Gamete quality in triploid Pacific oyster (Crassostrea gigas)

Triploidy induction in shellfish aims to obtain faster growth and sterility of reared individuals. Triploid Pacific oysters are most often not totally sterile, but have greatly reduced reproductive capacities compared to those recorded in diploid individuals. The description of gamete output in triploid aquatic animals is most often confined to the assessment of fecundity in females. The present work aims at further describing gamete quality of triploid Pacific oysters and comparing it to values observed in diploid individuals. Diploid and triploid oysters (produced by mating tetraploid males×diploid females) were reared in standard conditions and then transferred to the wild to allow the offspring to attain reproductive maturity. At the end of gametogenesis, the reproductive characteristics of both groups were estimated.After gonad sampling, gametes could be observed in 92.9% diploid oysters compared with 42.0% in triploids. A higher number of spermatozoa was produced in diploids (344±106×109 spermatozoa) compared with triploids (5±7×109 spermatozoa). Furthermore, the percentage of motile sperm and sperm swimming speed were higher in diploids compared with triploids. In contrast, a higher intracellular ATP content was found in triploids (99.1±34.0nmole10−9 sperm) than in diploids (63.6±20.7nmole10−9 sperm). A higher number of oocytes was collected from diploid oysters (19.1±3.8×106 oocytes), than from triploid ones (0.1±0.1×106 oocytes). The D-larval yield was 45% higher for crosses (female×male) triploid×diploid, than the control (diploid×diploid). Furthermore, the lowest D-larval yield was measured for triploid×triploid crosses.Considering the present data, a partial estimation of the reproductive potential of triploid Pacific oysters (triploid×triploid crosses) could be close to 0.06% of that of diploid individuals. However, this estimation is probably over-evaluated because it does not take into account the low sperm production of triploids (1.5% compared to diploids), the unknown frequency of spontaneous spawning in triploids and the low viability of the progeny. The consequences of the low reproductive potential of triploid Pacific oysters on natural populations and on hatchery practices are discussed. In conclusion, the present work confirms that triploidy leads to a limited reproductive potential as estimated by gamete characteristics and embryo developmental success.This study is relevant to aquaculture because triploids are produced by aquaculture and the consequences of their low gamete quality, described in this paper, on the natural environment but also on hatchery practices are discussed.