The monthly variations in various body component indices such as mantle, gill, adductor muscle, gonad and hepatopancreas of the edible oyster Crassostrea madrasensis collected from the natural bed of Pulicat Lake have been studied.

Among the five body components, mantle forms a prominent body component. The gonad index and the digestive gland index showed an inverse relationship during the entire period of observations. The digestive gland index is higher than the gonad index during the sexually inactive phase. The percentage of water content was found to be higher in the gills than the other body components. The water content in the gonad was lower than the hepatopancreas during the period of gametogenesis, but it increases immediately after spawning indicating a reciprocal relationship between the water content of the gonad and the hepatopancreas. The water content was low in the mantle when the oysters are sexually ripe, but increases steadily after spawning. There is a gradual increase in the total body weight during the period of gametogenesis reaching its peak during the fully ripe condition and decline sharply after spawning. The digestive gland index was found to increase as a result of accumulation of energy reserves when feeding was high, whereas it declined sharply as a result of supply of nutrients to the gonad during gametogenesis.

Among the environmental parameters, salinity plays a major role in the changes of the body component indices of the oyster.