Leviticus 11:9

Any fish, either from salt water or fresh, might be eaten if it had both scales and fins. but no other creature that lives in the waters. Shellfish of all kinds, whether mollusks or crustaceans, and cetaceous animals, were therefore prohibited, as well as fish which appear to have no scales, like the eel; probably because they were considered unwholesome, and (under certain circumstances) found to be so.