The effects of triploidy on Penaeus (Marsupenaeus) japonicus (Bate) survival, growth and gender when compared to diploid siblings

In this study the effect of triploidy on survival, growth and gender of the Kuruma shrimp, Penaeus (Marsupenaeus) japonicus (Bate), was assessed by making comparisons to diploid sibling performance. Triploidy was induced by preventing polar body II extrusion using 6-dimethylaminopurine (6-DMAP) chemical shock. Four families with a triploid induction rate greater than 70% were reared for approximately 300 days (10 months), during which time their survival, growth, gender ratios and individual ploidy level were assessed. All triploids were found to be female resulting in significantly skewed gender ratios. Males treated with 6-DMAP were either diploid or mosaic (having both diploid and triploid cell populations). 6-DMAP treated shrimp in families 1 and 2 had significantly poorer (P <0.05) survival performance from PL10 to tagging size (PL187) compared to controls. Survival of family 4 control animals from PL10 to tagging size (PL118) was also significantly greater than the treatment animals, however, there was no difference in the survival performance of control and 6-DMAP treated shrimp for family 3 (P >0.05) over the same period. Similarly, for all four families, survival of 6-DMAP treated and control shrimp was not significantly different (P >0.05) from tagging through to assessment age (PL251 for families 1 and 2, and PL173 for families 3 and 4). Treatment with 6-DMAP had a significantly negative effect on weight of females at assessment age when compared to controls of families 1, 2 and 3, and males of families 1 and 4. Diploid females that had received treatment with 6-DMAP were significantly heavier (P <0.05) than triploid females for family 3 at assessment age, and family 4 from assessment age through to the end of the rearing period. The discovery that all triploid P. japonicus are female could be of significant commercial interest in the future, as shrimp growth rates are sexually dimorphic with females growing faster and larger than males. However, triploidy induced by 6-DMAP chemical shock appears to have detrimental effects on growth and survival performance from sub-adult to reproductive size when compared to diploid siblings. The production of only female triploids supports the hypothesis that the female is the heterogametic gender for P. japonicus.