Tibetan Buddhists will not eat shellfish because far too many creatures—shrimp, mussels, clams, scallops, crabs-- must die for one person’s meal, a “holocaust on every plate.”
Kosher Jews do not eat shellfish claiming these, like pigs, are scavengers and thus unclean. Islam forbids eating shellfish as “vile things” although a debate continues on whether shrimp, non-scavengers abundant in the southern waters of the Mediterranean and arguably more fish than shellfish, are halaal (permitted) or haraam (forbidden). It’s a sect-by-sect, person-by-person decision.
An unorthodox explanation for the Jewish and Islamic taboo is: at the time of its creation, shellfish were such inefficient calorie producers for the foraging work involved, they had to be disparaged.