Disasterina abnormalis, D. leptalacantha, Tegulaster emburyi, Mithrodia clavigera and Coscinasterias calamaria were located only in intertidal regions. Linckia guildingii, L. multifora and Echinaster luzonicus were found predominantly in intertidal regions but some specimens were located subtidally. Culcita novaeguineae, Acanthaster planci, Fromia elegans, Gomophia egyptiaca and Neoferdina cumingi were located predominantly in subtidal habitats, but are known to occur intertidally. Culcita novaeguineae seemed to mainly inhabit the deeper coral pools adjacent to the lagoon. The low occurrence of Culcita novaeguineae on the intertidal traverses is because the traverses avoided this slightly deeper-water habitat. While Culcita novaeguineae, Fromia elegans, Gomophia egyptiaca, Linckia multifora and Echinaster luzonicus were sometimes found at the base of the reef slope, they were never observed on the sea floor away from the reef. There are no published records of these species from the off-reef floor zone (see Clark and Rowe, 1971).

“Reef” echinoderm species were separated from “mainland” species on the basis of their habitat requirements by Endean (1956) who discussed the biogeographical relationships of Great Barrier Reef species. With the exception of Ophidiaster confertus and Coscinasterias calamaria, which are essentially temperate species, 23 asteroid species found at Heron Reef can be regarded as coral-reef species and their distribution differs from species such as Astropecten polyacanthus, Iconaster longimanus, Pentaceraster regulus, Leiaster leachi, Nardoa rosea, Ophidiaster armatus, Tamaria megaloplax and Echinaster stereosomus. These latter species appear to be predominantly off-reef, sea-floor species that are widely distributed throughout the shallow waters of tropical and sub-tropical Queensland. The predominantly reefal distribution of the long-spined, corallivorous species, Acanthaster planci, contrasts with that of its generally deeper water, short-spined, molluscivorous relative, A. brevispinus. Only small fissiparous specimens of Coscinasterias calamaria were located on Heron Reef. Large adults of this and other forcipulatid species are predators in temperate communities. Both Ophidiaster confertus and Coscinasterias calamaria appear to be predominantly temperate species that occur in Australian mainland waters but which have extended their ranges to reefs at the southern end of the Great Barrier Reef.

The finding of Iconaster longimanus, Asteropsis carinifera, Dactylosaster cylindricus, Fromia elegans, Linckia multifora, Ophidiaster armatus, Ophidiaster lioderma, Ophidiaster robillardi, Tamaria megaloplax, Asterina anomala, Disasterina abnormalis, Tegulaster emburyi, Mithrodia clavigera, Echinaster stereosomus and Coscinasterias calamaria represent new records for Heron Reef. In some cases these represent new records for the Great Barrier Reef, and in other cases known ranges on the Great Barrier Reef have been considerably extended. This study has also provided the first record of the predominantly temperate species, Coscinasterias calamaria on the Great Barrier Reef.

The distinguishing characteristic of coral-reef species of starfish is their possession of a spatial distribution that never extends into the deeper parts of the off-reef floor zone. Such a spatial distribution would preclude between-reef migration by post-larval stages of these species. It is not known why some species of starfish are essentially restricted to coral reefs, but it is likely that such species would differ in their physiological and / or ecological requirements from species that occur elsewhere. While the intertidal region of a coral reef undergoes both temperature and salinity fluctuations (Maxwell, 1968), a substrate of coral sand and rubble (aragonite not calcite) would ensure complete carbonate saturation of the waters and hence the waters would be well buffered against pH changes. Some species of starfish that occur exclusively in association with coral reefs may have narrow pH tolerances. Other species may have evolved interdependencies that involve settlement or survival conditions that are only present within the coral reef ecosystem. Likewise, with respect to the coral reef ecosystem itself, it might be expected that species that occur predominantly in one of the major zones of a coral reef (e.g. the reef flat) would differ in their physiological and / or ecological requirements from species that occur in several of these zones. For example, they might differ in their degree of tolerance to sub-aerial exposure at low tide or in their biotic associations.

Patches of localised high density were observed within the populations of some of the smaller-bodied species of coral-reef starfish that were studied. However, each of these patches appeared to be restricted to a very small area. For example, the small-bodied starfish Disasterina abnormalis occurred at an average density of over eight individuals per square metre at one location on the northern reef crest but 100 metres away (still on the reef crest) its density was less than one individual per square metre. This region of high density of Disasterina abnormalis appeared to be confined to a narrow strip behind a rubble bank and this species was not found on 25 of the 72 traverses that were made. In this region, Disasterina abnormalis was highly clumped (at the metre square scale) in one sampling period and randomly distributed in another sampling period.

Echinaster luzonicus was the most abundant starfish found on the intertidal traverses and Linckia multifora was the next in order of decreasing abundance. Both of these small-bodied species were found in relatively high numbers in some regions of the reef crest. The large-bodied starfish Linckia laevigata was third in order of decreasing abundance on the traverses but its maximum density did not approach that of either of the preceding species anywhere at Heron Reef. The density of Linckia laevigata at Heron Reef appeared to be low compared with its density on reefs that are known to have carried an outbreak of Acanthaster planci (Laxton, 1974; Yamaguchi, 1977 a; Thompson and Thompson, 1982; Klumpp and Pulfrich, 1989). Laxton (1974) suggested that Linckia laevigata may either increase its numbers or extend its range following outbreaks of Acanthaster planci. Disasterina abnormalis was fourth in order of decreasing abundance and occurred at the highest local density of any species of starfish during this study.

The intertidal traverses made during this study covered an area of 125 hectares. Over 1400 individuals of Echinaster luzonicus were located and over 100 individuals of each of another 8 species were located. However, fewer than 25 individuals of each of the remaining 15 species were located. The low starfish density found at most locations on Heron Reef contrasts markedly with the high densities recorded for asteroids of temperate communities (Loosanoff, 1961; 1964; Mauzey et al, 1968; Menge, 1975; Dayton et al, 1977; Birkeland et al, 1982; Stevenson, 1992).

Traverse sampling resulted in the location of a total of 24 species of intertidal starfish. For 10 of these species, a sufficient number of individuals was obtained for reproductive analysis and for 7 of these species size-frequency variation was examined over different sampling periods. Traverse sampling enabled data to be gathered on a large spatial scale (125 hectares) which facilitated both the collection of sufficient specimens for reproductive and size-frequency analysis as well as the determination of large scale non-randomness in the spatial distribution of these species.

While the intertidal traverse data did not allow small-scale analysis of either spatial or temporal abundance variation, the starfish assemblage at Heron Reef clearly embraces a highly diverse and spatially heterogeneous group of species. Individuals of each species were extremely non-random (clumped) in their spatial distribution. Only Echinaster luzonicus was sufficiently abundant and widespread to be found on all but three of the traverses. Linckia laevigata and Nardoa novaecaledoniae were not located on 10, Nardoa pauciforis was not located on 19, Linckia multifora was not located on 22, Disasterina abnormalis was not located on 25, Asterina burtoni was not located on 26 and Linckia guildingii was not located on 34 of the 72 traverses made. Representatives of the remaining species were not found on the majority of these intertidal traverses.

With the exception of Echinaster luzonicus, the abundance distributions of all of the species had a modal traverse density of zero individuals per hectare. This indicated that, with the exception of Echinaster luzonicus, each coral-reef starfish species was not represented on a large number of the traverses. The more common of these species possessed a bimodal abundance distribution which indicated that they were non-random (patchy) in their spatial distribution. For these species, there were many traverses where both zero and a relatively large number of individuals per hectare were recorded and very few traverses where intermediate (mean) densities occurred.

Table 4.1 lists the mean density per hectare and the variation that occurred in the mean density of each species between traverses. In all species the standard deviation was greater than the mean density. These data together with the bimodal population distribution data (Figures 4.2 to 4.12) indicate that large scale aggregation occurs in all of the species with the possible exception of Echinaster luzonicus. A stratified-random sampling procedure, using multiple belt transects would have allowed a detailed comparison of starfish abundances between different habitats. However, when used on a reef that has low general starfish abundance, such a sampling method would not have located a sufficient number of individuals in the limited time available for field studies at Heron Reef to permit a statistically valid size-frequency and reproductive analysis.

A mode in the abundance distribution was recorded at between three and 10 individuals / hectare in six species (rank 1 – 6) and at between one and three individuals / hectare in another six species (rank 7 – 12). The remaining twelve species (rank 13 – 20) were encountered so infrequently that the only mode in the abundance distribution of each species was at zero individuals per hectare. Five species were sufficiently uncommon (rank 20) to be encountered on only one intertidal traverse during the entire study.

Culcita novaeguineae, Fromia elegans, Gomophia egyptiaca and Nardoa rosea were encountered much more frequently in sub-tidal traverses than they were on intertidal traverses. Disasterina leptalacantha was recorded more frequently at Heron Island by Endean (1957) than it was in this study, but there may have been confusion between the two similar congeneric species in the earlier study. Similarly the ecological distinction between Asterina anomala and Asterina burtoni is unclear. The observed variation in the abundance of Asterina burtoni at Heron Reef is consistent with the results of Achituv and Sher (1991), but the mode of reproduction appears to be different.

The very small and highly cryptic species Disasterina abnormalis occurred periodically with high abundance at one location on the inner reef crest. It was possible to sample this species in this localised habitat by means of metre square quadrat sampling (Table 4.2). The data obtained do not represent the abundance of this species generally, but serve to illustrate clearly the enormous spatial and temporal variation that occurs in the population distributions of this opportunistic species.

Although the diets of the coral-reef starfish species encountered were not studied in detail, many of them appeared to feed on epibenthic felt. In every coral reef zone, some species sought no refuge and occurred in exposed situations. Clear examples of niche (dietary or microhabitat) specialisation are known only for Culcita novaeguineae and the predominantly subtidal species Acanthaster planci both of which feed primarily on corals. Competitive interactions were not studied, but many species occurred at a sufficiently low density that they may not be resource limited.

Because of the patchy nature of the spatial distributions of all of the coral-reef asteroid species, size-frequency analysis over multiple sampling periods (Tables 5.1 to 5.12 and Figures 5.1a to 5.10d) was considered the most appropriate means of establishing the existence of population stability. Obvious changes in abundance due to either sexual or asexual recruitment, and significant changes in mean individual size were observed in the populations of Linckia multifora, Disasterina abnormalis and Echinaster luzonicus (Table 8.1 and Figures 8.1a to 8.3c). While some recruitment and some change in abundance was noticed in both Ophidiaster granifer (parthenogenetic) and Asterina burtoni (hermaphroditic), no significant change occurred in the mean individual size of either species. Linckia guildingii, Linckia laevigata, Nardoa novaecaledoniae and Nardoa pauciforis exhibited only small changes in mean individual size and these species did not fluctuate greatly in abundance during the period of study. Also, the population structure of these species appeared to be adult dominated and juveniles were encountered only rarely.

The remaining species were not found in sufficient numbers for meaningful statistical analysis of size-frequency data. Their populations were sparse and juveniles were not encountered except for one specimen each of Culcita novaeguineae, Fromia elegans and Gomophia egyptiaca. Their populations appeared to be adult dominated. Juveniles of Culcita novaeguineae and Fromia elegans were not encountered subtidally despite the existence of a subtidal population of adults. One juvenile of Acanthaster planci was located at the base of the reef slope.

