Gonadal development and spawning induction in artificially conditioned Manila clams Ruditapes philippinarum

Manila clams Ruditapes philippinaruin were maintained on Pavlova lutheri of mean density 11.0×107 cells/g (wet body weight)/day at 18°C for 3 months out of their spawning season. During this period, attempts to induce spawning were made at 1 and 2 weeks intervals; concurrently, clams were sacrificed for estimation of condition factor and gonadal development (histological assessment). Data were recorded on larval survival rates until the pediveliger stage. Ripe males and females became dominant 30 and 40 days respectively, after the commencement of the experiment. Abundant spawning, 33.1×104 eggs/female, which coincided with peak gonadal maturation, was evident on day 50. However, extensive resorption of sperms and oocytes, together with reduction of condition factor and number of spawned eggs, was observed from day 50 to the end of the experiment. Egg diameter increased and thickness of jelly layer decreased with the advance of maturity. Low larval survival was observed in eggs with smaller diameter. Variation in survival was observed in larvae hatched from eggs with larger diameter.