Phenotypic trait analysis of diploid and triploid hybrids from female Crassostrea hongkongensis × male C. gigas

• 100% allotriploids were successfully conducted through the hypotonic treatment.
• Diploid and triploid hybrids appeared to be different in growth.
• Triploid hybrids could effectively improve aquaculture performance.
• Gonadal development of hybrids exhibited the variable patterns of sterility.

The allotriploid induction is one of effective tools used for animal breeding in aquaculture. To improve the growth weakness of diploid hybrids between female Crassostrea hongkongensis and male Crassostrea gigas, allotriploid induction by inhibiting the second polar body of fertilized eggs of C. hongkongensis was conducted using a hypotonic treatment. Three replicates were successfully performed, and each replicate consisted of two intraspecific crosses (GG—C. gigas ♀ × C. gigas ♂ and HH—C. hongkongensis ♀ × C. hongkongensis ♂), a diploid cross HG (C. hongkongensis ♀ × C. gigas ♂) and a triploid cross HHG (C. hongkongensis ♀ × C. gigas ♂). The phenotypic traits of these crosses were evaluated in the aspects of survival, growth, gonad development and ploidy estimation. The egg cleavage rate of the HHG cross was similar to that of HG, and that of both crosses were significantly lower than that of the two intraspecific crosses (P < 0.05). The D larval rate of HHG was the lowest in all the groups. The survival heterosis of the diploid hybrids was always positive, despite the growth weakness. The allotriploid advantages of survival and growth were both negative in the larval stages and then converted to positive in the spat and adult stages. Most of the triploid hybrids exhibited sterility, with atrophic gonads and abnormal gametogenesis as observed in the diploid hybrids. Our finding suggests that triploid hybrids are viable, fast growing (in terms of the diploid hybrids), and highly sterile, and promise potential utilization in aquaculture as a new oyster stock.