Culcita novaeguineae and many other coral-reef starfish species were not encountered in sufficient numbers to warrant an examination of their population stability. The study of Laxton (1974) appeared to show a greater abundance of Linckia laevigata on the reef flat at Heron Reef than was observed in this study. Laxton suggested that this species may vary its distribution range following outbreaks of Acanthaster planci. It is possible that large-bodied species of starfish, such as Linckia laevigata, undergo large scale aggregation behaviour but the limited duration of this study precluded examination of such long period fluctuations.

Grassle (1973), Sale and Dybdahl (1975), Talbot et al. (1978) and Hutchings (1981) all found that most coral-reef species are rare. Endean and Cameron (1990 a) mention that the high incidence of rare species in the coral-reef community contributes markedly to species diversity. Some of the rarer species of coral-reef starfish are known from only a few specimens and their low-density populations defy our normal understanding of population dynamics and reproductive strategies. It is not clear how these species survive or whether their populations are predator, resource or recruitment limited. Species such as Tosia queenslandensis, Ophidiaster lioderma and Tegulaster emburyi have always been considered rare throughout their geographical range. Although nothing is known of their reproductive cycles, if they are truly rare and valid “biological” species, then they might be expected to exhibit mechanisms such as population aggregation, asexual reproduction, parthenogenesis or hermaphroditism that would facilitate their persistence at low population densities.

Inter-coelomic injection with the hormone 1-methyl adenine was used to determine the sex ratio, reproductive maturity and type of larval development of several of the species. It can be seen from Tables 6.1 to 6.8 and Figures 6.1 to 6.8 that eight of the more common species appeared to demonstrate an annual sexual reproductive cycle. Disasterina abnormalis possessed small (non-yolky) sticky eggs that adhered to the substrate immediately following their release from the gonopores. Small juveniles of Disasterina abnormalis were relatively common in one highly localised area at Heron Reef, but high settlement was not observed in any of the other species. The remaining seven species possessed eggs that dispersed and underwent either planktotrophic or lecithotrophic larval development. No species were observed to brood larvae.

Culcita novaeguineae, Acanthaster planci, Linckia guildingii and Linckia laevigata were observed releasing eggs that contained little yolk and underwent planktotrophic development. Fromia elegans, Gomophia egyptiaca, Nardoa novaecaledoniae, Nardoa pauciforis, Ophidiaster granifer and Echinaster luzonicus were observed releasing eggs that contained large amounts of yolk and underwent lecithotrophic development. Specimens of both Linckia multifora and Asterina burtoni were injected regularly, but did not release gametes during the entire study.

Vance (1973) and Yamaguchi (1973 a, 1973 b, 1977 b) suggested that lecithotrophic development is an adaptation to high predation or starvation of larva because with this development the length of larval life can be shorter than with planktotrophic development. On Heron Reef, and possibly the Great Barrier Reef in general, where many reefs exist in relatively close proximity, lecithotrophic genera such as Nardoa, Fromia and Echinaster might be expected to be better represented than they are on widely scattered atolls. At Heron Reef, the larger-bodied species namely, Culcita novaeguineae, Acanthaster planci, Linckia guildingii and Linckia laevigata all liberated dispersing, small eggs that underwent planktotrophic development while the smaller-bodied species, together with Nardoa novaecaledoniae and Nardoa pauciforis (both intermediate in body size), all liberated larger eggs that underwent lecithotrophic development. The small, sticky eggs of Disasterina abnormalis resulted in high localised settlement and this strategy appeared to be unique amongst the starfish species that were studied at Heron Reef.

In addition to the species that demonstrated a sexual reproductive cycle, Linckia guildingii, Linckia multifora, Ophidiaster robillardi and Echinaster luzonicus reproduced asexually and exhibited comet stages while Asterina anomala and Coscinasterias calamaria reproduced asexually by binary fission. All small specimens of these species exhibited the characteristics of either autotomous propagation (see Rideout, 1978) or binary fission. While all of the arms might look quite similar in some small individuals of autotomous species, the original arm from which the others regenerated was always apparent following closer examination. All specimens of fissiparous species showed signs of recent binary fission.

While specimens of both Linckia guildingii and Echinaster luzonicus were observed releasing gametes in response to injection with 1-methyl adenine, no sexually-propagated juveniles were observed in the populations of any species that reproduced asexually. With the exception of Linckia guildingii, large bodied species of coral-reef starfish do not appear to have a small scale (low dispersion) reproductive strategy. This could indicate that survival of offspring is more likely away from adult populations. The advantages of a high dispersion reproductive strategy must be balanced against the high dispersive loss resulting from the relative isolation of reefs of the Great Barrier Reef and elsewhere.

Linckia multifora and Echinaster luzonicus were the only asexually reproducing species in which high rates of autotomy were observed and the location of comet stages and adults in various stages of regeneration is evidence of relatively high asexual recruitment. These three species had the highest localised abundances of any of the coral-reef starfish but also had highly patchy spatial distributions. The remaining species never occurred at densities comparable with these species even though the average density of Linckia laevigata was higher than the average density of Disasterina abnormalis. While comet stages and adults in various stages of regeneration were observed in Linckia guildingii, this species did not show evidence of high asexual recruitment.

With the exception of Disasterina abnormalis (see Chapter 6), all the species of starfish at Heron Reef either possessed a planktonic dispersive larval phase or were not observed to reproduce sexually . The largest-bodied persistent species released planktotrophic eggs while the opportunist species were either lecithotrophic, hermaphroditic (Asterina burtoni), parthenogenetic (Ophidiaster granifer) or solely asexually reproducing (Linckia multifora). Nardoa novaecaledoniae, Nardoa pauciforis and Gomophia egyptiaca would appear to be of intermediate position and the taxonomic position of Asterina anomala is unclear.

All of the large-bodied species studied liberated either eggs or sperm directly into the water column and fertilisation was external. While possible pairing was observed in crowded aquaria (following injection with 1-methyl adenine), no species were observed mating in the field as has been recorded by Run, Chen, Chang and Chia (1988) for the tropical species Archaster typicus. Slattery and Bosch (1993) also recorded mating behaviour in an Antarctic species of starfish.

Ormond et al. (1973) discussed the consequences of spawning aggregations of Acanthaster and suggested that the increased proximity of adult starfish may enhance the chances of fertilisation, especially if synchronous spawning takes place. It was suggested by Lucas (1984) that a conspecific stimulus would induce synchronous spawning in Acanthaster planci and a delayed spawning activity in dispersed individuals of Acanthaster planci was observed by Okaji (1991). It was suggested that this delay reflected less frequent stimulus from conspecifics in dispersed populations compared with aggregated populations and that synchronous spawning induced by such stimulus would lead to higher rates of fertilisation when the animals formed an aggregation. Evidence of the existence of sexual pheromones in starfish was presented by Miller (1989).

The effect of sperm dilution, adult aggregation and synchronous spawning upon the fertilisation of sea-urchin eggs was reported by Pennington (1985). Pennington concluded that significant fertilisation occurred only when spawning individuals are closer than a few metres. The consequences of water mixing and sperm dilution for species that undergo external fertilisation were discussed by Denny and Shibata (1989) who found that only a small fraction of ova were fertilised other than in densely packed arrays. They commented that the low effectiveness of external fertilisation may change the way one views the planktonic portion of such life cycles and suggested that this could serve as a potent selective factor. For the rarer sexually reproducing species, it is apparent that aggregation resulting in the occurrence of an opposite sexed conspecific within the effective fertilisation distance is a condition precedent to successful reproduction. The degree of reproductive success may be strongly dependent on just how close the rare spawning individuals are to each other. While the results of Babcock and Mundy (1992) appear inconsistent with these previous studies, the population density and degree of adult aggregation would be highly relevant factors for both the synchrony of spawning and the level of egg fertilisation in externally fertilising dioecious species. If a low density starfish population is highly dispersed then the degree of egg fertilisation would be much lower than if aggregation occurred.

The above factors influence recruitment as do many other factors such as dispersion loss (Atkinson et al, 1982; Dight et al., 1990 a, b; Black and Moran, 1991; Wolanski, 1993) and starvation of larvae (Birkeland, 1982; Olsen, 1987). These factors, together with the mortality of juveniles prior to first reproduction (Endean, 1977; 1982; McCallum et al, 1989), might result in this assemblage being recruitment limited as suggested for certain species of coral-reef fish by Doherty (1982). If the process of recruitment is completed when an organism enters the breeding population, then a species could be regarded as recruitment limited if mortality of its larvae or juveniles was sufficiently great to maintain adult populations at a low density. This may occur as a result of either low egg fertilisation or high mortality of larvae or juveniles.

On reefs such as Heron Reef that have low adult starfish abundance, predation of adult starfish appears to be a rare event and was not studied because of logistic constraints. While the giant triton (Charonia tritonis) is a voracious predator of large juvenile and adult starfish (Endean, 1969; Pearson and Endean, 1969), no specimens of this species were observed at Heron Reef either subtidally or on intertidal traverses during the entire study. The giant triton is cryptic and it is extremely difficult to survey the population density of this predator. It is likely that there are other predators of coral-reef starfish, particularly fishes. Other predators (see Endean and Cameron, 1990 b) have been found for Acanthaster planci. If starfish populations are stable then mortality (including lethal predation) will match recruitment which appeared to be extremely low in the populations of large bodied coral-reef starfish. If starfish populations are maintained at a low adult density, then predation on pre-adults could be a major factor in controlling the assemblage.

An increase in anti-predatory structures with decreasing latitude was found by Vermeij (1978) and Blake (1983) suggested the existence of a similar pattern in sea stars. Pearson and Endean (1969) and McCallum et al. (1989) reported a high incidence of sub-lethal predation in populations of Acanthaster planci. Blake (1983) commented that the asteroid fauna of the Indo-West Pacific are dominated by the order Valvatida and members of this order have the best developed anti-predatory devices. Yamaguchi (1975 b) commented on the difference between adult and juvenile asteroid habits and suggested that the heavy armour of exposed adult asteroids might reflect heavy predation pressure.

In addition to the protection afforded by structural features, many species of starfish are protected from generalist predation by the possession of skin toxins (Riccio et al., 1982, 1985; Gorshkov et al., 1982; Minale et al., 1984; Narita et al., 1984; Noguchi et al., 1985 a,b; Miyazawa et al., 1985; 1987; Kicha et al., 1985; Shiomi et al., 1988; Shiomi et al., 1990; Zagalsky et al., 1989; Iorizzi et al., 1991; Bruno et al., 1993; Casapullo et al., 1993). These skin toxins have been shown to be toxic to some fish species (Rideout, 1975). The role of echinoderm toxins as a defence against predation has been discussed extensively (Bakus, 1974; Green, 1977). Cameron and Endean (1982) discussed the role of venomous devices and toxins as defences against predation and Endean and Cameron (1990 a) have noted that persisters are often toxic. There is little information available on the toxicity of juvenile starfish to potential predators. Eggs and juveniles of Acanthaster planci are known to carry toxins. It has been proposed that the production of toxins for defence incurs an energy cost which is balanced against the probability of mortality (Eckardt, 1974) but in some species, toxins might be metabolic by-products that incur no energy cost in their synthesis.

