Introduction and evaluation on the US West Coast of a new strain (Midori) of Pacific oyster (Crassostrea gigas) collected from the Ariake Sea, southern Japan

We describe the introduction, taxonomic identification and genetic parameters for farm traits of the Midori strain of the Pacific oyster Crassostrea gigas a possible alternative to the widely farmed Miyagi strain on the US West Coast. Broodstock (G0) oysters collected from the mouth of the Midori river and near Iwajima Island, Ariake Bay, Japan were identified as C. gigas based on the nuclear internal transcribed spacer (ITS) region and 28S gene as well as a mitochondrial (COI) marker. G0 and G1 generations and G2 larvae and spat were maintained in quarantine conditions and subjected to numerous histological exams and PCR tests for detection of pathogens of concern. After the G1 generation and G2 early life stages were found to be free of detectable diseases, G2 spat were planted at farm test sites and adults were used to produce a G3 generation. This G3 generation was cultured under non-quarantine conditions and families were planted at sub-tidal and inter-tidal test sites, in Oregon and California, respectively. Field performance traits were measured to determine the potential for improvement through selection. Overall, Midori oysters showed significantly lower values for harvest yield, survival, individual average weight and dry meat weight compared with oysters from a selected stock of the Miyagi strain of Pacific oyster; however, shell shape index (a measure of shell depth) was significantly greater in Midori oysters. Heritability estimates for group traits at harvest (survival, yield and individual average harvest weight) were high, whereas heritability estimates for individual traits (individual weight, wet meat weight and dry meat weight) and for shell dimension measurements (shell length, shell width and shell depth) were moderate-to-high. Family-by-site (GxE) correlations were low-to-moderate for all measured traits. Genetic correlations were positive and moderate-to-high among group traits and shell dimensions. In contrast, correlations between shell shape and other traits had small-to-moderate negative values but these were not statistically significant. Estimated potential genetic gain for dry meat weight was high (46%) whereas estimated gains for yield, individual average weight, individual weight were high-to-moderate (approximately 10%). In contrast, shell shape showed very little potential for genetic gain through selection. Overall, the Midori strain of the Pacific oyster shows promise as a deep-cupped oyster for the half-shell trade. Statement of relevance: We describe the introduction of a new strain of Pacific oyster (Midori) collected from southern Japan to the US West Coast. Rigorous quarantine protocols and disease tests were applied to ensure that the risks of introducing diseases were minimized. The Midori strain was found to have a deeper cup than the commonly farmed Miyagi strain but the potential estimated gain for improving this trait through selection was small.

Automated summary

The Midori strain of Pacific oysters was introduced from Japan to the US West Coast as a potential alternative to the widely farmed Miyagi strain, with genetic parameters and performance traits evaluated. Despite having slightly lower values for traits such as harvest yield and average weight compared to the Miyagi strain, the Midori oysters exhibited significantly greater shell depth, showing promise as a deep-cupped oyster for the half-shell trade.