The main thing to watch for when cleaning mussels, prawns and squid is that each one has a different part that needs removing: the beard on a mussel, the vein on a prawn's back, and the inner membrane and clear quill inside a squid. These parts are safe to eat, but removing them improves both texture and flavour.
How do you clean mussels?
The fibrous beard hanging out of a mussel's shell is pulled off firmly from close to the shell; this should be done right before cooking, since a mussel with its beard removed can spoil faster. Shells should be scrubbed against each other, and any that are cracked or do not close when tapped should be discarded, since they are more likely not to be fresh. Mussels should be rinsed in plenty of water a few times before cooking to remove any sand or mud.
How do you clean prawns?
The thin dark line visible along a prawn's back once it is shelled is its digestive tract, and it is usually removed by making a shallow cut along the back with a knife and pulling the vein out from one end. In small prawns this vein is thin enough not to notice even if left in, but in medium and large prawns leaving it can look unappealing and leave a faint gritty feel. Leaving the tail on while shelling both improves presentation and helps the prawn hold its shape while cooking.
How do you clean squid?
Once a squid's body is opened, a clear, plastic-like quill is found inside; this pulls out easily by hand and should never be left in before cooking. The thin purple-brown membrane on the outside of the body can be peeled off by rubbing with a fingertip; left on, it can shrink and toughen the texture during cooking. If squid rings are not sliced thinly enough, they risk turning rubbery when fried, so slice thickness matters just as much as cleaning for the final texture.
What matters after cleaning?
Cleaned mussels, prawns and squid should be cooked as soon as possible, or stored in a covered container in the fridge; once cleaned, seafood loses freshness much faster than it does still in the shell or whole. The knife and board used during cleaning have been in contact with raw seafood, so they should be washed with hot soapy water right away and not used for another ingredient straight after.