This 2014 Oct 14 article by Jolene Bouchon avouches:

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.

Jasper White avouched the opposite on Epicurious.com

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.)

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So who's correct, and why?

  • $\begingroup$ You may have better luck on the cooking stack. $\endgroup$
    – John
    Aug 25 '19 at 16:23

Do they close is a better predictor.

before cooking tap the bivalve if they remain open do not cook or eat them. Unresponsive bivalves of any kind not safe to eat are dead and there is no way to tell for how long. Whether they open during cooking is meaningless for mussels they can stay locked tight even if healthy and freshly killed. Smell is also a good predictor never eat shellfish that smell off.

The differences between wild and farmed mussels makes this even less useful, wild mussels may be filled with sand making them heavy even if dead. Of course if a mussel is comes pre-cooked only smell is useful.

float vs sink is not really useful since it relies on how tightly sealed a bivalve is and whether it trapped air when it closed. A air seal is not a guarantee with many bivalves, mussels included, nor is trapped air. It is just not a useful metric.


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