The reproductive biology of the sea cucumber Holothuria atra Jaeger, 1833 (Echinodermata: Holothuroidea) in Laucala Bay, Fiji, with notes on its population structure and symbiotic associations

The reproductive periodicity of the sea cucumber Holothuria atra Jaeger 1833 in Laucala Bay, Fiji was determined at two sites, Makeluva and Sandbank Reefs, through measurement of gonad index, sperm activity, oocyte diameter, and gonad tubule diameter over a 14 month period. Population structure and distribution patterns were compared between the two sites. Associates of H. atra were enumerated and factors affecting their abundance discussed. Holothuria atra is dioecious. It is an asynchronous spawner in Fiji with a main breeding period in summer (September to December). Gonad indices and mean maximum gonad tubule diameter trends showed that the breeding season of H. atra is confined to a few months in summer. Sperm activity, gonad staging and oocyte size frequency trends, however, showed that the breeding season was prolonged, with ripe oocytes present all year around. The increase in gonad mass was periodic, but the ripeness of gonads was prolonged. Reproductive seasonality is probably influenced by temperature. Reproductive effort was greater in female H. atra from Makeluva Reef than females from Sandbank Reef. However, there was no significant difference in reproductive effort between males at both sites. Size at first sexual maturity was about 80 g gutted weight or 19 cm total length. H. atra is fissiparous at the sites studied but fission rates were very low. Holothuria atra is the most abundant holothurian on Laucala Bay reefs with a unimodal size structure. There was a noticeable absence of females early in the study and no juveniles (<6 cm) were found. The Makeluva population of H. atra had a larger mean body size than the Sandbank population. H. atra occupying different zones of the same reef had different mean body sizes. The size distribution of H. atra across Sandbank Reef was not random and density was related to exposure and substratum type. Females dominated the larger class sizes at both sites but the sex ratios were approximately 1:1. The unreliability of measuring holothurians was not fully resolved, and it was suggested that gutted weight be used as the base parameter as it is the least variable. However, the minimum legal length should be designated by a wet legal length. Symbiotic associations between Holothuria atra and invertebrate symbionts in Laucala Bay included the polynoid scale-worm, Gastrolepidia clavigera, the sabelliphilid copepod Scambicornus ? modestus, the harlequin crab, Lissocarcinus orbicularis, and the eulimid gastropods, Peasistilifer mitidula, and Melanella aciculata. Symbionts were more abundant at the Sandbank Reef site. In some months, host animals with larger body sizes had greater numbers of some symbiotic species. Size limits for all processed holothurian species in Fiji are presently based on a minimum legal dry length of 7.62 cm which is an impractical measure of size. Size at first sexual maturity from this study is used to set minimum size limits based on length. Findings of parts of this study will help formulate new management practices and set the basis for reproductive studies of other commercially important holothurian species in Fiji.