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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.

Pacific oyster hatchery production remains highly variable, partially due to inconsistent gamete quality. However, little attention has been paid to oocyte quality in Pacific oyster compared to important vertebrate aquaculture species such as fishes. The aims of the present study were to identify criteria of oocyte and trochophore quality used as predictive tools for embryo developmental success at the D-larval stage. Twenty-six potential proxies of oocyte and trochophore quality were studied on 25 mature females, as determined using histology. Some morphological and biochemical characteristics of mature oocytes were described. Protein appeared to be the main constituent of fully mature Pacific oyster oocytes. Neutral lipids accounted for 61% of the total lipids and triglycerides for 85% of the neutral lipids. Oocyte fertilization capacity was assessed by determining the D-larval yield. High inter-individual variation of D-larval yield was observed (ranging from 1 to 58%). D-larval yield was positively correlated with the percentage of motile trochophores (R2 = 0.237) and with trochophore Velocity of the Average Path (VAP; R2 = 0.255) and negatively correlated with oocyte total steryl esters (R2 = 0.244). The use of a multivariate model, including D-larval yield, trochophore VAP and total steryl esters, led to a higher coefficient of determination (R2 = 0.352), highlighting that the prediction of embryo development in molluscs is a complex process requiring sophisticated multi-factor models.