Studies on triploid oysters in Australia .6. Gonad development in diploid and triploid Sydney rock oysters Saccostrea commercialis (Iredale and Roughley)

Triploid Sydney rock oysters, Saccostrea commercialis (produced using cytochalasin B to treat newly fertilised eggs), were sampled from 22-34 months of age to encompass a full reproductive season. A comparative assessment of the extent of gametogenesis was made for both diploid and triploid Sydney rock oysters. It was necessary to develop separate staging criteria for the triploids. Triploid gametogenesis was highly retarded compared to the diploids. The extent of retardation of gametogenesis in the triploids was more severe than that observed for triploid Pacific oysters. In particular, triploid females developed abnormally and follicle branching was severely retarded with few mature ova present. Development in the male triploid appeared to hall at spermatocyte formation; on the few occasions that spermatids were present they were sparsely distributed throughout the gonad. In the thirteen months of sampling, there was no evidence of spawning in the triploids. Very few hermaphrodites were found in either diploids or triploids sampled. Sex ratios showed little change over time for diploids and for triploid males but the incidence of triploid females increased between January and May. The gonad area indices for triploids (sexes combined) were consistently much lower than for those of either diploid males or females.