It's an empty shell, but it's actually sticky sea mud.

Dead mussels will open because the tendons connecting the two shells lose their elasticity. And when you pick it, you must choose an exposed antenna, which will shrink when you touch it. You can't shoot without exposure.

It's easy to tell whether the clam is dead or not. When you see the speaker showing their tentacles, touch their tentacles with your hands. If they shrink, it means they are still alive, otherwise they are dead. Then you can't eat it. Eating stale seafood will upset your stomach.