Leviticus 11:10
But all creatures in the seas or streams that do not have fins and scales — whether among all the swarming things or among all the other living creatures in the water — you are to regard as unclean. The comprehensive coverage — all creatures without fins and scales, whether swarming creatures or other living things in the water — ensures that the clean/unclean distinction covers every possible water creature without exception. The swarming creatures of the water (shellfish, crustaceans, mollusks) and the larger creatures without scales (eels, sharks, rays) are all unclean. The comprehensiveness of the category leaves no grey area in the water creature regulations.