Major Radius (R) / Frequency – Echinaster luzonicus (total)
Chapter 5. Size Structure
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.
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).
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.
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
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).