Effects of salinity and temperature on the timing of germinal vesicle breakdown and polar body release in diploid and triploid Hong Kong oysters, Crassostrea hongkongensis, in relation to tetraploid induction

Tetraploids are crucial important for commercial production of all-triploid Crassostrea hongkongensis. Successful production of tetraploid C. hongkongensis requires precise control of fertilization and applying treatment. The germinal vesicle breakdown (GVBD) ratio of stripped eggs before fertilization influences the efficiency of tetraploid inductions. According to our research, in both diploids and triploids, the time to 50% polar body 1 (PB1) release was significantly reduced if a GVBD ratio of 100% was reached before fertilization (p < 0.05). We consider the time at which there is 100% GVBD of stripped oocytes to be the optimum time point to start fertilization. 50% PB1 release is an important time point to apply the treatment for inducing tetraploids. Since lowering salinity (from 25 to 15 ppt) and increasing temperature (from 25 to 31 degrees C) both accelerate the GVBD and the PB1 release rate of eggs, 31 degrees C and 15 ppt are the most appropriate temperature and salinity level in which to perform tetraploid induction of C. hongkongensis. In these conditions, we found that there was significantly more variation in timing of 100% GVBD and 50% PB1 release among eggs from different triploids compared to different diploids (p < 0.05), creating difficulties in controlling fertilization and duration of treatment in tetraploid induction using triploid eggs. For reducing the variation, we recommend that treatments for tetraploid induction of C. hongkongensis should use eggs from individual triploid females.