 The sexes are usually separate in bivalves but some hermaphroditism is known . The gonads are located close to the intestines , and either open into the nephridia , or through a separate pore into the mantle cavity . The ripe gonads of male and females release sperm and eggs into the water column . Spawning may take place continually or be triggered by environmental factors such as day length , water temperature , or the presence of sperm in the water . Some species are " dribble spawners " , but others release their gametes in batches or all at once . Mass spawning events sometimes take place when all the bivalves in an area synchronise their release of spawn . 
