it is a sign of stress or a survival instinct. what have you done or changed recently?
you need to clean your skimmer and make sure it is running 100%, prepare a water change and run some carbon if you are not. they can foul the water pretty bad and actually cause other clams to sawn which compounds the problem.
I don't think so I have had the three clams in my tank for over a year. The sperm release was followed by what appears to be eggs. The anthias were in a feeding frenzy as were the corals.
The water parameters check out fine.
twenty is correct, usually clams in our tanks spawn at times of stress, which in our tanks, usually means a quick jump in water parameters. It could be anything- temp, salinity, alk, etc etc. One clam spawning often inducing the other to follow suit. Although the fish will eat some, the majority will just pollute the water, so as twenty said, be ready for some serious water cleaning of some sort.