You really should not deshell them prior to cooking. Debeard them - yes (removing the bushy stringy thing from the shell crack) but leaving them in the shell makes the dish look and taste better! And besides, you won't damage the mussels trying to remove them as you would prior to cooking.
As I said before, the dish will taste better, look better and is not that difficult to eat (and it is fun) with the shells on.
I have never heard of a mussel dish with the shells removed prior. Don't do it!