Abstract

The Manila clam Ruditapes philippinarum is an ecologically and economically important bivalve in Korean tidal flats. To examine seasonal patterns in reproduction and health, we monitored monthly changes in gonadal development, condition index (CI), trematode infection, and Perkinsus olseni infection intensity at two tidal flats on Yeongheungdo Island in the west. Histology indicated that gametogenesis initiated in early spring, peaked in late spring to early summer, and was followed by spawning during summer. The CI increased prior to spawning and declined during autumn and winter, reflecting reproductive effort and food availability. Histology also revealed that heavy infection by the flatworm Cercaria tapidis leads to gonad castration during the spawning period. Trematode infection exhibited moderate seasonal variation but did not mask reproductive patterns. P. olseni infection intensity, measured by gill assays, displayed a significant spatial variation, as the intensity was significantly higher at Naeri tidal flat (p < 0.05), although P. olseni infection showed no significant correlation with CI or gonad maturation at either site. These findings suggest that, under the natural environmental conditions observed, P. olseni infection did not exert measurable adverse effects on the condition or reproductive development of the clams. This study emphasizes the substantial impact of seasonal environmental factors on the reproductive cycles of R. philippinarum, while suggesting that P. olseni infections may not directly affect reproductive performance at the observed infection levels.