When preparing mussels most people abide by the rule that any mussels still closed after cooking should be thrown away. However, some scientists claim this is simply an old wives' tale and that the rule is actually a myth. Nick Ruello, an Australian fisheries biologist, cooked and ate more than 30 batches of mussels for a seafood report. Of those mussels, 11.5 percent were still closed after cooking, but Ruello found he could safely eat each one of them after forcing the mussels open with a knife.
The abductor muscles in live mussels keep the two halves of the shell together. When the shellfish are cooked, the heat denatures the proteins of (pardon the pun) mussels' muscles, so they come unstuck from their shells. However, even if the abductor muscles refuse to disintegrate in the heat, the meat has been cooked at a high enough temperature that it is safe enough to eat.
The one exception? If mussels are open before they've been cooked (and don't close up tightly when tapped), then they're dead, and not fresh enough to eat under any circumstance. Have you ever pried open a cooked mussel that was closed shut? Will this new piece of information change the way you eat the shellfish?