Population density, size-frequency and reproductive data on an assemblage of shallow water, coral-reef starfish (Asteroidea) were gathered over several years at Heron Reef. Heron Reef is a reef in the Capricorn Group at the southern end of the Great Barrier Reef. It has not been known to carry an outbreak of the corallivorous crown-of-thorns starfish (Acanthaster planci) and its coral cover is well developed. Specimens required primarily for size-frequency and reproductive analysis were collected by means of quadrats, general searches and intertidal traverses carried out at the western end of the reef. Most traverses included both reef flat and reef crest zones and all exposed starfish within a four meter width were collected for the length of the traverse. In addition, a selection of large and small, dead coral slabs were overturned and cryptic specimens located beneath these slabs were collected.
The finding of Asteropsis carinifera, Dactylosaster cylindricus, Fromia elegans, Linckia multifora, Ophidiaster armatus, Ophidiaster lioderma, Ophidiaster rohillardi, Asterina anomala, Disasterina abnormalis, Tegulaster emburyi, Mithrodia clavigera and Coscinasterias calamaria represent new records for Heron Reef. This study has also provided the first record of the predominantly temperate species, Coscinasterias calamaria on a reef of the Great Barrier Reef. Essentially, the Heron Reef asteroid fauna is comprised of widely ranging West Pacific and Indo-West Pacific species plus a few species that appear endemic to the reefs of the Capricorn Group or are sub-tropical, rocky-shore species that have extended their ranges to include the southernmost reefs of the Great Barrier Reef.
It was found that Heron Reef carries a rich and diverse asteroid fauna and the linearity of the species : (log) area relationship indicates that additional species are still to be located. Of the 25 starfish species found on Heron Reef, 17 (Asteropsis carinifera, Dactylosaster cylindricus, Fromia milleporella, Linckia laevigata, Nardoa novaecaledoniae, N. pauci foris, 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. An additional three species (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 have been recorded intertidally. 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. The preceding species can be regarded as coral-reef species and their distribution differs from that of species such as Astropecten polyacanthus, Iconaster longimanus, Pentaceraster regulus, Leiaster leachi, Nardoa rosea, Ophidiaster armatus, Tamaria megaloplax, Echinaster stereosomus and Euretaster insignis that are found in the deeper off-reef waters in the Heron Island region. The observation that most species of coral-reef starfish found on Heron Reef appear to be confined to the reef top (reef flat or reef crest) does not appear to have been noted previously.
The coral-reef asteroids found on Heron Reef showed some inter-specific variation with respect to diet but many species appeared to feed on epibenthic felt. The Heron Reef asteroids also showed some inter-specific variation with respect to habitat but some species occurred in exposed situations. Clear examples of niche specialisation (dietary or micro-habitat) are known only for the corallivores Culcita novaeguineae and Acanthaster planci. The Heron Reef asteroids did not appear to be resource limited.
Competitive interactions involving the Heron Reef asteroid assemblage were not observed during the five year period of the study as most species occurred with densities that were low. Indeed, the majority of species in this assemblage are considered to be rare or very rare. Even Echinaster luzonicus, the most abundant species, had an average density of only 16 specimens per hectare. Individuals of the more common species were patchily distributed (clumped). Only four species of starfish showed clear changes in density during the study period. These species were Linckia multifora, Asterina burtoni, Disasterina abnormalis and Echinaster luzonicus.
The small-bodied starfish Disasterina abnormalis occurred at an average density of over 8 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.
Juveniles of the relatively common, large-bodied, sexually reproducing asteroids Linckia laevigata, Nardoa novaecaledoniae and N. pauciforis were rare and their populations were adult-dominated throughout the study period. In all large-bodied species studied, distinct year classes were not observed in the population size structure and mortality was rarely observed. These species appear to possess low recruitment and low adult mortality. Juveniles were more common in the populations of Linckia multifora, Asterina burtoni, Disasterina abnormalis and Echinaster luzonicus and resulted from either sexual or asexual reproduction.
In the commoner of the large-bodied species, Linckia guildingii, L. laevigata, Nardoa novaecaledoniae and Nardoa pauciforis, little or negligible change in mean size was observed over the study period of five years indicating that these species are long-lived (persisters). In the same period, Linckia multifora, Disasterina abnormalis and Echinaster luzonicus showed mean size variation that was highly significant, this variation being the result of periodicity in either sexual or asexual reproduction. Such species are shortlived (opportunists). In the other coral-reef asteroid species encountered, abundances were too low for statistically valid comparisons to be made.
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. 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. On the other hand, 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. In these species, the population structure appeared to be adult-dominated and juveniles were encountered only rarely.
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.
Small sexual recruits of Disasterina abnormalis were relatively common in one highly localised area at Heron Reef, but high sexual recruitment was not observed in any of the other species. Disasterina abnormalis possessed small (non-yolky) sticky eggs that adhered to the substrate immediately following their release from the gonopores. All the remaining species possessed eggs that dispersed and underwent either planktotrophic or lecithotrophic larval development and 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 ncvaecaledoniae, Nardoa pauci foris, 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 bertoni were injected regularly with 1-methyl adenine, but did not release gametes during the entire study. In addition to tie species that demonstrated a sexual reproductive pattern, Linckia guildingii, Linckia multifora, Ophidiaster robillardi and Echinaster luzonicus reproduced asexually and exhibited conet stages while Asterina anomala and Coscinasterias calamaria reproduced asexually by binary fission.
With the exception of Disasterina abnormalis, all the species of starfish at Heron Reef either possessed a planktonic dispersive larval phase or did not reproduce sexually. The largest-bodied persistent species released planktotrophic eggs while the opportunist species were either lecithotrophic, hermaphroditic, parthenogenetic or solely asexually reproducing. No opportunistic species was observed to possess planktotrophic development.
Sone 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 including short life, high recruitment and high mortality.
Other species, namely Culcita novaeguineae, Linckia laevigata, Linckia guildingii, Nardoa novaecaledoniae and Nardoa pauciforis could be regarded as persistent species as proposed by Endean and Cameron (1990 a) and were characterised by less abundant populations with relatively smaller fluctuations in mean individual size than the opportunist species. These medium to large bodied species demonstrated all of the typical persister characteristics which include rarity, long life, low recruitment and low mortality. Despite searches over wide areas of different habitat, at different times over a 5 year period, a large proportion of the Heron Reef starfish species were sufficiently uncommon to preclude any analysis of either their relative abundance or size distributions. They have the attribute of rarity, which is characteristic of persisters, and are placed in this category pending further investigation. The persistent species in. the Heron Reef asteroid assemblage appear to be recruitment limited.
The observed level of numerical and size-frequency stability in the persistent 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 results presented 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 opportunistic species characterised by fluctuating population sizes and size structures.