Studies on triploid oysters in Australia: effect of initial size on growthof diploid and triploid Sydney rock oysters, Saccostrea commercialis (Iredale & Roughley)

In a a-year grow-out trial, triploid Sydney rock oysters, Saccostrea commer cialis (Iredale & Roughley), from two initial size grades grew faster (in t erms of both mean whole weight and shell height) than the equivalent initia l size grades of sibling diploids (P < 0.05). Small size grade triploids ca ught up with and had significantly heavier (P < 0.05) final whole weights t han large size grade diploids after a 2-years grow-out period. The initial size grade had a significant effect on final mean whole weight and shell he ight for both ploidy types. After the 2-years grow-out trial, the final mea n whole weights (but not shell heights) of small and large diploids (35.8 /- 0.68 and 39.4 +/- 0.58, respectively) were significantly different (P < 0.05). Small and large triploids grew at a similar rate for the first 18 mo nths despite the significantly (P < 0.05) heavier final mean weight of larg e grade triploids (48.4 +/- 0.8 g and 61.2 +/- 0.7 g, respectively). The ef fect of the initial size grade on subsequent growth of both diploid and tri ploid oysters which was demonstrated in the present study is of significant commercial value to hatchery and nursery operators as well as growers of s ingle seed oysters, In addition, small-grade triploids appeared to be more valuable in terms of potential growth rate than all diploid grades. There w as no significant difference in the final percentage triploidy between smal l and large grade triploids, A large proportion of diploid/triploid mosaici sm was detected in adult oysters.