Kumamoto oysters (Crassostrea sikamea) are highly regarded on the US Pacific Coast due to their deep-cupped shells (Figure 1) and firm summer meats. Previously, it had been reported that some commercial Kumamoto stocks were highly inbred and were contaminated by accidental hybridization with Pacific oysters. Consequently, in 2006, Kumamoto oysters were collected from the Ariake Sea, Japan, to supplement existing commercial Kumamoto stocks and to develop a new breeding population for the Molluscan Broodstock Program (MBP) – a breeding program to improve US Pacific Coast oyster broodstock.
The G1 generation and early life stages of the G2 generation of the new Japanese Kumamoto stock were maintained in quarantine conditions and subjected to numerous histological exams and genetic tests to ensure the absence of detectable diseases of concern, before release and planting of G2 spat at farm test sites.
Harvest traits (yield, survival, growth, shell dimensions and shape) of families of the G2 and G3 generations were determined at a sub-tidal farm site and compared to those of established Pacific Coast commercial stocks. A single MBP Kumamoto breeding population was produced in the G5 generation by crossing individuals from Japanese families and Pacific Coast stocks. Analysis of the ITS1/2 genomic regions of samples from the G5 and previous generations indicated no genetic hybridization with Pacific oysters.
Mean individual weights and shell lengths of the G3 Japanese families were significantly less (Tukey-Kramer; p<0.05) than those of families produced from established Pacific Coast commercial stocks; however, other harvest traits (yield, survival, average individual weight, shell dimensions and shape) were not significantly different. Harvest traits of crosses between the Japanese and Pacific Coast populations were intermediate in value compared with those of the two parental populations. Heritability values for G5 harvest traits were high, ranging from 0.83 for shell depth to 0.97 for survival, indicating strong potential for genetic improvement through selection. Genetic correlations were generally positive among harvest traits, except for shell shape (depth/(width + length) which showed negative (but non-significant) correlations with individual weight and meat content, indicating that selection for deep shell cupping may have a negative effect on these other traits.