Reproductive Cycle Dynamics of Subtropical Manila Clams (Ruditapes philippinarum) Cultured in Temperate Waters: Temperature Thresholds and Bimodal Spawning Patterns

The Manila clam Ruditapes philippinarum is a commercially important bivalve worldwide, with China being the leading producer. While the reproductive biology of this species has been extensively studied in its native or long-established ranges, knowledge of how populations cultured from non-native seed sources adapt their reproductive cycles to new environmental conditions remains limited. In this observational study, we investigated the annual reproductive cycle of a Manila clam population originating from subtropical waters (Zhejiang Province, Southern China) that was cultured in temperate aquaculture grounds in Zhuanghe Bay, Northern China. Monthly histological examination of 50 clams demonstrated that the gametogenic cycle synchronized between male and female clams. Gametogenesis started in March when seawater temperature exceeded 5.7 °C, and most gametes matured by May. A distinct bimodal spawning pattern was observed: a minor spawning event occurred from May to July, followed by a major spawning phase from September to November after a one-month gonadal recovery period in August. The condition index (CI), analyzed monthly in 30 clams, effectively reflected reproductive stages, increasing during gametogenesis and declining sharply during spawning, with its amplitude indicating spawning intensity. Seawater temperature was identified as the primary regulatory factor driving reproductive development from gametogenesis to spawning, while food availability (indicated by chlorophyll a concentration) played a crucial role in supporting gonadal recovery during summer. These results align with observations in other temperate populations, demonstrating that subtropical-origin clams can successfully acclimate their reproductive cycles to temperate environmental conditions. This study provides the first comprehensive description of the reproductive biology of transplanted Manila clams in Northern China, offering critical benchmarks for optimizing hatchery production schedules and informing sustainable fishery management practices in the region.