ATP content and viability of spermatozoa drive variability of fertilization success in the Pacific oyster ( Crassostrea gigas )

Fertilization of the oocyte is a critical process of sexual reproduction depending among other factors on spermatozoa functionality. Mitochondria participate in many crucial processes for spermatozoa motility and fertilizing ability. The Pacific oyster, Crassostrea gigas, is characterized by a high inter-individual variability of its reproductive success which has been shown to be partially due to the variability of gamete quality. The present study explored spermatozoa characteristics such as: i) relationships among mitochondrial functionality (ATP content, mitochondrial membrane potential (MMP), reactive oxygen species production); sperm viability (percentages of live, dying, and dead); and motility; and ii) their involvement in inter-oyster variability of fertilizing ability demonstrated using simple and multiple regressions. Our results showed that ATP content depends on both MMP and viability of spermatozoa. Using multiple regressions, 61% of the variability of the trochophore-larval yield was explained by a model combining the ATP content and the percentage of dying spermatozoa (P < 0.001). Our results reveal that capacity of spermatozoa to maintain a high level of ATP via OXPHOS partly explains the inter-individual variability of fertilization success in the Pacific oyster. Sperm ATP content and viability assays will provide valuable tools for assessing sperm quality of this species in aquaculture production, cryopreservation, and bioassays.