Hybridization among Crassostrea species: a review

Numerous attempts at hybridization in the genus Crassostrea have been reported. Most suffer from one or more of the following: (1) ambiguities in the classification of oysters; (2) contamination of experimental cultures by extraneous gametes or larvae; (3) the absence of control experiments for assessing the quality of gametes as well as larval viabilities; (4) the absence of genetic confirmation of hybrid status. We conclude that to date there is no unequivocal evidence for the existence of viable interspecific hybrids among Crassostrea species. Examples of taxonomic ambiguities that cloud interpretation of hybridization experiments include the frequent misclassification of the Kumamoto oyster ( C. sikamea) as the Pacific oyster ( C. gigas), the probably unjustified distinction between C. gigas and the Portuguese oyster C. angulata, and the lumping of C. iredalei with C. rivularis. Low-level contamination of cultures is a common occurrence that may account for many reports of successful hybridization. Experimental controls are essential to properly assess gamete quality and zygote growth and survival, yet are often not included. Finally, few reports provide genetic confirmation that the zygotes produced are actually hybrids. To date, most genetic analyses have demonstrated that putative interspecific hybrid oysters were not hybrids. Use of adequate controls and genetic analysis of parents and offspring are essential to hybridization experiments, and are demonstrated elsewhere.