In some groups of starfish behavioural mechanisms are used as defences against predation and Blake (1983) suggested that both Luidia and Astropecten have broad open ambulacral furrows because they were predators on active solitary forms where increased skeletal mobility was essential. Because both these active, hunting genera live on and within unconsolidated sediment they avoid predation by burrowing which is facilitated by the paxillose nature of their aboral surface.

Another behavioural defence possessed by asteroids is the autotomy of arms. Of the coral-reef starfish studied, Linckia guildingii, Linckia multifora, Ophidiaster robillardi and Echinaster luzonicus are capable of regenerating a complete individual from the distal section of one arm. These autotomous species were extremely aggregated in their spatial distribution, suggesting that population growth occurs with little dispersal of individuals.

In species of starfish that do not reproduce by autotomy, specimens are often observed in various stages of regeneration following loss of one or more arms. McCallum et al (1989) reported that 40% of the adult individuals in a population of Acanthaster planci showed signs of arm regeneration. Cameron and Endean (1982) suggested that autotomy is an adaptation to predation and Birkeland et al (1982) observed autotomy in their study of asteroid predatory interactions. At Heron Reef, many individuals were observed in various stages of regeneration following autotomy of one or more limbs. A number of tropical asteroids are known to undergo regular autotomy (Rideout, 1978; Yamaguchi, 1975 b) and Blake (1983) commented that interpretation of the skeleton can be difficult as it has more than one function and protection against predation can be accomplished by many mechanisms (e.g. Bullock, 1953; Feder, 1963; Mauzey et al., 1968; Ansell, 1969; Birkeland, 1974; Phillips, 1976; Dayton et al., 1977; Jost, 1979; Schmitt, 1982; Stevenson, 1992; Iwasaki, 1993).

In the species that reproduce by autotomy, it is not known to what extent the autotomisation of a limb is caused by physical disturbance such as predation. While direct predation was not observed, large individuals of the large-bodied species of starfish often had their arms intertwined with the substrate such that they were difficult to dislodge. In the large-bodied species that only reproduce sexually, parts of a limb and even one or two whole limbs were observed to be missing from some individuals. The existence of such behaviour together with the observations of missing arms in species that do not reproduce asexually, indicates that sub-lethal predation does occur. Whether it is significant in the regulation of the Heron Reef asteroid assemblage will depend on the age structures of the populations. Sub-lethal predation of adults will be especially important if a species is long lived.

This study has examined the population dynamics of both relatively common and relatively rare species of coral-reef starfish. Although some species were not sufficiently numerous to provide statistically satisfactory numbers of records, data were gathered on their habitat, size, spatial pattern and relative abundance. It is clear that the majority of species of intertidal starfish at Heron Reef were sufficiently uncommon to preclude small scale methods of population examination. There is considerable disagreement over the accuracy of large scale methods (manta tow) to examine subtidal populations of starfish (Fernandes, 1990; Fernandes et al., 1990; Moran and De’ath, 1992 a,b). However, the determination of large scale, non-random variation in the distribution of any species is a condition precedent to the determination of its overall abundance. In the estimation of average density, methods of both sampling and analysis must adequately consider the high standard error of the mean. All conclusions must have due regard to the bimodality and skewness of the abundance distributions of starfish.

Some species, namely Disasterina abnormalis, Asterina burtoni, Ophidiaster granifer, Linckia multifora and Echinaster luzonicus, could be regarded as opportunist species as they were characterised by possessing relatively abundant populations with relatively large fluctuations in mean individual size. These invariably small-bodied species demonstrated all of the typical opportunist characteristics which are short life, high recruitment and high mortality (see Endean and Cameron, 1990 a).

Other species, namely Culcita novaeguineae, Linckia laevigata, Linckia guildingii, Nardoa novaecaledoniae and Nardoa pauciforis could be regarded as persistent species and were characterised by less abundant populations with relatively smaller fluctuations in mean individual size. These invariably medium to large bodied species demonstrated all of the typical persister characteristics which are long life, low recruitment and low mortality. A large proportion of coral-reef starfish were sufficiently uncommon to preclude any analysis of either their abundance or size distributions. Apart from the knowledge that they remained rare through the study period of 5 years, little is known of their natural history. Because of their extreme rarity, which is a characteristic of persisters, they might be placed in this category pending further investigation. Of the 25 intertidal species of starfish, five species (20 percent) were characteristic opportunist coral-reef species and 18 species (72 percent) were characteristic persister coral-reef species (stable abundance and size distribution or remained uncommon throughout study). Only two species (8 percent), namely Ophidiaster confertus and Coscinasterias calamaria were sub-tropical, rocky-reef (mainland) species that had extended their ranges to embrace the southernmost reefs of the Great Barrier Reef.

The longevity of a species is determined by the relative probability of juvenile and adult survivorship. In the simplest case, if the probability of a sexually mature organism’s survival from one reproductive season to the next is greater than the probability of one of the offspring reaching sexual maturity, then the species will exhibit iteroparity (see Cole, 1954; Murphy, 1968; Goodman, 1974; Stearns, 1977; Roff, 1981; Ebert, 1982). Although neither predation nor mortality was observed during this study, both low adult mortality and relative longevity can be inferred from the stability of the size-frequency distributions of the persistent species studied. This contrasts with the large population fluctuations and instability of the population structure of the opportunist species studied.

Most marine benthic invertebrates have a high energy cost associated with reproduction (Mileikovsky, 1971). Under differing selection pressures, it has been suggested that long life can be associated with either variable recruitment (Sterns, 1977) or fixed low recruitment (Charnov and Schaffer, 1973; Schaffer, 1974; Ebert, 1982). McCallum (1987) and McCallum et al. (1989) have suggested that Acanthaster planci is recruitment limited by juvenile and sub-adult predation.

A model relating to our perception of the life history of all organisms, referred to as r- versus K- strategy, was reviewed by Stearns (1977). The different survival characteristics in the model were thought to have evolved in response to specific types of environments (Murphy, 1968; Hairston, Tinkle and Wilbur, 1970). The spectrum of existing life history attributes, apparent in any community study (see e.g. Menge, 1975; Vance, 1973), was considered to represent many points on a continuum between the conceptually ideal r- strategists and K- strategists.

It has been suggested that the dispersal stage of a population spreads the risk of local extinction in space and time (Den Boer, 1971; Scheltema, 1971; Strathmann, 1974). Opportunists survive by being able to colonise regions quickly following disturbance. In this regard, an important distinction must be made between equilibrium and non-equilibrium populations in terms of adaptive characteristics (Caswell, 1982; Ebert, 1985). High spatial and temporal variation in population size seems to characterise the typical opportunists.

The degree of spatial and temporal stability in the population of a species determines its position on a theoretical opportunist – persister continuum. Each species was viewed in this context and a basic dichotomy was observed. Because it does not require presumptions of carrying capacity, and inferences about competition, the opportunist / persister model of Endean and Cameron (1990 a) seems to best describe this low density assemblage of coral-reef starfish. Stable ecosystems should be characterised by small fluctuations of their component species. However it is clear that the apparent stability or instability of any biological system is dependent not only on the spatial and temporal scales of observation (Bradbury and Reichelt, 1982; Sale, 1984; Weiss, 1969) but also on the particular subset of species that is examined.

The observed level of numerical and size-frequency stability in the persistent coral-reef asteroid species is consistent with a model of community equilibrium. It is clear that mortality, dispersion, larval survival and settlement phenomena did not result in widely varying size structures or greatly differing adult numbers from one year to the next over a period of 5 years. The vast majority of species of coral-reef starfish in the assemblage studied were characterised by continuing low abundance. It would appear that when a rare, large-bodied starfish is established in its adult population, it is likely to be long lived. Acanthaster planci is a member of this coral-reef starfish assemblage and Cameron (1977) has suggested that only when the coral reef ecosystem is drastically altered can such a rare and long-lived carnivore undergo population outbreaks. This restriction may also apply to other persistent species in the coral-reef starfish assemblage.

Factors such as high gamete dilution (Rothschild and Swann, 1951; Pennington, 1985; Denny and Shibata, 1989; Epel, 1991), as well as basically unpredictable environmental factors such as larval mortality and enormous potential larval dispersion can affect the number of larvae reaching a reef. Because the area of coral reef in the Great Barrier Reef region is relatively small compared with the area of sea surface in the region, the probability of a planktonic starfish larva reaching a coral reef is quite low. Also, if predation on post-settlement juveniles is intense then recruitment will be low. In low density starfish populations, the aggregation of adults prior to spawning may be essential to the reproductive success of a rare species. Because successful recruitment implies that post-settlement juveniles must survive to enter the breeding population, predation on juveniles as well as sub-lethal predation of adults (when loss of gonad affects fecundity) are both forms of recruitment limitation.

The results presented in this study are in accord with the hypothesis of Endean and Cameron (1990 a) that complex, high diversity assemblages of coral-reef animals are characterised by a predominance of rare, long-lived species with relatively constant population sizes and size structures and a minority of relatively common, short-lived opportunistic species characterised by fluctuating population sizes and size structures.

 

Relative Abundance and Diversity

Figure9

Chapter 9. Relative Abundance and Diversity

9.1 Introduction

In addition to the high diversity of the coral reef ecosystem, a feature of this ecosystem is the large number of rare species within each taxonomic group. The general relation between the number of species and the number of individuals in a sample of a population was discussed by Fisher, Corbet and Williams (1943), who commented that species are not equally abundant, even under conditions of considerable uniformity. They went on to state that the majority of species are comparatively rare while only a few are common.

It is not known whether the rarity of a species is indicative of its low competitive ability or alternately whether the species is restricted to specialised microhabitats with excess recruitment eliminated by predators (Hairston, 1959; Kunin and Gaston, 1993). The relative abundances of the species in a diverse assemblage are often distributed over many orders of magnitude. As a result, qualitative representations of abundance such as common, moderately abundant or rare must be arbitrary in their assignment.

Many different mathematical models have been proposed to describe satisfactorily the relationship that exists between the relative abundances of different species in an assemblage. While each model has been criticised extensively (Hurlbert, 1971; Abbott, 1983; Connor and McCoy, 1979; Connor, McCoy and Cosby, 1983; Martin, 1981; McGuiness, 1984; Pielou, 1981; Sughihara, 1981), each attempts to quantify the degree of variation in the relative abundances of the different species. The most noticeable result of this abundance variation is the different rates at which species accumulate with increased sampling in different assemblages.

9.2 Methods

The population density of each species and the relation between sample area and the number of individuals in the sample was calculated in Chapter 4. The relation between sample area and the total number of species in the sample (the species : area curve) was also calculated from the traverse data. The cumulative number of species was compared with the cumulative area of the traverses (starting at the completion of Traverse 1 and continuing through to the completion of Traverse 72). This comparison was also undertaken with the natural logarithm of the cumulative area of the traverses.

Shannon’s Evenness Index (see Pielou, 1981) which is the expression (S P(log P)) / log S, where P is the proportion of each species in the community, and S is the total number of different species, is often used to display the relative richness of various communities. Shannon’s Evenness was calculated for each traverse individually and cumulatively starting with Traverse 1 and ending with Traverse 72.

