Studies on the reproduction of Marcia opima (Gmelin)

The annual reproductive cycle of the clam, Marcia opima (Gmelin) from two geographically separated populations at Tuticorin and Ashtamudi was studied. It was observed that M. opima spawns twice a year at both the places. But the spawning periods are May to June and November to December at Tuticorin and March to May and November to January at Ashtamudi. Fully matured condition of the clam was associated with an increase in the values of gonad index and condition factor and a decrease in the digestive gland index. A slight increase in temperature initiated the spawning activity of M. opima at both the places. The sex ratio at Ashtamudi showed a male dominance. Attempts to induce the maturation of dams showed that fed clams matured faster than the unfed. The gonad index, condition factor and oocyte diameter of clams, kept at 28°C with proper feed was more than that kept at 23°C. Spawning and larval rearing of M. opima were successfully carried out in the hatchery. The settlement of larvae took place on 11111 day. On day 75, the spats attained an average size of 2.81 x 2.37mm. The growth and survival of spats at varying salinities was studied. Maximum growth rate and 100% survival of spat was reported at salinity 25 %0. During biochemical analysis, it was noticed that protein level increases to maximum in the fully matured clams. A low level of carbohydrate was observed during gametogenesis. Lipid level was more in the female gonad than the male gonad. Studies revealed that wet meat weight of clam is directly related to oxygen consumption, ammonia excretion and protein catabolism. The clam fishery at Tuticorin bay constituted a number of species like Marcia opima, Semele striata, Mesodesma glabratum, Gafrarium tumidum, Mactra cuneata and Paphia sp. Allometric relationships of M. opima from both the sites showed a correlation coefficient near to unity between various morphological characteristics. Parasitic infestation was observed in the clams during some months of the sampling period in both the populations