Aspects of the ecology and life history strategies of holothurian species were investigated at Heron Reef, southern Great Barrier Reef, and in Moreton Bay. Of the six epifaunal holothurian species whose distribution patterns were studied at Heron Reef, five (Holothurian atra, H. impatiens, H. edulis, H. leucospilota and Stichopus chloronotus) were zoned across the reef flat, and one (S. horrens) was not zoned. Distribution patterns of the six species were related to the distribution across the reef flat of five habitat categories, i.e. H. atra was numerically dominant in the "sand" habitat, H. edulis, H. leucospilota and H. impatiens in the "boulder" habitat, S. chloronotus in the "coral platform" habitat and S. horrens in the "rubble" habitat. No species studied were abundant in the "live coral" habitat.
Four Holothuria species, H. atra, H. impatiens and H. edulis at Heron Reef and H. scabra in Moreton Bay, showed considerable variation in reproductive parameters, namely ovum size and number, gonad index and seasonality of spawning. At Heron Reef, H. atra spawned biannually in summer and winter, and females produced a relatively large number of small ova; H. impatiens spawned annually in spring with females producing a small number of large ova; and H. edulis showed no cyclic spawning pattern and produced an intermediate size and number of ova. In Moreton Bay a large proportion of the H. scabra population spawned in summer with a small proportion spawning in autumn, and females produced a very large number of relatively large ova. Of the four species studied, the value of the mean mature gonad index ranged from 6.3% in H. scabra in Moreton Bay to 2.7% in H. atra, 2.6% in H. impatiens and 1.3% in H. edulis, at Heron Reef. Data for reproduction of H. atra from four reef sites, the shallow lagoon, inshore gutter, middle reef flat and reef rock rim indicate that the magnitude of gonad index is site dependent and is greater in the middle reef flat and inshore gutter sites than in the shallow lagoon and reef rock rim sites.
Asexual reproduction by transverse binary fission occurred in individuals of H. atra, H. edulis, S. chloronotus and S. horrens at Heron Reef. The proportion of H. atra individuals sampled that were detectable products of asexual reproduction ranged from 6% to 70%. Fission frequency varied significantly between sample sites and was greatest in the middle reef flat and inshore gutter sites, lower in the shallow lagoon site and least in the reef rock rim site. Fission frequency also varied significantly with sample time with highest fission frequency in August, following the winter spawning.
Growth in H. atra was investigated by two methods, first by analysis of changes in populations' size frequency distributions over time, and secondly by analysing changes in the size of tetracycline-labelled calcareous parts over time. The analyses were complicated by the effect of frequent asexual reproduction on recruitment to the population, and in altering the relative size of body parts. Such complications make traditional analyses of growth and mortality rates inapplicable to studies of the dynamics of E. atra populations.
Aspects of the life history traits of the four major holothurian species studied were quantified, and the species were ranked with respect to a spectrum of life history strategies generated from r/K selection theory, and from the specialist/opportunist theories of Grassle (1973). Life history traits of the sub-tropical (Moreton Bay) species, H. scabra, were the most opportunistic and r-selected of the four species studied, and the species apparently does not undergo asexual reproduction. Of the three coral reef species studied, H. atra was the most opportunistic and r-selected and frequently reproduced asexually, while H. impatiens was least opportunistic and relatively K-selected, and did not undergo asexual reproduction. Because of the potentially dispersive phase of H. atra’s life cycle, observed differences between life history parameters of E. atra at different Heron Reef sites are likely to be environmental, or the result of short-term selection, rather than being the result of long-term selection for different optimal life history strategies in genetically isolated populations at different reef sites.