How do you know if your bivalves are alive? Immediately get rid of anything with broken or damaged shells. Clams and mussels shells should be slightly open, and should shut quickly when you tap on them. If they're closed, don't shut, or float in water, they're dead. Introduce them to the trash. Oyster shells, on the other hand, should be closed tightly. And, as with all fish and shellfish, your bivalves should have a fresh, oceany smell with no hint of fishiness or ammonia.
Screenshot below hails from 0:15.
When submerging the steamers in the water, watch for any that float—this means that they're already dead and should be discarded. (Note that when preparing mussels, the opposite is true: They clamp shut tightly, trapping air in the shell, and therefore should float when alive. Discard any that sink.)
So who's correct, and why?