Effects of thyroxine on embryonic and larval growth and development of Fujian oyster (Crassostrea angulata)

Thyroid hormones (THs) are key regulators of early development and metamorphosis in vertebrates, implying possible developmental roles in invertebrates. Given the importance of embryonic growth and survival to oyster seed quality and aquaculture success, understanding TH function during oyster embryonic development is therefore essential. This study investigated the effects of thyroxine (T4) concentration and exposure duration on the embryonic and larval development of Fujian oyster ( Crassostrea angulata ) . T4 was administered both by immersing female broodstock and by directly exposing embryos to T4-enriched seawater. Additionally, thiourea was used to inhibit endogenous TH synthesis in broodstock, followed by T4 treatment of their embryos. Each concentration-duration treatment was performed in triplicate, with 0.01 N NaOH serving as the vehicle control. Results showed that low to medium T4 concentrations (1.29 × 10 −9 mol/L, 1.29 × 10 −8 mol/L, 1.29 × 10 −7 mol/L) significantly enhanced the hatching rate and shell length (μm) of D -shaped larvae compared to controls ( P < 0.05). In contrast, high T4 concentrations (1.29 × 10 −5 mol/L, 1.29 × 10 −6 mol/L) exhibited significant inhibitory effects ( P < 0.05), which intensified with prolonged exposure durations. Using a validated Chemiluminescence immunoassay, results indicated that prolonged exposure may lead to cumulative T4 accumulation, contributing to developmental inhibition. Compared to direct embryonic exposure, the maternal approach potentially appeared to mitigate the inhibitory effects associated with direct high-dose exposure. Additionally, in the 1.29 × 10⁻⁸ mol/L group, an elevated triiodothyronine to T4 ratio was observed from the cleavage to the blastula stage, potentially explaining the observed developmental advantages. In conclusion, this study clarifies the dose- and time-dependent regulatory roles of T4 in early bivalve embryogenesis. While limited to the D -larval stage, these findings provide preliminary theoretical insights into TH dynamics that may inform future aquaculture seed production strategies. • Embryonic or broodstock exposure to T4 (10⁻ 9 –10⁻ 7 M) enhances larval development. • High T4 (10⁻ 6 -10⁻ 5 M) exposure inhibited larval development, intensifying over immersion time. • Prolonged exposure may cause cumulative T4 effects, leading to developmental inhibition. • Broodstock T4 immersion suggests advantages for studying TH effects. • Elevated T3/T4 from cleavage to blastula stage may contribute to larval growth advantages.