Effect of sperm density on hybridization between Crassostrea virginica, Gmelin and C-gigas (Thunberg)

In this study, effects of sperm density on hybridization between Crassostrea gigas and C. virginica were studied. Two-by-two crosses, C. gigas x C. gigas (GG), C. gigas x C. virginica (GV), C. virginica x C. gigas (VG), C. virginica x C. virginica (VV), were conducted. Five levels of sperm density, which were measured by the relative number of attached sperm per egg (AS/egg), were produced and rested in 3 replicates. At each of the 5 levels (2-3, 5, 10, 20, and 30 AS/egg), the affinity between sperm and eggs, fertilization success, and survival and yield at 48 h were examined and determined. Reciprocal differences in fertilization in GV and VG crosses were found, with GV less successful than VG. In GV crosses, 20 or 30 AS/egg could not be attained, while in VG, GG, and VV crosses all 5 AS/egg levels were accomplished. More sperm were needed to attain a given AS/egg level in GV crosses than in VG, VV, and GG, and likewise, in VG crosses than in VV and GG at more than 10 AS/egg. The affinity between C. virginica sperm and C. gigas eggs was significantly weaker than between C. gigas sperm and C. virginica eggs. The ratio of sperm to eggs at 10 AS/egg was significantly higher than at 2-3 or 5 AS/egg. Survival and yield at 48 h in GV crosses were significantly lower than in GG, VV, and VG crosses. There was no significant difference among the first 3 AS/egg levels in fertilization, survival and yield at 48 h, but embryo deformation was found from higher than 10 AS/egg (>10%). The results showed that the affinity between heterologous gametes was weaker compared to homologous gametes. We suggest that 2-5 AS/egg was a safe and effective range for both pure crosses and hybrid trials between these 2 species, although more sperm is needed for hybrids.