Influence of ploidy and environment on grow-out traits of diploid and triploid Hong Kong oysters Crassostrea hongkongensis in southern China

Crassostrea hongkongensis has great economic value and is widely cultured in southern China. Like many other oyster species, diploid C. hongkongensis undergo slow growth and high mortality during gametogenesis. Promoting the commercial culturing of triploids may be one way to solve these problems. We present the first study of the effects of ploidy, site and their interaction on the grow-out traits of C. hongkongensis reaching market size. According to our results, triploid larvae grew significantly faster (P < 0.05) than diploids, but there was no significant difference in survival rate between diploid and triploid larvae. Within each site, triploids often had higher shell height than diploids (P < 0.05), and there were significant differences (P < 0.001) in shell height among sites. Additionally, there were significant differences (P < 0.001) in whole weight based on both site and ploidy, but no significant interactions between site and ploidy were observed. The incremental survival rates of triploids were significantly higher (P < 0.001) than those of diploids in December 2015, April 2016, August 2016 and April 2017, likely because triploids appeared less sensitive to environmental variations and displayed an obviously lower degree of gonad development than diploids. Moreover, there were significant differences (P < 0.05) in incremental survival rate among sites, suggesting that variations in environmental conditions may have great impacts on the survival rate of C. hongkongensis. In the reproductive phase, the relative expressions of the genes belonging to the insulin-like pathway were significantly higher (P < 0.05) in diploid C. hongkongensis, which is likely related to the high-energy consumption of gametogenesis in diploids. In the non-reproductive phase, the significantly higher (P < 0.05) expressions of insulin-like pathway genes in triploid C. hongkongensis matched their significantly faster growth, which may provide a new explanation for this fast growth of triploids. In conclusion, this study confirms the clear advantage of triploid C. hongkongensis over diploids in terms of growth, whole weight and survival rate, and supports the commercial promotion of triploid C. hongkongensis.