EARLY DEVELOPMENT OF THE ESTUARINE MOLLUSK POLYMESODA-SOLIDA (PHILIPPI, 1846) (BIVALVIA, CORBICULIDAE) IN LAKE MARACAIBO, VENEZUELA

The estuarine clam, Polymesoda solida (Philippi, 1846), from Lake Mara caibo, Venezuela, was spawned and reared under laboratory conditions t o monitor its early development. Spawning was induced via salinity cha nges and serotonin injections. Serotonin injections were more effectiv e than salinity changes yielding viable gametes in 64% of the specimen s used. Eggs and sperm were released into the water and fertilization was achieved by mixing 5 ml of eggs with 1 ml of sperm. After fertiliz ation, the first two embryonic divisions occurred at 1 and 1.5 hr, tro chophore larvae appeared between 24 and 30 hr, and free swimming strai ght-hinge veligers developed two days after the trochophores. Umbo sta ges with limited movement were seen after the fifth day. Trochophore a nd straight-hinged veliger stages were observed surviving inside the g elatinous envelope that surrounds the eggs. This envelope dissolved in most cases when the trochophore stage was reached, but persisted in t hose experiments where the culture environment deteriorated. We postul ate that this envelope plays an important role in protecting the early developmental stages under adverse conditions.