The relation between the numerical abundance of each species and the rank abundance of each species was calculated by ordering the numerical abundance from most common (rank 1) to least common (equal rank 20 for five species). Percent relative abundance was the ratio of the numerical abundance of each species to the total asteroid abundance.

9.3 Results

Table 9.1 lists the numerical, relative and rank abundances of each species located on the intertidal traverses. Figure 9.1a graphs the relation between the numerical abundance of a species and its rank abundance. Figure 9.1b graphs the relation between (log) relative abundance and rank abundance. Figures 9.2a,b graph the species : area and species : (log) area relation. Figures 9.3a,b graph the relation between Shannon’s Evenness and cumulative area and cumulative (log) area. Natural logarithms were used in all these calculations. Shannon’s Evenness as a measure of diversity has the advantage that the index is a ratio of attained diversity over maximum possible diversity and is therefore independent of the base of logarithm which has been chosen.

Table 9.1

The numerical abundance, relative abundance and abundance rank of inter-tidal asteroids at Heron Reef.

SPECIES                 NUMERICAL     RELATIVE      RANK 
                  
Culcita novaeguineae          15          **          13    
Asteropsis carinifera          3          *           19
Dactylosaster cylindricus      1          *           20
Fromia elegans                16          **          12
Fromia milleporella            1          *           20 
Gomophia egyptiaca             6          *           16 
Linckia guildingii           116          ***          8
Linckia laevigata            509          ***          3
Linckia multifora            522          ***          2
Nardoa novaecaledoniae       326          ***          5
Nardoa pauciforis            187          ***          7
Nardoa rosea                   1          *           20
Ophidiaster armatus            4          *           17
Ophidiaster confertus          4          *           17
Ophidiaster granifer         116          ***          9
Ophidiaster lioderma           1          *           20
Ophidiaster robillardi        24          **          10
Asterina anomala              17          **          11
Asterina burtoni             208          ***          6
Disasterina abnormalis       500          ***          4
Disasterina leptalacantha      7          *           14
Tegulaster emburyi             1          *           20     
Echinaster luzonicus        1402          ****         1 
Coscinasterias calamaria       7          *           14
*        Very rare <10 :** rare 11-100 :*** common 101-1000 :**** abundant>1000    

9.4 Discussion

The generally low abundances of most of the species of starfish at Heron Reef precluded the use of quadrats in general sampling. Because the traverse method will miss many cryptic individuals and provide only an approximate area measurement, the species diversity and species accumulation figures are only approximate. It would appear from Table 4.1 that most species occurred at a density that was less than one individual per hectare, with many species being far less abundant. It should be noted that traverse sampling will underestimate the density of all cryptic species, and will also fail to detect species that are both rare and cryptic.

McGuiness (1984) suggested that the use of species : (log) area or (log) species : (log) area for the display of the species : area relationship should be based on the underlying relative abundances of the species. The slope of the species : (log) area relationship, the slope of the (log) relative abundance : rank abundance relationship and Shannon’s Evenness index are all indices of diversity. These allow a direct comparison to be made between different assemblages. Not only do these indices consider the number of species, they also express the inherent range of abundance between most common and least common within the assemblage (Connor and McCoy, 1979; Connor and Simberloff, 1979; Connor et al., 1983).

Figures 9.1a,b show that the four most common species account for 70% of the total number of individuals in this assemblage. However, even Echinaster luzonicus, the most abundant species, had an average density of only 16 specimens per hectare. Of the 24 species of asteroid that occurred in the traverse samples, five species occurred only once. Presumably the species which were not found during this study, but which are known from the locality, occur with even less frequency than these five. Less than ten specimens of each of another six species were located on the intertidal traverses. Hence, 11 of the 24 species are regarded as very rare. Less than 25 specimens of another three species were found and these are regarded as rare. Thus a majority of the asteroid species found at Heron Reef are rare or very rare.

The slope of the regression line in Figure 9.1b is a measure of the diversity of this asteroid assemblage. The steeper the line the greater the range of relative abundance within a certain group of species. The less equal the relative abundances, the lower the diversity as measured by most diversity indices. Community studies often show a log-normal relationship in relative abundance, in which most species occur with close to the average abundance (Pielou, 1981). This assemblage of coral-reef asteroids does not clearly demonstrate this relationship, but this result may be attributable to an inadequate number of both species and individuals in the present study. The order of the species in Figures 9.1a,b is that of numerical abundance. If biomass or some other parameter was chosen as a measure of abundance, then the order of the species may change but the slope of the regression line might not alter greatly.

Figures 9.2a,b illustrate the species : area curve for the Heron Reef asteroid assemblage. The slope of the (log) area regression line is independent of the units used to measure area. Whether they be square metres or hectares, providing the habitat continues, the species will accumulate at a rate determined only by the relative abundances of the species in the assemblage. If there is some finite species pool which obviously cannot be exceeded, then the curve will become asymptotic.

The pronounced dips in Figures 9.3a,b are a result of small scale patchiness in the distribution of Echinaster luzonicus and Disasterina abnormalis. After continued sampling, the effect of this high localised abundance was rendered insignificant in the total diversity.

Figures 9.1a to 9.3b all relate to the one ecological parameter, namely the relative abundances of the species within this assemblage. This will determine the rate at which the species accumulate in a species : area curve, as well as the diversity as measured by most diversity indices.

The richness of the coral-reef asteroid assemblage at Heron Reef is unable to be compared directly with that of other coral-reef asteroid assemblages either on the Great Barrier Reef or elsewhere. This is because the extent of sampling has not been quantified in the majority of biogeographical studies. Because the area sampled determines the number of species in a sample of any assemblage (Fisher, Corbet and Williams (1943), the large number of species found at Heron Reef may be a result of the intensive sampling. Even so, it would appear from the linearity of Figures 9.2b that additional species of starfish occur intertidally at Heron Reef, but these species are either extremely rare or cryptic.

It is apparent that Heron Reef carries a rich and diverse asteroid fauna, 24 species belonging to six families having been found intertidally in 120 hectares during this study. The linearity of the species : (log) area relationship for the intertidal asteroid assemblage at Heron Reef indicates that additional species are still to be found. Indeed, Mithrodia clavigera was located subsequent to the traverses and Endean (1956) found three species (Acanthaster planci, Ophidiaster watsoni and Anseropoda rosacea) in the area of the traverses that were not found during the current study.

 

Constancy of Mean Size in Starfish

Figure8Composite

Chapter 8. Constancy of Mean Size

8.1 Introduction

The fluctuations that occur in animal populations have been regarded as a measure of community stability (MacArthur, 1955; Frank, 1968; Den Boer, 1971; Jacobs, 1974; Goodman, 1975; Brown, 1981). Many authors believe that complex high diversity systems are characterised by relative constancy of species composition (e.g. Dunbar, 1960; 1972; Leigh, 1965; Margalef, 1963; 1974). They propose that populations of the component species do not vary to the extent demonstrated in more simple communities. Additionally, the interaction of competitors and predator / prey situations might prohibit the resource monopolisation so characteristic of dominant species in less diverse systems. Other authors (e.g. Connell, 1978; Sale, 1976, 1977; Sale and Douglas, 1984) believe that there is temporal variability in the community structure of the coral-reef organisms they have studied.

A paucity of juveniles characterises the population structures of large bodied, coral-reef starfish (Yamaguchi, 1973 a). It is possible that populations are maintained either by continual low recruitment or occasional high recruitment, each coupled with iteroparity. The juveniles are cryptic and their apparent absence or rarity indicates that reproductive success is either constantly low, sporadic or both. Amongst coral-reef species, population outbreaks have been well documented for only Acanthaster planci. However, population changes in Linckia laevigata following Acanthaster outbreaks have been suggested by Laxton (1974) and Asterina burtoni is known to have extended its range into the Mediterranean Sea following its introduction through the Suez canal (Achituv, 1969).

In most common species of coral-reef asteroid that have been studied, their reproductive strategy was directed towards the production of enormous quantities of gametes which were released directly into the surrounding water (Yamaguchi, 1973 a; 1977 a). If the mortality of the resulting larvae varied greatly from year to year, then we would expect years of noticeable recruitment followed by one or more years of little or no recruitment success. If the adult population is short-lived (e.g. two years), then the recruitment necessary to maintain the population must occur within this short period. If there were another consecutive year of recruitment failure then the species would become locally extinct. In these short-lived species, juveniles should occur in sufficient numbers to be detected. If, however, the adults are long-lived, then the level of recruitment required to maintain the adult population could be extremely low per annum and we might expect to see juveniles only occasionally.

If periods of high recruitment are required to maintain the population structure of a common species, the mean individual size of the populations should vary as a consequence of the influx of juveniles. In Ophidiaster granifer, when periods of recruitment occurred at Guam, the mean individual size of the population decreased (see Yamaguchi and Lucas, 1984). The mean individual size should increase progressively throughout the interval between periods of recruitment. In a large bodied species, such as Linckia laevigata, the mean individual size should increase slowly (dependent on growth rate), but might reach a size equilibrium determined by the availability of food (Paine, 1976). If periods of recruitment occurred, the mean individual size of the population should decrease. If recruitment did not occur, the mean individual size should increase slowly. The rate of increase in mean individual size will be determined by the average individual growth rate. This might be very slow in a species that is long lived.

8.2 Methods

The size data for the more common species were analysed to see if there was any temporal variation in mean size for each species. A one way ANOVA (ratio of variance in mean major radius (R mm) between and within sampling periods) for each species was calculated and the results are listed in Tables 5.1 – 5.10.

The mean size variation required to produce a probability level of .05 or .01 was very small for these relatively common species. Any probability not less than 0.001 represented only a small mean size variation compared with the size variation within each of the populations.

8.3 Results

Table 8.1 The significance of temporal size variation for each of the more common species. The grand mean (R mm), sample size (N), F statistic, degrees of freedom and probability level of the size variation over several sampling periods are tabled. See Figure 8

SPECIES                    MEAN R     N      F    d.f.    P
Linckia guildingii           134     131    1.1   9,82     N/S
Linckia laevigata            127     516    1.7  11,459   <.05
Linckia multifora             38     396    9.5  10,329  <.001
Nardoa novaecaledoniae        88     361    3.2  11,295   <.01
Nardoa pauciforis            104     233    2.2  11,179   <.05
Disasterina abnormalis        15    1109   15.8 12,1054  <.001
Echinaster luzonicus          48     988   11.4 10,883   <.001 

8.4 Discussion

From Tables 5.1 – 5.10, Figures 5.1a – 5.10a and Table 8.1, it can be seen that four of the large-bodied species, Linckia guildingii, Linckia laevigata, Nardoa pauciforis and Nardoa novaecaledoniae, did not vary their mean size greatly over the entire study period of five years. In two of these species, Linckia laevigata and Nardoa pauciforis, the mean size variation was significant at 0.05. In Nardoa novaecaledoniae, while significant at 0.01, this variation still represented only a small change in the mean size of the population over the entire study period.

Although four of the seven common species maintained a size distribution that did not vary greatly during the study period, the possibility that many of the species might demonstrate occasional high recruitment success, when observed on a much larger time scale, cannot be rejected. If such recruitment occurred, it would manifest itself as oscillations in the mean individual size of that species. Asteroids are also known to possess highly plastic growth rates which can effectively disguise annual year classes.

