Bring the water, peppercorns and lemon slices to a boil in a big pot and add the scallops.
Cook over a moderate heat for 2 minutes, then remove the scallops with a slotted spoon and put them in a colander.
Bring the water back to a boil and add the shrimp.
Cook covered for 4 minutes, then transfer them to the colander to drain and cool.
Bring the water back to a boil and add the squid. Cook for 1 minute or until opaque, then add the squid to the colander.
Bring the water back to a boil and add half the mussels. Cook for 2 minutes or until they open, then add them to the colander.
If the colander is full, you can transfer the drained seafood from the colander to a bowl, to make space for more seafood.
Cover and cook the remaining mussels until they open. Discard any which do not open.
Discard the cooking water, peppercorns and lemon slices.
Peel and devein the shrimp unless they are ready-peeled.
Remove the mussels from their shells.
Toss all the cooled seafood together briefly.
Combine the extra-virgin olive oil, lemon juice, lemon zest, garlic, salt and black pepper, to make a dressing.
Toss the seafood with the dressing and celery.
Cover and chill for 1 or 2 hours, or for up to 24 hours for a more intense flavor.
Remove the seafood salad from the refrigerator 30 minutes before serving.
Stir in the parsley just before serving and garnish with lemon wedges.
