SUMMARY: Effects of ultraviolet (UV) irradiation on genetical inactivation and morphological features of sperm were examined in the Pacific oyster. Ultraviolet light (254 nm) was effective for inactivating sperm chromosomes. Irradiation for 60 s at a UV intensity of 72 erg/mm 2 per s was the optimum dose to achieve haploid gynogenesis. The rates of the fertilization and the development of D‐shaped larvae decreased with increasing irradiation time, and the development of the eggs fertilized with the genetically inactivated sperms terminated before reaching the D‐shaped stage. Electron microscopy showed clear destruction of the sperm acrosome and flagellum in the UV‐irradiated sperms. As the duration of irradiation increased, the acrosome of sperms tended to suffer greater damage until the sperms eventually lost their flagella. Abnormalities in these structures have appeared to account, at least in part, for the decline of the fertilization rate of eggs inseminated with UV‐irradiated sperms.

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