It should be noted that a stable size distribution does not necessarily imply low recruitment and low mortality, but can result from a balance of high recruitment and high mortality. Under conditions of high mortality and low recruitment, a population with a low growth rate can also manifest a stable size distribution but it would show a simultaneous decline in population density. This was not observed in the present study and the small change in the mean size of the large-bodied species suggests that Linckia guildingii, Linckia laevigata, Nardoa novaecaledoniae and Nardoa pauciforis are long-lived.

During this period, Linckia multifora, Disasterina abnormalis and Echinaster luzonicus showed mean size variations that were highly significant. This size variation was the result of periodicity in either sexual or asexual reproduction. In Linckia multifora and Echinaster luzonicus, the difference in size resulted from autotomy. High recruitment of juveniles was observed in only one small bodied, sexually reproducing species, Disasterina abnormalis. In the remaining species, the abundances were low, and statistically valid comparisons of what might have been temporal mean size variation could not be justified. This applied to Ophidiaster granifer that showed periodic recruitment when studied at Guam (Yamaguchi and Lucas, 1984), and Asterina burtoni which did not show significant mean size variation in the present study.

The relative stability of the size distributions of the common large-bodied species can be explained by assuming very slow growth of a predominant year class or a balance of recruitment and mortality within each of the species. It seems likely that a combination of both is involved. The paucity of juvenile asteroids, and the constancy of the size distributions in all the large bodied sexually reproducing species can be explained only by a life-history model which incorporates low adult mortality and includes the assumption of longevity.

The variation in mean size between populations of Linckia laevigata at different localities in the Indo-West Pacific could be caused by the presence of geographically asynchronous, dominant year classes. However, this is unlikely as this species did not alter its mean size greatly during the period of the present study. The highly plastic growth rate may be influenced by nutrition (see Wolda, 1970) or other factors (e.g. disturbance) may cause both the higher density and smaller mean size. Dwarfism, resulting from high salinity, was described in Asterina burtoni by Price (1982).

The results of this study of coral-reef asteroids contrasts with data relating to laboratory rearing of Acanthaster planci which are claimed to demonstrate individual senescence at an age of approximately five years (Lucas, 1984). This finding, which appears to be inconsistent with the general biology of an often rare, large-bodied and venomous animal, can be attributable to the laboratory rearing conditions (see Endean and Cameron, 1990 b). Additionally, a specimen of Acanthaster planci held in an aquarium at the Heron Island Research Station decreased to two-thirds of its original size within a period of 6 months. When adequate food is not available, regression in size might occur in many coral-reef asteroid species. At Heron Reef, the coral-reef asteroid community is not dominated by violently fluctuating size structures as might be expected from the work of Lucas (1984). All the large-bodied, sexually reproducing asteroids in this study existed with a stable size structure for the entire study period.

 

Asexual Reproduction in Starfish

UW209

Chapter 7. Asexual Reproduction

7.1 Introduction

Some species of coral-reef Asteroidea are known to exhibit both sexual and asexual reproduction (Yamaguchi, 1975 b). The species known to reproduce asexually are detailed by Emson and Wilkie (1980). Rideout (1978) has shown that asexual reproduction is the chief form of reproduction in the asteroid Linckia multifora at Guam. Achituv and Sher (1991) have suggested that Asterina burtoni reproduces only by asexual reproduction in the Mediterranean Sea. However, the relative roles of sexual and asexual reproduction in the population maintenance of other coral-reef asteroid species have not been studied. It is not known whether asexually reproducing species have a regular alternation of sexual and asexual activity, or if sexual reproduction is strongly reduced or even absent in any of these species.

In asteroids, asexual reproduction involves either the splitting of the disc (fission), or the casting off of arms that regenerate new starfish (autotomy). In some species, autotomised arms need not contain any section of disc or madreporite for their survival (Clark, 1913; Edmondson, 1935). In such species, regeneration of a mouth and basic digestive organs, must occur while the regenerating arm is metabolising stored reserves of energy (Lawrence et al., 1986). A description of the stages of regeneration, following autotomy in Linckia multifora, is provided by Rideout (1978).

The period of regeneration following autotomy, before the regenerated arms are ready for reautotomy, might be prolonged greatly in species larger than Linckia multifora. Autotomy is reduced when individuals are infected with the parasitic gastropod Stylifer as this parasite inhibits autotomy of infected arms. Because the mortality of regenerating individuals is high (Davis, 1967; Rideout, 1978), though not as great as expected by Clark (1913), this inhibition of autotomy results in greater survival of the parasite (Davis, 1967).

An alternation of sexual and asexual activity has been observed in Nepanthia belcheri (Ottesen and Lucas, 1982). Some sexual activity has been recorded in the asexually reproducing species, Ophidiaster robillardi (Yamaguchi and Lucas, 1984). Other species, such as Asterina anomala and Linckia multifora, often remain small and sexually immature through the continuing process of asexual autotomy or fission. Some sexual activity has been noticed in Linckia multifora and Linckia guildingii (Mortensen, 1937, 1938) but the contribution of this as a means of population maintenance may be outweighed by that of autotomy (Rideout, 1978).

7.2 Methods

Individuals of any species that exhibited signs of recent autotomy or fission were identified, collected and measured. The presence of comets was considered indicative of asexual reproduction by means of autotomy, and recent disc fission, followed by regeneration of more than half the disc, indicative of reproductive binary fission. The frequency of occurrence of asexual products was recorded for each sampling period. Measurements were taken routinely of the number of arms and the length of the longest arm of each individual. Periodically, all arms of asexual specimens were measured and details of obvious regeneration or autotomy were recorded.

In Linckia multifora and Echinaster luzonicus, the mean major radius (mean R mm), mean minor radius (mean r mm) and mean major/minor radius (R/r) were calculated for each sampling period. The variation throughout the study in mean major radius of these species is discussed further in Chapter 8.

7.3 Results

Six species of asteroid occurring at Heron Reef exhibited signs of asexual reproduction either by autotomy or binary fission. Specimens of Linckia guildingii, Linckia multifora, Echinaster luzonicus and Ophidiaster robillardi were observed in varying stages of regeneration following arm autotomy. Specimens of Asterina anomala and Coscinasterias calamaria were observed in varying stages of regeneration following binary fission. Autotomous species can regrow a complete individual from the distal half of an arm, with no need for any portion of the disc or madreporite to be included.

The first stage of regeneration in the autotomised arm is called a comet after its characteristic shape. Comets were encountered frequently in these species and most of the individuals collected in this study had recently autotomised at least one arm, with the remaining arms being in various stages of regeneration. Individuals with all arms of equal length were very uncommon. On several occasions, recently autotomised arms were found alongside the parent animal in the field. In large specimens of Echinaster luzonicus, the process of autotomy can proceed very quickly, and if the animals are handled roughly, arms can be autotomised within seconds. If high water quality and a temperature comparable with that which occurs naturally, are not maintained when Linckia multifora and Echinaster luzonicus are kept in aquaria, freshly autotomised arms die.

During the period of study, large changes in mean major radius were observed in both Linckia multifora and Echinaster luzonicus. At times of smaller mean individual size, the frequency of comet stages in the population was generally higher, but this varied between years. In 1978, the number of comet Echinaster luzonicus found in the field corresponded well with periods of lower mean individual size. For example, in May 1978, 29% of individuals were comets, whereas in August 1978, the proportion of comets was only 15%. This declined to about 5% in June 1979, and did not vary greatly over the remainder of the study. In Linckia multifora the proportion of comets in May, August and November 1978 was 32%. This declined to 8% over the following two years. In 1981 and 1982, the proportion of comets increased to 18%.

The difficulty in locating comet stages will have biased these results. Because the frequency of comets and autotomised limbs was difficult to both sample and analyse (and was not studied in detail), variation in the mean major radius (R) and mean major/minor radius (R/r) was considered a more reliable index of the frequency of autotomy.

In both Linckia multifora and Echinaster luzonicus, variation in the mean major radius (R) and mean major / minor radius (R/r) is illustrated in Figures 7.1 and 7.2. The significance of this variation in mean major radius (R) is discussed further in Chapter 8. While specimens of Linckia guildingii and Ophidiaster robillardi that had recently undergone autotomy were observed throughout the study, only Linckia multifora and Echinaster luzonicus were common enough for this variation in mean size to be analysed.

At Heron Reef, Asterina anomala and Coscinasterias calamaria reproduce by binary fission which results in two halves each regrowing to form two complete individuals. In this case a portion of disc containing a madreporite is always present as both species possess several madreporites on the disc.

The remaining species in the Heron Reef asteroid assemblage, while possessing great powers of regeneration, showed no evidence of using autotomy or fission as a form of asexual reproduction. If parts of the body of these species are autotomised, these parts die and the remaining body regenerates the lost limbs.

7.4 Discussion

Asexual reproduction can offset the effects of intense benthic and planktonic larval predation, as well as those caused by the general vicissitudes of planktonic life which include dispersal loss and starvation (see Yamaguchi, 1975 b; Rideout, 1978; Franklin, 1980; Ottesen and Lucas, 1982; Yamaguchi and Lucas, 1984; Olsen, 1987). On coral reefs, benthic predation of larvae and newly settled recruits might be too high for benthic larval development or brooding behaviour (see Menge, 1975; McClary and Mladenov, 1989; McClary, 1990; Bosch, 1989; Bosch and Pearse, 1990; Komatsu et al., 1990) to be a viable survival strategy.

Two modes of reproduction are employed by some asteroids, one mode allowing the species to disperse, the other, the build-up of population numbers once colonisation is established (Yamaguchi and Lucas, 1984). Cameron and Endean (1982) suggested that autotomy is an adaptation to predation and Birkeland et al (1982) observed autotomy in their study of asteroid predatory interactions. A number of tropical and temperate asteroids are known to undergo regular autotomy (Yamaguchi, 1975 b; Rideout, 1978; Emson and Wilkie, 1980; Crump and Barker, 1985; Mladenov et al., 1989; Dubois and Jangoux, 1990).

The size distribution of Linckia multifora and Echinaster luzonicus varied significantly over the study period (see Table 8.1). While the autotomy rate varied within and between years, the frequency of comet stages in the population was also determined by the survival rate of autotomised arms. This seemed to vary considerably from one year to another. Although comet stages of Linckia guildingii and Ophidiaster robillardi were found, there was no obvious temporal variation in their occurrence. The abundance of these species was not sufficient to allow analysis of the change in mean individual size. Although individuals of Asterina anomala and Coscinasterias calamaria were found in varying stages of regeneration, once again, there appeared to be no temporal pattern in the occurrence of asexual stages (compare with Muenchow, 1978; Ottesen and Lucas, 1982). All specimens of Asterina anomala were small and fissiparous and this species appeared to be distinct from Asterina burtoni, at Heron Reef.

At Heron Reef, sexual reproductive effort appears to be very low in Echinaster luzonicus and Linckia guildingii. No sexual activity was recorded in Linckia multifora at all. Small individuals of these three species resulted invariably from autotomy. It would appear that asexual reproduction is the chief means of population maintenance in these three species. Ophidiaster robillardi is less common than either Linckia guildingii, Linckia multifora or Echinaster luzonicus at Heron Reef. On adjacent Wistari Reef, the density of Ophidiaster robillardi in small patches on the reef crest was considerably higher than at Heron Reef. The spatial distributions of Linckia multifora, Ophidiaster robillardi and Echinaster luzonicus were highly clumped. This patchiness in abundance might be a result of local population increases following an initial sexual colonisation (Ottesen and Lucas, 1982; Mladenov and Emson, 1984; 1990; Crump and Barker, 1985; Johnson and Threlfall, 1987). Linckia multifora and Echinaster luzonicus, together with Disasterina abnormalis (which liberates sticky eggs) occurred at the highest local densities recorded during this study.

