Successful cryopreservation of Pacific oyster (Crassostrea gigas) oocytes

Protocols for cryopreservation of sperm and oocytes would provide the ultimate control over parental crosses in selective breeding programmes. Sperm freezing is routine for many species, but oocyte freezing remains problematic, with virtually zero success in aquatic species to date. This paper describes the development of a successful protocol for cryopreserving high concentrations of Pacific oyster (Crassostrea gigas) oocytes. Ethylene glycol (10%) and dimethyl sulfoxide (15%) were found to be the most effective cryoprotectants resulting in post-thaw fertilization rates of 51.0±8.0 and 45.1±8.3%, respectively. Propylene glycol was less effective and methanol resulted in zero fertilization post-thaw. The use of Milli-Q water rather than seawater as a base medium significantly improved fertilization (20.4±3.0 and 8.7±2.2%, respectively) as did the inclusion of a 5min isothermal hold at −10 or −12°C (35.9±5.0 and 31.9±4.6%, respectively). The optimal cooling rate post-hold was 0.3°Cmin−1, with virtually zero post-thaw fertilization with cooling rates of 3 and 6°Cmin−1. Using an optimized protocol, post-thaw fertilization rates for oocytes from eight individual females ranged from 0.8 to 74.5% and D-larval yields from 0.1 to 30.1%. For three individuals, larvae were reared through to spat. Development of D-larvae to eyed larvae and spat was similar for larvae produced from unfrozen (24.8±4.1% developed to eyed larvae and 16.5±3.2% to spat) and cryopreserved (28.4±0.6 and 18.7±0.5%, respectively) oocytes. The ability to cryopreserve large quantities of oyster oocytes represents a major advance in cryobiology and selective breeding.