In a medium pot, cook the calamari, seppia, and octopus in boiling water for 8 10 minutes, drain, and set aside to cool. Boil the shrimp for 2 3 minutes, drain, and set aside to cool.
Cut the red and yellow peppers, removing the seeds and stem, slice julienne style. Chop the celery stalks into small pieces.
In a large bowl mix the octopus, calamari, shrimp, peppers, celery, garlic, parsley, lemon juice, olive oil, salt and pepper. Refrigerate at least 1 hour for flavors to combine and chill.
Serve on a bed of mixed greens with sliced tomato.