Studies On Reproductive Physiology Of Sinonovacula Constricta And Cyclina Sinensis

The reproductive cycle of marine bivalves is closely related to cycles of nutrient storage and use and to environmental factors such as water temperature and food availability. Gametogenesis is an energy-demanding process that requires mobilization of nutrients from ingested food or the reserves from the body tissues. In general, when food is abundant, reserves accumulate in the tissues prior to gametogenesis in the form of lipid, glycogen and protein substrates, and subsequently are utilized in the production of gametes when metabolic demand is considerably high. The particular importance of these substrates, where they are stored and the timing of their use, varies among species, as well as among populations of the same species. Therefore, analysis of the biochemical composition of separate organs which allows the identification of nutrient storage sites and determination of nutrient use during reproduction is essential for a complete understanding of reproductive strategy in marine bivalves.The Yellow River delta is situated in the northeast of Dongying City, Shandong Province in northern China, facing the Bohai Sea in the North, and is one of the three largest estuary deltas in China. In this paper, two commercially important bivalve species in this area were studied: the razor clam Sinonovacula constricta (Lamarck 1818) and the venus clam Cyclina sinensis (Gmelin). Seasonal variations in the environmental factors, the reproductive cycle and seasonal changes in biochemical composition (lipid, glycogen, protein, RNA/DNA ratio) in different tissues (gonad-visceral mass, foot, adductor muscles, mantles, siphons) of the male and female razor clam; seasonal variations in estradiol-17βand testosterone levels in the gonad of the razor clam; the reproductive cycle and biochemical composition (lipid, glycogen, protein, RNA/DNA ratio) in different tissues (gonad-visceral mass, adductor muscle, mantle) of the male and female Venus clam were investigated from April 2007 to March 2008. The major results of the study are presented as follows:1.Histological analysis indicated that the gametogenesis of the S. constricta comprised two phases: a resting phase (February-May) and gametogenesis, including ripeness and spawning, during the rest of the year. Gametogenesis of S. constricta began from June-July and was completed during September-October when food was abundant. A significant decrease in the glycogen content of all tissues both in the female and male during sexual maturation suggests that carbohydrates play the most important role in, reproduction of this species. The protein and lipid content increased in association with the gametogenesis in the female gonads, indicating that they are the major organic component of oocytes. Conversely, the protein and lipid decreased in the male gonads and other tissues, demonstrating that they also support reproduction after the glycogen is depleted. Seasonal change in RNA/DNA ratio suggests that it is a valuable indicator of maturation both in the female and male.2.Estradiol-17βand testosterone levels were determined by enzyme-linked immunosorbent assay (ELISA) in the gonad of S. constricta (Lamarck 1818). Estradiol-17βlevels varied between 224.1 and 769.7 pg/g wet weight during gametogenesis and were near 10 times higher than testosterone levels. Estradiol-17βconcentration in the female gonads was higher than that of the male gonads and testosterone concentration in the male gonads higher than that of the female gonads during sexual maturation. A positive correlation between estradiol-17βand mean oocyte diameter was observed during the study period (r = 0.664, P < 0.05), indicating that estradiol-17βmay have an important role during vitellogenesis. Estradiol-17βand testosterone increased during sexual maturation in female and male, respectively, and then decreased markedly after spawning. Thses findings indicated that the fluctuations in the levels of both steroids closely related to the reproductive cycle, thus estradiol-17βand testosterone could have a role as endogenous modulators in sex determination, development and maturation of S. constricta.3.According to histological observations, gametogenesis began in January when the water temperature was low. Gametes riped mainly in June-July when the condition index (CI) and mean oocyte diameter peaked. Spawning occurred mainly in August when the temperature was highest and coincided with phytoplankton bloom. The CI and oocyte diameter decreased sharply after spawning occurred, as mostly larger, mature gametes have been released. In autumn, the plentiful phytoplankton and higher water temperature were fit for the larvae to grow. Biochemical analysis indicated that gametogenesis took place at the expense of reserves accumulated in the tissues previously during winter. A slight increase in lipid and protein content in female gonads during sexual maturation was observed, demonstrating that the lipid and protein would be accumulated as vitellin in oocytes. Conversely, the lipid and protein content in the male gonads decreased during sexual maturation. The biochemical compositions in the adductor muscles and mantles varied during the study period suggest that they could support the reproduction and growth. Seasonal variation in RNA/DNA ratio suggests that it can not reflect the situation of gonad development in this species.The data generated in this study provide useful information not only for developing management strategies of S. constricta populations, but also for optimizing the hatchery-based seed production.