It is my understanding (and I could well be wrong) that sperm release is triggered by another clam releasing eggs. In the wild a "female" will release her aggs and the other clams will release sperm, then another "female" will release her eggs and the males will again release sperm and so on until all "females" have released eggs.
Note, Tridacnids are hermaphrodites and a clam is male when it matures, once it reaches a certain size depending on species, it will develop female sex organs but not lose its male organs.