Effects of salinity on larvae of the oysters Crassostrea ariakensis, C. sikamea and the hybrid cross

Crassostrea sikamea and C. ariakensis are two oysters from Asia that are sympatric and differ slightly in their environmental preference. They can hybridize in one direction, although the fertilization level and survival of hybrid larvae to spat are low. To understand the possible effects of salinity on cross-fertilization and the survival of hybrid larvae, we conducted hybridization experiments at four salinities (20, 25, 30 and 35 ppt). At all salinities, about 10% of C. sikamea eggs were fertilized by C. ariakensis sperm, while the reciprocal cross completely failed to fertilize. Survival of fertilized eggs to D-stage in the hybrid cross (C. sikamea female x C. ariakensis male, SA) was significantly (P<0.05) higher at 35 ppt than at lower salinities, while the effect of salinity was not significant in two pure-species crosses. High salinity negatively affected larval survival to day 7 in C. ariakensis and the hybrid cross. Larval growth in all groups was negatively correlated with salinity (P<0.01). Larval shell height at day 7 as well as daily increment was significantly smaller for hybrid than for purebred larvae (P<0.05). These results indicate that hybrid larvae prefer high salinity during the early stages of development but low salinity at later stages. The low and one-directional fertilization represents a pre-zygote barrier to hybridization, which may play a role in sympatric speciation.