Reproductive cycle and biochemical composition of the Zhe oyster Crassostrea plicatula Gmelin in an eastern coastal bay of China

The reproductive cycle and biochemical composition of the oyster Crassostrea plicatula in Rushan Bay, an eastern coastal bay of China, were investigated between March 2004 and February 2005 in relation to environmental factors (temperature, salinity and chlorophyll a). Histological analysis, combined with oocyte examination and measurements of protein, glycogen and lipid levels and RNA/DNA ratio from gonad, adductor muscle and mantle tissue of each sex were performed. The gametogenic cycle of C. plicatula comprised two phases: a resting phase (November–February) and reproductively active phase, including ripeness and spawning, during the rest of year. Gametogenesis in spring and summer was associated with an increase in protein and lipid contents in the ovaries, and took place at the expense of reserves (mainly glycogen) accumulated previously during winter. The RNA/DNA ratio is a good indicator of sexual maturity in the ovary; the increasing RNA/DNA ratio in the ovary appears to show the rising synthetic activity of vitellin within the ovary. Spawning occurred at the moment when water temperature and chlorophyll a levels were highest, and salinity lowest throughout the year. High variation in glycogen contents during storage and gametogenic development suggests that carbohydrates are the main respiratory substrate in this species. The results demonstrated that C. plicatula may be considered a conservative species in gametogenic pattern.