Factors driving the settlement of Pacific oyster Crassostrea gigas larvae in Hiroshima Bay, Japan

Hiroshima Bay, located in the western part of the Seto Inland Sea, is the largest Pacific oyster Crassostrea gigas farming area in Japan. Rafted aquaculture using natural spat of the native oyster is conducted in this bay and spat collection during the spawning season in summer is a key process for sustainable farming. In order to elucidate the favorable environmental window for oyster spat collection, we investigated water quality parameters (water temperature, salinity and nutrients), abundance of larval food and success of larval settlement during the period of spat collection in Hiroshima Bay. We also examined the species composition of <10 μm size phytoplankton in sea water, the potential food for larvae, by metabarcoding analysis using next generation sequencing. The results showed that water temperature and abundance of larval food correlate with larval settlement. Successful larval settlement was found to be achieved when D-shaped larvae were exposed to >26.9 °C and chlorophyll a concentrations for <10 μm size phytoplankton of >5 μg L−1. The results of metabarcoding analysis indicated that increase in <10 μm size diatom Cyclotella spp. in sea water led to a successful larval settlement in both 2019 and 2020. Furthermore, C. choctawhatcheeana was abundantly detected in the digestive tract of oyster larvae collected in the field, suggesting that Cyclotella spp. sustain the recruitment of oyster spat as an essential food.