 

Sexual Reproduction in starfish

Figure6Composite

Chapter 6. Sexual Reproduction

6.1 Introduction

Most asteroids possess 10 gonads, two in each ray with gonoducts opening in the interradii. In some coral-reef genera, such as Linckia and Nardoa, the gonads are arranged serially with numerous gonoducts existing along the length of the arm. When an individual is ready to spawn, the gonads can occupy the whole length of a ray. In most species, the sexes are separate, but hermaphroditism has been reported in Asterina burtoni (Achituv, 1972; Achituv and Malik, 1985; Achituv and Sher, 1991). In all coral-reef species studied previously, fertilisation is external with gametes being released directly into the water. An off-reef species, Euretaster insignis, belongs to the family Pterasteridae, other members of which are known to brood their young within the supra-dorsal membrane (McClary and Mladenov, 1989; McClary, 1990). The spawning of one individual might trigger other individuals to spawn, thus increasing the chance of fertilisation (Okaji, 1991). Alternately, synchronous spawning might be triggered extrinsically (see Yamaguchi and Lucas, 1984; Minchin, 1987).

The sexual reproductive cycle has been studied in some of the commoner coral-reef species e.g. Asterina burtoni, (by Achituv, 1972; James, 1972; Achituv and Malik, 1985), Linckia laevigata (by Yamaguchi, 1977 a), and Ophidiaster granifer (by Yamaguchi and Lucas, 1984). The type of larval development has also been studied in several species e.g. Astropecten polyacanthus (by Oguro et al., 1975), Acanthaster planci (by e.g. Henderson and Lucas, 1971), Gomophia egyptiaca (by Yamaguchi, 1974), Leiaster leachi (by Komatsu, 1973), Ophidiaster granifer, Ophidiaster robillardi and Ophidiaster squamous (by Yamaguchi and Lucas, 1984). The known forms of reproduction along with the type of larval development of most Guam species are tabled by Yamaguchi (1975 b).

The use of the hormone 1-methyl adenine to produce final maturation and subsequent release of gametes in asteroids is well documented (Kanatani, 1969, 1973). Yamaguchi (1977 a) described the injection of the hormone into the coelomic cavity of Linckia laevigata to assess the stage of development of the gonads. This procedure was also used by Yamaguchi and Lucas (1984). The presence and strength of response to treatment have been shown to depend upon the stage of maturation of the gametocytes (Kuborta et al., 1977).

The time required before a response is produced, following treatment, depends upon the proximity of the natural breeding season (Kanatani, 1969). A delayed response in the genus Echinaster was described previously by Turner (1976). Additionally, it is possible that not all individuals within a population are at exactly the same stage of gamete development at any time (see Pearse, 1968).

6.2 Methods

The reproductive analysis of each of the species entailed the injection of 1-methyl adenine into the arms of a sample of the population several times during each year. The chemical was obtained as anhydrous powder in 10 mg tubes from Sigma Chemicals. The anhydrous powder was kept frozen following delivery. The concentration of 1-methyl adenine used for injection was 0.0001 M. dissolved in sea-water. A working solution was prepared freshly for each sampling period and stored in a refrigerator at 4°C. It was necessary to warm the sea-water temporarily to 40°C to facilitate the dissolution of the chemical as the working solution was being prepared. Once the chemical was dissolved (requiring about five to ten minutes with stirring) it was immediately placed in the refrigerator. The solution was not warmed again before injection into the starfish.

The quantity injected per individual depended on the mean size of the species tested. It ranged from one millilitre per individual in small species such as Ophidiaster granifer to five millilitres per individual in large species such as Linckia laevigata. In large species, the hormone was injected into three of the arms of each animal while it was temporally removed from the aquarium. In smaller species the injection was administered aborally into one interradius. Following injection, each animal was returned to its aquarium along with other conspecifics which had also been injected. The test animals were then observed for several hours and any release of gametes through the gonopores was recorded. To maintain visibility within the test aquaria, specimens were removed and placed in a larger tank once they commenced spawning. Spawning always continued following the transfer. Gametes that were fertilised in aquaria were never released in the field.

The procedure used by Kanatani (1969) required the extraction of gonad for in vitro treatment with 1-methyl adenine. Other methods of determining reproductive periodicity, such as gonad index or histological examination require the test individuals to be killed, and the gonads removed. Many species of coral-reef asteroid occur in low abundance and the regular killing of test individuals would have required the use of much smaller sample sizes to ensure that the population was not reduced by periodic testing. In the present study the convenience of an in vivo treatment, that allowed the rapid testing of a large number of individuals of each species, was considered to outweigh the limitations imposed by the lack of detailed histological information.

Possible variability in response within samples required sample sizes in the vicinity of 20 to 30 individuals to ensure statistical significance of the different spawning frequencies, at different times of the year. Histological study would have provided direct qualitative evidence of gametogenesis. The dichotomous (presence / absence) spawning data obtained in the present study required larger sample sizes to demonstrate any periodicity conclusively. The G Test was used to establish that the observed response in the breeding season was significantly different from the null (low all year round) response. While in some cases, the “expecteds” were as low as two, the G statistic appeared sufficiently high to indicate a significant spawning response.

6.3 Results

Release of gametes required no longer than 3 hours after injection except in Echinaster luzonicus. However, for every species that was studied, the response time varied throughout the year. This time ranged from just under three hours, two months before the breeding season, to as little as 15 minutes at the peak of the season. At this peak, if the water temperature within aquaria was allowed to rise above that of the reef flat (as it would on a very hot day), spontaneous spawning was observed in species of Linckia and Nardoa. Spawning in the field was not observed during this study. The result with 1-methyl adenine was always reduced if the test animals had undergone previous spawning.

Tables 6.1a to 6.8a list the spawning response to injection with 1-methyl adenine and Figures 6.1 to 6.8 graph the annual spawning pattern of each of the common species over the study period. Tables 6.1b to 6.8b show the results of the G test, comparing the spawning response in four seasons. Table 6.9 lists the reproductive strategies of each species.

Providing that the water did not become too cloudy, it was always possible to determine the sex of the individuals by the type of gamete released. The size, number and development of eggs was not studied in detail, but varied among species depending on the type of larvae produced.

The results of observations, relating to 1-methyl adenine injections, on the sexual reproductive cycles of the commoner asteroid species at Heron Reef are outlined below.

Females of Fromia elegans release from 100 to 200 very large (approximately 2.0 mm diameter) eggs from gonopores, two of which are located in each interradius. The eggs are bright red in colour and are of neutral buoyancy. Egg release may take up to three hours following injection but can occur after only 30 minutes. The males spawn within one hour and sperm are released through gonopores which are located slightly higher in each interradius than in the female. The fertilised eggs undergo lecithotrophic development. The peak of sexual activity occurred in early summer (November, December).

Females of Linckia guildingii release large numbers of small (approximately 0.1 mm diameter), colourless and negatively buoyant eggs through gonopores located serially along the arms. At no time was the sexually active proportion of the population very high. The peak in sexual activity that was apparent occurred in mid summer (December).

The reproductive cycle of Linckia laevigata, at Guam, was studied by Yamaguchi (1977 a). Females release very large numbers of small (approximately 0.1 mm diameter), colourless and negatively buoyant eggs through gonopores located serially along the arms. At Heron Reef, L. laevigata showed a positive response to treatment (approximately 1 million eggs shed) from mid-winter to mid-summer. Spontaneous spawning occurred only in the summer months (November, December).

Females of Nardoa novaecaledoniae release approximately 1000, large (approximately 1.0 mm diameter), orange and positively buoyant eggs from gonopores arranged serially in each arm. The eggs undergo lecithotrophic development. The peak of sexual activity occurred in late summer (January) but specimens dissected in mid-winter (July) showed extensive gonad development. No response to 1-methyl adenine injection could be produced at this time. It would appear that this species can spawn and undergo complete gametogenesis within six months, but spawning was observed only once a year.

Females of Nardoa pauciforis produce eggs which appear very similar to that of Nardoa novaecaledoniae. They are the two common species of this genus on the Great Barrier Reef and are both very similar as adults. They are distinguished by the compression of the distal plates of the arms in Nardoa novaecaledoniae. The peak of sexual activity occurred one month earlier (December) in Nardoa pauciforis than in Nardoa novaecaledoniae, indicating a degree of reproductive isolation in these species at the southern end of the Great Barrier Reef.

Ophidiaster granifer produces eggs which undergo parthenogenetic development (Yamaguchi and Lucas, 1984) and in this study only females (total of 7 individuals) were observed to spawn. Small numbers (20-60 per female) of large (0.6 mm diameter), neutrally buoyant, bright red eggs underwent at least initial development despite no obvious sperm having been released in the water. The only spawning activity in this species was observed in early summer (November, December).

Disasterina abnormalis does not appear to be abundant on the Great Barrier Reef other than in the Capricorn Group at its southern end. At Heron Island this species is abundant behind an extensive rubble bank on the northern side of the reef. The eggs of this species are small (approximately 0.1 mm diameter), colourless and sticky. They sank to the bottom of the aquarium and adhered to the glass, from which they were hard to dislodge. The type of development is unknown. The peak of sexual activity occurred in late spring (October).

Echinaster luzonicus liberates small numbers (20-100 per female) of positively buoyant and approximately 1.0 mm in diameter red eggs during late summer (February). Specimens with fully developed arms were selected for the injection of hormone as arm regeneration following autotomy, which is common in this species, may be at the expense of gonad development.

