There is no need to worry about eating mussels as long as you follow the following guidelines: - Mussels should be closed before cooking - Mussels should be fully open after cooking If you come across an open mussel when cleaning you can give it a tap with the side of your knife and it should close after a few minutes, or you can run it under cold water and it should close a few seconds. If the mussel shows no sign of life then you can discard the shell as it is not fit for eating.