I bought some frozen mussels from the super market they were allready open and the top shell gone?!

I bought some frozen mussels from the super market they were allready open and the top shell gone?

does this mean that they are already cooked if yes how should they be served?
if not should it be cooked the same way as usual with the closed ones?
thank you

Hi there, those mussels are the new zealand green lip, they have been steamed only just, so they are almost raw, you can do with them anything that you would do with live ones, my favourite way , mince or whizz garlic,parsley, softened butter or margarine and pepper(everything to taste) then place a Little on the shell and grill till half cooked, enjoy.