The remaining species either showed no response to 1-methyl adenine or were not sampled in sufficient numbers to establish reproductive periodicity. The following responses to treatment occurred:

Iconaster longimanus no response July (N=1)

Culcita novaeguineae one female February (N=4) no response October (N=4)

Asteropsis carinifera no response July (N=1)

Gomophia egyptiaca two males and one female December (N=3) no response February (N=1), May (N=1), July (N=2)

Linckia multifora no response throughout study (N=120)

Nardoa rosea one male February (N=1) no response July (N=2), November (N=4)

Ophidiaster armatus one male July (N=2) no response February (N=1), October (N=1), December (N=1)

Ophidiaster confertus no response July (N=1)

Ophidiaster robillardi no response May (N=7), June (N=1), October (N=4), November (N=1), December (N=3)

Tamaria megaloplax no response July (N=1)

Asterina burtoni no response throughout study (N=59)

Echinaster stereosomus no response July (N=2)

Table 6.9 The primary type of reproduction (REPRO) and the type of larval development (DEVEL) where known are shown for all asteroid species recorded from Heron Reef. – = not known; PLANK = Planktotrophic; LECITH = Lecithotrophic A = Achituv (1972); B = Barker (1977); Y = Yamaguchi (1975); * = this study

SPECIES                         REPRO.	DEVEL.	SOURCE
                                                
Astropecten polyacanthus        SEXUAL	PLANK	Y
Iconaster longimanus            -	-    
Culcita novaeguineae            SEXUAL	PLANK	Y
Acanthaster planci              SEXUAL	PLANK	Y
Asteropsis carinifera           SEXUAL	PLANK	Y
Dactylosaster cylindricus       -	-
Fromia elegans                  SEXUAL	LECITH	*
Fromia milleporella             -	-
Gomophia egyptiaca              SEXUAL	LECITH	Y
Linckia guildingii             ASEXUAL	PLANK	*
Linckia laevigata               SEXUAL	PLANK	Y
Linckia multifora              ASEXUAL	PLANK	Y
Nardoa novaecaledoniae          SEXUAL	LECITH	*
Nardoa pauciforis               SEXUAL	LECITH	*
Nardoa rosea                    SEXUAL	LECITH	*
Neoferdina cumingi              -	-
Ophidiaster armatus             -	-
Ophidiaster confertus           -	-
Ophidiaster granifer            SEXUAL	LECITH	Y
Ophidiaster lioderma            -	-
Ophidiaster robillardi         ASEXUAL	-
Ophidiaster watsoni             -	-
Anseropoda rosacea              -	-
Asterina anomala               ASEXUAL	-	A
Asterina burtoni                SEXUAL	-	A
Disasterina abnormalis          SEXUAL	-	*
Disasterina leptalacantha       -	-
Tegulaster emburyi              -	-
Mithrodia clavigera             SEXUAL	PLANK	Y
Echinaster luzonicus           ASEXUAL	LECITH	*
Coscinasterias calamaria       ASEXUAL	PLANK	B

6.4 Discussion

The species of coral-reef asteroids studied at Heron Island showed differences in the length of their breeding season and this may reflect on their colonisation ability (Mileikovsky, 1971). The length of the breeding season within a species might vary with latitude and the further the population is from the equator, the shorter may be the season for summer breeders. However, except for Linckia laevigata, the species studied at Heron Reef generally showed a one to two month breeding season. In two species, Linckia multifora and Asterina burtoni, no sexual activity was observed throughout the study. This might result from lower than required water temperature at Heron Reef for most of the year (see Mladenov et al., 1986). In this regard, Mortensen (1937) was able to obtain eggs from Linckia multifora in the Red Sea where the water temperature is higher than at Heron Reef. It might also be correlated with an increased emphasis on asexual reproduction in Linckia multifora once a reef has been colonised by a few sexually reproduced individuals. Although Asterina burtoni was not observed to undergo either sexual or asexual reproduction, the distinction between A. burtoni and the small fissiparous A. anomala is unclear. It is possible that A. anomala is an asexually reproducing form of A. burtoni.

Nardoa novaecaledoniae and N. pauciforis possess arms swollen with gametes for much of the year but still have only a limited breeding season. At Heron Reef, at the southern end of the Great Barrier Reef, Nardoa pauciforis is reproductively mature earlier in the summer than is Nardoa novaecaledoniae and its eggs are released in November or early December. At this time Nardoa novaecaledoniae is not capable of releasing eggs and sperm. Although both species look similar they appear to have limited interbreeding, at least over part of their geographic range. In general, temperature seems to be an important factor in gametogenesis, but the final spawning trigger is dependent on lunar/tidal cycles in many species (Pearse, 1970, 1975; Yamaguchi and Lucas, 1984).

In coral-reef asteroids the range in fecundity is extremely large. Fromia elegans, Gomophia egyptiaca, all species of Nardoa, Ophidiaster granifer and Echinaster luzonicus produce large, buoyant, highly pigmented and yolky eggs. While larval development was not studied in detail, the initial phases of lecithotrophic development were observed in these species. The large reserves of yolk should ensure that the resulting larvae need not feed while in the plankton. The number of eggs produced with this development was not studied in detail but appeared to be relatively small (that is, less than 1000 and sometimes much fewer per individual). The eggs are buoyant, opaque and 0.6 to 2.0 mm in diameter. For any species, the egg size, in combination with number of eggs liberated, is an index of reproductive effort. The energetic fecundity in relation to body size of different species, might represent qualitatively different reproductive strategies.

Planktotrophic larvae are produced by Astropecten polyacanthus, Choriaster granulatus, Culcita novaeguineae, Acanthaster planci, Asteropsis carinifera, all species of Linckia, Mithrodia clavigera, Leiaster leachi and Coscinasterias calamaria (Yamaguchi, 1975; Barker, 1977). With this type of larval development, many (up to 1 million), small (0.1 to 0.2 mm), non-yolky, transparent eggs are produced. These eggs appeared less buoyant than eggs that contain large yolk reserves. This may influence dispersion.

Larvae of species of Nardoa and other genera which undergo lecithotrophic development may be less likely to die of starvation in the plankton compared with those of species that undergo planktotrophic development and have an obligate larval feeding stage before settlement (see e.g. Thorson, 1950, 1966; Vance, 1973; Barker, 1977; Strathmann and Vedder, 1977; McEdwards and Janies, 1993). Lecithotrophic larvae have shorter development times, but the planktonic stage can be extended if suitable settlement sites are not available (Strathmann, 1978; Yamaguchi, 1974; Yamaguchi and Lucas, 1984). However, these larvae cannot remain in the plankton for as long as larvae with planktotrophic development. Their dispersal ability and genetic exchange is lower (Scheltema, 1968, 1971; Nishida and Lucas, 1988; Nash et al., 1988; Mladenov and Emson, 1990; Benzie and Stoddart, 1992 a,b). Yamaguchi (1975 b) has commented on the low abundance of lecithotrophic species on oceanic atolls. However, such species are well represented at Heron Reef, a situation that might result from the proximity of adjacent reefs, which would allow short lived larvae from one reef to settle on nearby reefs ( see Fisk and Harriott, 1990). Any species may have difficulty colonising over distances greater than its larval dispersal capacity and lecithotrophic species might suffer local extinction following large scale destruction or alteration of coral reef habitat.

Despite the high sexual reproductive effort displayed by most of the large-bodied species, there is little evidence of high recruitment of starfish at Heron Reef. Loosanoff (1964) observed periodic high recruitment during a 25 year study of the temperate species Asterias forbesi. Periodic high recruitment has also been observed in the coral reef species Acanthaster planci.

It is possible that many eggs are never fertilised when adult populations exist at low densities, such as at Heron Reef. Many fertilised eggs or subsequent larvae would also die from predation or starvation in the plankton (Jackson and Strathmann, 1981; Olsen, 1987). The availability of suitable settling substrate or post-settlement benthic predation might also limit the recruitment of juveniles.

It might be expected that planktonic regulation would be less constant than benthic regulation because of the relative unpredictability of small scale water circulation and the extremely patchy distribution of planktonic predators compared with a possibly more regular and species-specific mortality caused by benthic predators. The post-settlement survival of small juvenile Acanthaster planci was examined by Keesing and Halford (1992) and Keesing and Cartwright (1993) who found a difference in survivorship between caged specimens compared with uncaged specimens. For the less common species at Heron Reef, it is possible that many of their eggs are not fertilised. Adult numbers will be further regulated by a combination of either larval starvation or larval and juvenile mortality.

When the energy content of a liberated egg is considered, two different reproductive strategies are apparent. Sexual recruitment can follow either planktotrophic or lecithotrophic larval development (Hendler, 1975; Yamaguchi, 1977 b; Lessios, 1990; McEdwards and Chia, 1991). Because many small eggs can represent the same investment of energy as a few large eggs, the energetic fecundity per unit body weight can be similar in both strategies, despite the difference in numerical fecundity.

It has been suggested (Vance, 1973; Yamaguchi, 1973 a, 1973 b, 1977 b) that lecithotrophic development is an adaptation to high predation or starvation of larva. With this development it is possible the length of larval life can be shorter and hence larval survival should be favoured. On oceanic atolls, species with lecithotrophic development are never abundant and this could result from their poor dispersal ability (Yamaguchi, 1975 b). On Heron Reef, and possibly the Great Barrier Reef in general, where many reefs exist in relatively close proximity, lecithotrophic genera such as Nardoa, Fromia and Echinaster appear to be more abundant than they are on atolls. However, planktotrophic development is favoured where high dispersal is required or when planktonic predation is low and planktonic food is predictable (Menge, 1975; Mileikovsky, 1971; Vance, 1973; Yamaguchi, 1977 b). Many species with this type of larval development occur on oceanic atolls but they occur also on the reefs of the Great Barrier Reef.

Disasterina abnormalis liberated small sticky eggs which sank and adhered to the substrate. The type of development was not studied, but the low dispersion capacity of its eggs might explain its apparently limited distribution. This species was abundant locally. In all other sexually reproducing species, juveniles were uncommon and there was no evidence of either periodic high recruitment or mortality.

The extent of larval dispersion is an important factor in our understanding of the community dynamics within a reef or reef system. If high between-reef larval dispersal occurs, then the adaptive significance of the dispersal phase is the location of spatially and temporally isolated patches in the survival mosaic of each species. Alternately, if larvae recruit primarily into the parent reef population, as a result of circular water movement patterns (Atkinson, Smith and Stroup, 1982; Dight et al., 1990 a, b; Black and Moran, 1991; Black, 1993 but see Wolanski, 1993), then the adaptive significance of the dispersal phase is the avoidance of planktonic or benthic predation in shallow water.

The development time of one month, possessed by many asteroid larvae with planktotrophic development (Yamaguchi, 1977 b; Williams and Benzie, 1993), allows potentially high dispersal, and this development time can be extended further if no suitable settlement site is available. Larvae with lecithotrophic development are capable also of extending the length of the pelagic phase (Yamaguchi, 1974; Yamaguchi and Lucas, 1984). The larvae of coral-reef starfish generally require a solid substrate to complete their development, and a coralline algal substrate has been observed as the chosen settling surface for many species (Yamaguchi, 1973 b; Johnson et al., 1991). More complex species specific signals, located by sensitive chemosensory receptors might ensure settlement in habitats which are conducive to survival of post-settlement stages (Burkenroad, 1957; Morse, 1984). Yamaguchi (1977 c) showed that some juvenile starfish have exponential growth during the period following settlement and proposed that juveniles are subject to high mortality during this period. The juveniles transform to adult morphology at a certain size and before this size is attained may look quite different from adults (e.g. Culcita novaeguineae illustrated by Clark, 1921).

The phenomenon of aggregation (Ormond et al., 1973), parthenogenetic development (Yamaguchi and Lucas, 1984), hermaphroditism (Achituv, 1972) or asexual reproduction (Rideout, 1978) may be correlated with survival at low population densities. In low density, spatially dispersed populations of starfish, there is a low probability of locating a conspecific of the opposite sex at breeding time.

 

What size are starfish at Heron Reef?

Major Radius (R) / Frequency – Echinaster luzonicus (total)

Echinaster luzonicus Size

Chapter 5. Size Structure

5.1 Introduction

Many workers who have studied coral-reef asteroid populations have noted the adult-dominated size structure of these populations (Clark, 1921; Ebert, 1972; Yamaguchi, 1977 a). Similar findings have been made for asteroid populations from other communities (e.g. Paine, 1976). Although juvenile asteroids have been encountered on coral reefs, their abundance was so low and their appearance sufficiently different from that of the adults in some species, that juveniles have been placed in a species different from the adult (see Yamaguchi, 1975 b).

