Early Larval Development and Annual Gametogenesis of the Brooding Oyster Ostrea circumpicta (Pilsbry, 1904) in the Shallow Subtidal Benthic Ecosystem in Jeju Island, Off the South Coast of Korea

The subtropical brooding oyster Ostrea (= Striostrea) circumpicta (Pilsbry, 1904) occurs at high density in the shallow, subtidal, rocky bottom in Jeju Island, off the south coast of Korea, where the sea surface temperature and salinity varies annually from 13 to 25°C and 30 to 33 ppt, respectively. In this study, the annual gametogenesis and early larval development of O. circumpicta was examined, using histology and scanning electron microscopy (SEM). Histology indicated that the females and males initiated gonial mitosis in September, shortly after sexual resting in August. In December, ripe eggs first appeared in the follicles, and most of the females exhibited fully mature oocytes in May, as the water temperature reached 17 to 18°C. Spawning females were dominant in June and July, when the trochophore and strait-hinged veliger larvae were also identified in the branchial chambers, their size ranging from 111 to 130 μm and 135-205 μm in diameter, respectively. The veliger larvae in the brooding chamber exhibited a well-developed velum and digestive tract, suggesting that the larvae are engaged in feeding in the branchial chamber. Unlike other marine bivalves in temperate coastal ecosystems, O. circumpicta has a long period of gonad maturation and a short resting phase. It has been believed that such a long period of reproductive maturation is associated with a low level of food in the environment and the comparatively large size of the oocytes, which may require a relatively longer time to accumulate necessary nutrients to produce large eggs in a food-poor environment.