Muscles that are OPEN before cooking

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So bought net of muscles yesterday. Just come to wash them and majority are open in the net.

I have tapped a few and some have clised fully some still slightly open

Are these safe to cook and eat

I'm sure you're supposed to discard mussels that are open before cooking, then discard any cooked ones that have remained closed.

If they are actually moving to close, they are fine as it means they are still alive.
It's the ones that stay wide open before cooking (or remain closed after cooking) that need binning.

They're better if they're mainly shut but they're fine to eat if they close (even slowly) if you give them a tap. Don't eat any that don't or that have broken shells