The population structure of some of the more common, large-bodied species of coral-reef asteroids such as Linckia laevigata has been studied (Yamaguchi, 1977 a; Laxton, 1974; Thompson and Thompson, 1982). In these studies, the size distributions of the asteroid were unimodal indicating either a large overlapping of generations or a dominant year class. If the latter alternative is true, the variation in growth rates within the population must be extraordinary to produce the observed size range. Most of the studies on Acanthaster planci occurring under non-outbreak conditions have shown a primarily adult population with small juveniles occurring only occasionally (Yamaguchi, 1973 a; Zann et al, 1987). In studies of large-bodied coral-reef asteroids, few individuals were found which were smaller than half the average size (Yamaguchi, 1973 a, 1973 b; 1977 a). In this chapter, the maximum size attained and the population structure of each of the commoner species constituting the asteroid fauna of Heron Island Reef will be investigated.

5.2 Methods

Specimens collected on traverses were allowed to resume an extended shape in a plastic bucket and were then measured using a plastic ruler. When animals were measured both the length from the mouth to the tip of the longest arm, and the average length from the mouth to the interradius were recorded to an accuracy of one millimetre. These are called major radius (R) and minor radius (r) respectively and are expressed in millimetres (mm). “R” was always measured along the ambulacral groove. After measurement, individuals were placed along with conspecifics in glass aquaria that were provided with fresh running sea water if they were needed for later experiments relating to their reproduction.

The major radius (R) is used as a measurement of overall size. The ratio of major radius (R) to minor radius (r) is referred to as “R/r” or “R:r”. Because it is a ratio it has no units. It is a measurement of the degree of arm elongation and is of taxonomic significance.

In addition to specimens collected on traverses at Heron Reef many sub-tidal specimens of Fromia elegans and specimens of Disasterina abnormalis found during the quadrat study were measured. This is the reason for the difference in sample size between the tables of abundance and mean size.

Juvenile asteroids of most common species were located under boulders on the reef crest. Their identification, although initially difficult, was always possible following microscopic examination and reference to earlier studies (Clark, 1921; Yamaguchi, 1975 a, 1975 b, 1977 a).

5.3 Results

Tables 5.1 – 5.10 summarise the mean size data of all individuals recorded in each sampling period for each of the common species. ANOVA tables showing the significance of variations of major radius (R) are included. Figures 5.1a – 5.10a graph the frequency distribution of major radius (R). Figures 5.1b – 5.10b graph the frequency distribution of major radius / minor radius (R/r). Figures 5.1c – 5.10c graph the relation between minor radius (r) and major radius (R). Figures 5.1d – 5.10d graph the relation between major radius / minor radius (R/r) and major radius (R). The relation between these two radii is variable, but it is frequently used as a taxonomic distinction. Minor radius (r) was not measured in August and November 1978 and these data are excluded from the ANOVA. Table 5.11 is a summary of the size data for each of the 24 species that occurred on intertidal traverses.

Juveniles of Fromia elegans, Linckia laevigata, Nardoa novaecaledoniae and Nardoa pauciforis occurred rarely. The size-frequency distributions of major arm radius (R) show clearly the paucity of small individuals (R less than half the mean R) in the populations of these species. Linckia guildingii, Linckia multifora, Ophidiaster robillardi, Asterina anomala, Echinaster luzonicus and Coscinasterias calamaria reproduce asexually. Small individuals of these species, resulting from autotomy or fission, occurred intermittently throughout the period of the study . Juveniles of Asterina burtoni and Disasterina abnormalis occurred throughout the period of the study. The size- frequency distributions of major arm radius (R) show the abundance of small individuals in these species. Ophidiaster granifer was not common, but its highly skewed size-frequency distribution shows the presence of medium sized individuals (R half the mean R). With the exception of one specimen each of Culcita novaeguineae (R=50 mm) and Gomophia egyptiaca (R=10 mm), juveniles of less common species did not occur.

Table 5.11

The mean major radius (MEAN R mm), mean major radius / minor radius (MEAN R/r) and sample size (N) of the species that occurred on traverses at Heron Reef.

                            MEAN R mm    MEAN R/r        N

Culcita novaeguineae         109           1.3          12
Asteropsis carinifera         74           2.8           3
Dactylosaster cylindricus     83            -            1
Fromia elegans                32           3.9         183
Fromia milleporella           30           2.3           1
Gomophia egyptiaca            45           4.8          10
Linckia guildingii           134           9.8         131
Linckia laevigata            127           6.5         516
Linckia multifora             38           7.3         396
Nardoa novaecaledoniae        88           5.7         361
Nardoa pauciforis            104           6.6         233
Nardoa rosea                  88           6.2           7
Ophidiaster armatus           55           6.3           7
Ophidiaster confertus         86           8.2           4
Ophidiaster granifer          27           4.3         121
Ophidiaster lioderma         105            -            1
Ophidiaster robillardi        37           6.9          21
Asterina anomala               4           1.6          16
Asterina burtoni              13           1.8         203
Disasterina abnormalis        15           1.9        1109
Disasterina leptalacantha     13           2.0           8
Tegulaster emburyi            18           2.3           1
Echinaster luzonicus          48           5.8         988
Coscinasterias calamaria      19           4.4           8

5.4 Discussion

The species of starfish that occurred on the traverses can be grouped according to general body size. The species that had a maximum major radius (R) greater than 100 mm were regarded as large-bodied. The large-bodied species (maximum major radius in parenthesis) are Culcita novaeguineae (130 mm), Linckia guildingii (240 mm), Linckia laevigata (190 mm), Nardoa novaecaledoniae (115 mm), Nardoa pauciforis (135 mm), Nardoa rosea (101 mm), Ophidiaster confertus (102 mm) and Ophidiaster lioderma (105 mm).

Small-bodied species had a maximum major radius (R) that was less than 100mm. These species were Asteropsis carinifera (85 mm), Dactylosaster cylindricus (83 mm), Fromia elegans (46 mm), Fromia milleporella (30 mm), Gomophia egyptiaca (64 mm), Linckia multifora (90 mm), Ophidiaster armatus (73 mm), Ophidiaster granifer (48 mm), Ophidiaster robillardi (50 mm), Asterina anomala (7 mm), Asterina burtoni (23 mm), Disasterina abnormalis (27 mm), Disasterina leptalacantha (20 mm), Tegulaster emburyi (18 mm), Echinaster luzonicus (100 mm) and Coscinasterias calamaria (30 mm).

With the exception of Ophidiaster confertus and Ophidiaster lioderma, large-bodied species possessed an extremely tough body wall. Ophidiaster confertus is a temperate species and Ophidiaster lioderma (an extremely rare species) is covered with a greatly thickened skin. The cut-off distinguishing large-bodied from small-bodied starfish at maximum R = 100 mm is arbitrary, and both Asteropsis carinifera and Dactylosaster cylindricus could be included in this large-bodied group if the distinction was based on mean size. The mean size of the two next largest small-bodied species, Linckia multifora and Echinaster luzonicus, remained about half the maximum size through a continuing process of autotomy.

It can be seen from the data presented in Tables 5.1 to 5.11 and Figures 5.1a to 5.10a that juveniles of the relatively common, sexually reproducing, large-bodied asteroids, Linckia laevigata, Nardoa novaecaledoniae and Nardoa pauciforis were rare and the populations of these species were adult dominated throughout the study period. Relatively small specimens of Linckia guildingii resulting from asexual reproduction were observed but these individuals represent only a minor component of the adult dominated population. In all large bodied species, distinct year classes were not observed in the population size structures. While a highly variable growth rate can disguise a dominant year class, the fact that neither numerous small individuals (indicating high recruitment) nor obvious population declines (evidence of high mortality) was observed over a period of several years, suggests strongly that these species are long-lived (persistent).

Small individuals were more common in the populations of Linckia multifora, Echinaster luzonicus and Disasterina abnormalis, and to a lesser extent in the populations of Ophidiaster granifer and Asterina burtoni. These juveniles resulted from either sexual or asexual reproduction. While small specimens occurred in the sub-tidal population of Fromia elegans, its population structure was still adult dominated throughout the period of study.

Many hypotheses have been proposed to explain the apparent paucity of juveniles amongst coral-reef echinoderms. Juveniles may occupy such different habitats from the adults that they have not been adequately sampled or the adult animals may be long lived and recruitment low (Yamaguchi, 1977 a). Recruitment may also be patchy in both time and space (Yamaguchi, 1973 a, 1973 b, 1977 b). In studies of some fishes, it has been shown that each reproductive season large quantities of sperm and eggs are released but owing to the rigours of planktonic life and the uncertainty of locating settling substrate, most larvae are lost before settlement (Sale, 1976, 1977). Endean and Cameron (1990 b) proposed that the mortality that regulates the adult population density of Acanthaster planci occurs on post-settlement stages.

The giant triton (Charonia tritonis) and other members of the genus Charonia are known predators of many species of starfish (Chesher, 1969 b; Endean, 1969; Laxton, 1971; Noguchi et al., 1982; Percharde, 1972) and other predators of starfish include shrimp (Glynn, 1974), a worm (Glynn, 1984), fish (Ormond et al., 1973) and other starfish (Mauzey et al, 1968; Dayton et al, 1977; Birkeland et al, 1982). In most species at Heron Island, evidence of starfish mortality was hard to find. This has also been true for Acanthaster planci, even following population outbreaks. A high incidence of sub-lethal predation on adult Acanthaster planci was reported by McCallum et al. (1989), who suggested that lethal predation could account for the paucity of juveniles in populations of Acanthaster planci.

Parasitism of starfish by molluscs is well known (see e.g. Davis, 1967; Elder, 1979; Egloff et al., 1988). At Heron Reef, the incidence of infection by molluscs was low, except in Linckia multifora and Ophidiaster granifer. Bouillon and Jangoux (1985) recorded a high proportion of Linckia laevigata infected, but a high rate of infection of this species did not occur at Heron Reef.

Yamaguchi (1977 a) showed a much higher abundance of Linckia laevigata at Guam (a reef known to carry A. planci outbreaks) but the mean individual size was much smaller than in the present study. Thompson and Thompson (1982) and Laxton (1974) also found a smaller mean size of Linckia laevigata compared with that found in this study. Thompson and Thompson’s study was conducted at Lizard Island, Queensland (a reef known to carry A. planci outbreaks) and a greater spatial variation in density and mean size was found than was evident at Heron Reef. While Laxton’s samples were taken from Heron Reef in approximately the same habitat as was used in this study, the abundance of Linckia laevigata is not stated and there appears to have been confusion with Linckia multifora. Observations made at Lady Musgrave Reef in the Bunker Group where Linckia laevigata was much more abundant than at Heron Reef also show a smaller mean size of specimens from some habitats. Lady Musgrave Reef is known to have carried a minor outbreak of Acanthaster planci. The data indicate that abundance and mean individual size may be inversely correlated, but abundance appears to be more closely regulated on a reef such as Heron Reef that is not known to have carried an A. planci outbreak. Apart from the destruction of the hard coral cover caused by such outbreaks, great changes occur in the fauna and flora of reefs following A. planci population outbreaks (Endean and Cameron, 1990 b).