Influence of asexual reproduction on Holothuria atra and Stichopus chloronotus populations on the Great Barrier Reef

Densities and individual wet weights of Holothuria atra and Stichopus chloronotus were measured on near-shore reefs of the central Great Barrier Reef, to determine the effect of asexual reproduction by transverse fission on the population structure of holothurians. Permanent stations were established at Fantome Island, Brook Island and at two sites in Little Cannon Bay, Great Palm Island.

Holothurian densities were estimated every 1 to 3 months between March 1995 and August 1996 and again in April 1997 after Cyclone Justin, which occurred in March 1997. Fixed transect lines were established parallel to the shore and for each survey, a number of sub-transects, each 10 m long were deployed at random intervals, perpendicular to the transect line and all individuals within 1 m either side of each 10 m transect were counted. Only Holothuria atra was counted at Brook and Fantome Islands as Stichopus chloronotus does not occur at these sites.

At Brook Island, transect lines were established at 5-15 m and 25-35 m seawards from the mean high tide level and 10 sub-transects were deployed per line. Twenty sub-transects were deployed from a single 100 m transect line at Fantome Island. At Great Palm Island, 10 sub-transects were deployed along a single transect line at one site and along two transect lines at a second site, which were set 10-20 m and 30-40 seaward of the mean high tide level. Additional 50 x 2m line transects were counted at both sites on Great Palm Island on a number of occasions.

Weight frequency distributions were determined for Stichopus chloronotus from both sites at Great Palm Island and for Holothuria atra from Fantome Island and Brook Island. Random samples of each population, including intact individuals and fission products were collected and wet weights were measured on shore after allowing individuals to eject water from their respiratory trees. In addition, 151 recently divided Holothuria atra and 120 Stichopus chloronotus were weighed to determine the size range of fission products.

All weights were grouped into 10 or 15 g size classes and biomass for each sampling month was calculated as the frequency of each weight class x the modal value of the class x the density. For months where size structure or density was not estimated, values for closest sampling occasion were used in calculations.

A conceptual model based on these and previous findings was developed and 5 factors (mortality, habitat stability, optimum individual size, food availability and larval supply) were hypothesized as having an influence on promotion or repression of transverse fission.