If they are open before you cook them, chuck them. If they are shut after you cook them, chuck them.
I once chanced my luck with some half open ones when my portion of mussels had a disproportionate number of empty shells and the contents were not swilling unshelled at the bottom of the dish. I will not reduce the tone of the thread by describing the next two days. Just lets say, I now always veer on the side of caution!
In Normandy we go to the beach to a restaurant in an old boat shed with tables and chairs tumbling down on to the beach. You have your swim and walk straight up to the restaurant in your cossie and sand. Served with frites and accompanied by cider. Heaven.
I worked in Brussels for a year, and 'moules frites' was one of the best things about it. They bring you a wine bucket full of mussels. I cheat now, and buy them ready buttered and garlicked in the supermarket.
Grannylin That is seriously gross. I do hope Mr Grannylin is okay – although he doesn't deserve to be. There is no such thing as a "slightly" off mussel. It's either fresh i.e. live and the shell will shut when tapped sharply, or preserved (frozen, canned).
Just for the record, in case mussel lovers don't know, you can make a delicious but different equivalent of moules marinières with dry cider instead of white wine – Moules à la Normande.
That's a coincidence nonu but my kitchen was full of the smell of slightly 'off' mussels at lunchtime.OH put some in a bowl of cold water and more than half floated.I told him to discard them as they had been in the fridge for 2 days-I even checked on the Internet.But no! he knows best, so he ate them...but I didn't...we'll have to wait and see what happens