Studies On Reproductive Physiology Of Mactra Veneriformis And Gomphina Aequilatera

The reproductive strategy of marine bivalves is affected by surrounding seawaterconditions (physical, chemical, biological parameters) and interior parameter(nerve-endocrine system regulation). The gametogenesis of marine bivalves is closely relatedto nutrient storage and environmental factors. Gametogenesis is an energy-demanding process,accompanied by energy storage and utilization. Gametogenesis uses the energy storedpreviously in some somatic tissues, or the energy obtained from food directly. The energyresource and the stored tissues varied among species as well as among populations of thesame species. Combining the seasonal variation of environmental conditions, analysis of thenutrient storage of separate organs monthly would help to determine the dynamics of theenergy storage-exploitation during its reproductive cycle and illustrate the reproductivestrategy in marine bivalves.The dynamics of the surf clam Mactra veneriformis nutrient storage and exploitationduring its reproductive cycle was studied in relation to environmental conditions from April2007to March2008in the north coast of Shandong Peninsula (China). Seasonal variation inthe environmental parameters (temperature, chlorophyll a concentration, salinity) andbiochemical composition (lipid, glycogen, protein, RNA/DNA ratio) in different tissues(mantle, adductor muscle, foot, gonad-visceral mass) of the male and female surf clam weredetermined monthly throughout the year. The reproductive cycle of Gomphina aequilaterawas studied in relation to environmental conditions from April2008to March2009inHaiyang, Shandong Province. Seasonal changes in the station environmental parameters(temperature, chlorophyll a concentration, salinity) and biochemical composition (lipid,nucleic acid, RNA/DNA ratio) in different tissues (mantle, adductor muscle, foot,gonad-visceral mass) of the male and female G. aequilatera were investigated in eachdevelopmental stage. The major results of the study are presented as follows:1. Histological analysis indicated that gonads of males and females developed concurrentlyand gametogenesis began in December (early winter). The spawning period occurredonly once a year, starting in July and appearing to continue into August and September.Temperature and chlorophyll a concentration were in high level during the maturationand spawning period,21-30℃and18.23-24.35μg/L, respectively. The majority ofindividuals were resorbing their gonad, or alternatively in a period of gonadal inactivity in October and November. The monthly mean oocyte diameters increased gradually fromthe start of gametogenesis to maturation, and reached a maximum before spawning. Thesex ratio of females to males (1.138∶1) were significantly different from the expectedratio (1∶1; P <0.05) with the excess of females. All the surf clams sampled weredioecious, and no evidence of hermaphroditism was found in the surf clam examined.The study of condition index calculated as dry flesh weight/dry shell weight showed anapparently recovering period from November to January, reaching a peak of11.95.Separate biochemical analyses of the mantle, adductor muscle, foot and gonad-visceralmass in each sex were undertaken. The majority of individuals were resorbing their gonad,or alternatively in a period of gonadal inactivity in October and November. Utilization ofglycogen took place during the spawning period, whereas protein was biosynthesized asthe mature gametes formed. The glycogen value increased during the resting stage(autumn-winter). The reproductive pattern exhibited by M. veneriformis based on thestorage of reserves during the late resting stage, and the gonad maturation period, seemsto be conservative and opportunistic.2. According to histological analysis, the reproductive cycle of Gomphina aequilatera in theeast coast of Shandong Province followed an annual cycle. The gametogenesis initiated inJanuary when the water temperature (2oC) and chlorophyll a concentration (1.77μg/L)were at the lowest level of the year. As the increasing of the temperature and chlorophylla concentration, the majority of individuals matured in June. A massive gametesspawning occurred in July-August when water temperature was high (27~29oC) and foodwas abundant (18.55~25.19μg/L), followed by spent/resorbing or inactive stages. Nucleicacid analysis indicated that the RNA/DNA ratios in gonad-visceral mass of the female G.aequilatera did not change significantly among stages. Therefore, the ratios in female G.aequilatera could not be used to identify sexual maturity and oogenesis. The DNAcontent in gonad-visceral mass of the male individuals increased dramatically during thegametogenesis and peaked in the ripe stage. In contrast, the RNA/DNA ratios decreasedsharply during the gametogenesis and hit the dip in the ripe stage. The characteristics ingonad-visceral mass indicated that the DNA content and RNA/DNA ratios in male G.aequilatera are good indicators of sexual maturation and spermatogenesis. The lipidcontents in gonad-visceral mass of female individuals were significantly higher thanthose of males’ through out the gametogenesis. The information on M. veneriformis reproductive strategy obtained in this study providesa base for fisheries resource administrators to design biologically sound managementmeasures for this species, and is crucial for initiating its aquaculture.