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.