Pacific oysters (Magallana gigas, Thunberg 1793) preferentially consume Isochrysis galbana, increasing biomass and upregulating biomineralisation gene nacrein

Microalgae are the foundation of oyster diets in aquaculture. As demand for oysters increases, so does the need for nutritionally complete diets. Isochrysis galbana is considered the optimal oyster diet and is often supplemented with other algae like Nannochloropsis to provide complementary nutrients, but which diet do the oysters prefer, and what effects do the diets have on physiology? This study performed feeding experiments with single (I. galbana or Nannochloropsis) and mixed (both genera combined) diets in Pacific oysters (Magallana gigas). Oysters fed exclusively I. galbana had greater biomass gains but reduced shell growth, evidenced by a lower Oyster Condition Index, and gene expression analysis showed compensatory upregulation of the biomineralisation gene nacrein in this group. Oysters fed mixed diets showed higher algal cell clearance and pseudofaeces production, and within the mixed diet, a preference for I. galbana. This suggests that whilst a mixed-algae diet is traditionally used, a single species diet of I. galbana can significantly enhance oyster growth, reducing the need for complex multi-species algal culture. Culturing one alga is more time- and cost-effective, but stage-specific diets could promote specific physiological factors. These findings can help to optimise oyster feeding in a world with increasing demand for oysters.