Q. Since my hands are fairly weak, I use a pliers to rid the mussel of it's beard. I have, on occasion pulled so hard that the shell is slightly opened. Is this a problem? Shall I discard the poor thing?
Oh no. Just apologize and toss it in the pot. Mussels spoil quickly after expiring, so don't pull out the beards more than a few hours before they are to be cooked. But whether the mussel is open slightly or tightly closed is not the issue. The question is whether it is spoiled. You should be able to tell by smell. A fresh mussel should smell like a clean beach.
When you start working with mussels, if one is gaping wide open, it's probably dead and somewhere along the journey to spoiling. Ones that are open only slightly, a quarter of an inch or so, can be tapped, and should close within a half minute or so. James Peterson, author of Fish & Shellfish (Canada, UK), says the surest way to determine if a mussel is still among the living is to firmly push the shells sideways in opposing directions with your thumb and forefinger (not your pliers). If the mussel is dead, the shells will fall apart in your hand.