Effects of Stocking Density and Feed Types on Growth and Survival of Tetraploid Larvae of Crassostrea gigas

To investigate the optimal stocking density and dietingredient of tetraploid Crassostrea gigas larvae, in this study, we used diploid larvae as a control group and established three larval density gradients (4, 8, and 12 individuals/mL) and four diet treatments, Total Isochrysis galbana (Iso) group (T-Iso group, 100% Iso), Mixed α group (70% Iso+30% Tetraselmis subcordiformis (Tet)), Mixed β group (30% Iso+70% Tet) and Total Tet group (T-Tet group, 100% Tet). The effects of stocking density and diet on larval growth and survival were analyzed. The results demonstrated that both growth and survival of tetraploid larvae were significantly lower than those of diploids, with tetraploids being more sensitive to high-density stress and having higher dietary requirements. The density experiment revealed that a moderately higher density (8 individuals/mL) showed an advantage at the early developmental stage, while a low density (4 individuals/mL) was most conducive to the growth and survival of tetraploid larvae in the later stage, yielding the maximum shell height ((270.79±15.07)μm) and survival rate ((30.55±4.44)%) in 20 days post-fertilization. In contrast, high density (12 individuals/mL) led to growth retardation and a significant decrease in survival. The diet experiment indicated that the mixed diet (Mixed α group) was the most effective, resulting in the greatest shell height ((330.87±15.19)μm) and the highest survival rate ((70.30±2.81)%) of tetraploid larvae in 20 days. The T-Iso group and the Mixed β group were less effective while the single diet of Tet performed the poorest. In summary, under the experimental conditions applied, a moderately higher stocking density (8 individuals/mL) is more favorable for the growth and survival of C. gigas tetraploid larvae at the early developmental stage, while a lower density (4 individuals/mL) is more significantly advantageous in the later phase. After reaching the umbo larval stage, Tet can be supplemented as part of the diet at a recommended feed ratio of 70% Iso and 30% Tet.