Studies On Reproductive Biology Of Mactra Chinensis And Meretrix Meretrix

The reproductive strategy of marine bivalves can be considered as an adaptation to ambient environmental factors such as water temperature and food availability. Marine bivalves show seasonal cycles of energy storage and utilization that are closely related to reproductive activity. In general, when food is abundant, reserves accumulate prior to gametogenesis in the form of glycogen, lipid and protein substrates, and subsequently are utilized in the production of gametes when metabolic demand is high. The particular importance of these substrates, how the timing of their consumption is related to gametogenesis and where they are stored varies among species as well as among populations of the same species. Therefore, detailed knowledge of the spawning periodicity and consequent energy storage- exploitation in different organs of each sex in relation to the environmental factors is of much importance for a complete understanding of reproductive strategy in marine bivalves.In this paper, two commercially important bivalve species were studied:the clams Mactra chinensis and Meretrix meretrix along the coast of Shandong Province. We investigated monthly variations in the environmental parameters (temperature, chlorophyll a concentration, salinity); the reproductive cycle and seasonal changes in biochemical composition (glycogen, lipid, protein, RNA/DNA ratio) in different tissues (mantle, adductor muscle, foot, gonad-visceral mass) of the male and female M. chinensis; the reproductive cycle and seasonal changes in biochemical composition (glycogen, lipid, protein) in different tissues (mantle,adductor muscle, foot, gonad-visceral mass) of the male and female M. meretrix. The major results of the study are presented as follows:1. According to histological analysis, the reproductive cycle of M. chinensis in Shandong Province followed an annual cycle, comprising two phases: a resting phase (November-February) and gametogenesis, including ripeness and spawning, during the rest of the year. During gonad maturation, the synchronism between males and females was observed and the gametes of both sexes were spawned at about the same time. Histological evidence showed that the gametogenesis of M. chinensis began in March when the water temperature was 7℃. After rapid growth in April and May most of M. chinensis matured from June to July and a massive gametes release occurred in August. From September to October the majority of the population was in the spent stage followed by a 4 months inactive stage until a new gametogenic cycle began. The glycogen content of all tissues both in the male and female decreased considerably during sexual maturation indicates that gametogenesis of M. chinensis depended largely on the glycogen stored during their growing season thus behaved as a conservative species. The lipid and protein content increased as the female gonad matured suggests that the female gonads could accumulate protein and lipid as vitellin within the gonad. Conversely, the protein content decreased in the male gonads and other tissues as gamete developed, demonstrating that they may be utilized to produce energy for the process of gametogenesis after carbohydrate reserves were depleted. Seasonal changes in RNA/DNA ratios suggest that they are good index of gonad maturity.2. Histological evidence showed that the gametogenesis of the clam M. meretrix in the Yellow River delta initiated in January when the water temperature was 2℃. The clams matured mainly in July, followed by a massive gametes spawning in August-September when water temperature was high and food was abundant and the appropriate environmental conditions improved the possibility of larval success. The mean oocyte diameter and condition index (CI) of M. meretrix increased during sexual maturation, peaked when the gametes matured and declined as the massive spawning happened. The correlations suggest the mean oocyte diameter and condition index (CI) can be used as good indicators for identifying the gonadal maturity of M. meretrix. Biochemical analysis indicated that carbohydrates are the major energy reserves, which play the most important role in the reproductive process of this species. The lipid and protein content increased as the female gonad matured, suggesting they are the major organic component of oocytes. The decline of the lipid in the adductor muscle and the protein in the foot of both sexes indicates that they also supported gametogenesis of M. meretrix after the glycogen was depleted.The useful information obtained in this study is not only important for sustainable management of M. chinensis and M. meretrix's wild stock, but also can help to initiate aquaculture activities in the species.