Charles Ellicott Commentary Leviticus 11:10

"And all that have not fins and scales in the seas, and in the rivers, of all that move in the waters, and of all the living creatures that are in the waters, they are an abomination unto you," — Leviticus 11:10 (ASV)

Of all that move in the waters. —That is, apart from the fishes exhibiting the above-named signs, all other inhabitants of the water are forbidden. Hence all shellfish, whether molluscs or crustaceans, and cetaceous animals, are unclean.