Fertility of cryopreserved spermatozoa of the Japanese pearl oyster, Pinctada fucata martensii

The aims of this study were to compare the fertility of fresh and cryopreserved spermatozoa of the Japanese pearl oyster, Pinctada fucata martensii and to develop artificial fertilization methods using the cryopreserved spermatozoa. The optimal egg density for the fertilization test was found to be 1.0 × 105 eggs ml− 1 seawater. When 1.0 × 105 eggs ml− 1 seawater were mixed with various numbers of fresh or cryopreserved spermatozoa (from 0.041 to 63 × 107 spermatozoa), high fertilization rates (about 60%) were obtained following addition of more than 3.5 × 107 spermatozoa when using either fresh or cryopreserved spermatozoa. With reduced numbers of spermatozoa of less than 1.1 × 107 (cryopreserved) or 0.12 × 107 (fresh) spermatozoa, the fertilization rates gradually decreased to 40% or less. More than 10 times the number of cryopreserved spermatozoa was necessary to obtain similar fertilization rates to those arising from the use of fresh spermatozoa.