Determining the Biological Zero and Effective Accumulated Temperature for Gonadal Development in Diploid and Tetraploid Pacific Oysters, Crassostrea gigas

To calculate the biological zero and the effective accumulated temperature (EAT) for gonadal development in one- and two-year-old diploid and tetraploid Pacific oysters (Crassostrea gigas), four indoor heating experiments were carried out in 2022–2024. The biological zeros of diploids and tetraploids were 7.84 °C and 7.45 °C, respectively. The EAT in diploids was lower than that in tetraploids, at 389.05 °C×d and 546.95 °C×d, respectively. Histological analysis indicated that gonadal development was more rapid in diploids than in tetraploids. Simultaneously, one- and two-year-old diploid and tetraploid oysters had the same EAT. The proportion of females dominated all four study groups, especially the tetraploids. Age and ploidy did not significantly influence the cumulative survival rate (CSR). Gonadal development and warm temperatures may be the primary causes of mortality. The incremental survival rate (ISR) during the first half of the entire experimental cycle (ISR1) was markedly higher than that of the second half (ISR2) in both the diploid and tetraploid groups. These results suggest that one- and two-year-old tetraploid oysters could be used for the large-scale production of triploids by providing indoor heating.