Shellfish are forbidden

Jews affirm that shellfish and other water creatures lacking fins and scales are forbidden. The Torah permits from the waters only what has both fins and scales (Leviticus 11:9-12; Deuteronomy 14:9-10) and calls everything else an abomination. Shrimp, lobster, crab, clams, oysters, and the like therefore fall outside the permitted category, as do fish such as the eel that lack true scales. The rule is one criterion within the wider system of kashrut governing which creatures may be eaten.

The fins-and-scales test has generated genuine rabbinic disagreement at its margins, most famously over sturgeon and swordfish, whose scales, if present at any life stage, differ structurally from the easily removable scales typical of unambiguously kosher fish; nineteenth and twentieth-century authorities split over whether such scales satisfy the halakhic definition, with some Orthodox authorities permitting sturgeon and others prohibiting it, a live disagreement that still produces different kosher certifications for the same species depending on which rabbinic ruling a given supervision agency follows. The fins-and-scales criterion is worked out at length in the Talmud, specifically tractate Chullin, where the sages state as a general principle, at 66b, that any fish possessing scales will also be found to have fins, making scales the practical test a buyer or a rabbinic authority actually applies, with the fins largely assumed rather than separately checked.

As with the other food laws, observant Jews keep this prohibition as an act of obedience and sanctification rather than for a stated reason, and it is held across the observant denominations. The clear, physical test, fins and scales, makes the category easy to apply and hard to blur, which is part of its function in marking a distinct way of life. It belongs with the prohibitions of pork and of forbidden birds and insects as part of the single fabric of dietary holiness the Torah lays out.

The rule produces an instructive contrast with Catholic fasting practice, where all seafood, shellfish included, counts as permitted non-meat food during Lenten abstinence, since the Western Christian category of forbidden meat was built around warm-blooded land animals rather than around the biblical categories of clean and unclean creatures Leviticus sets out; the same shellfish a Catholic might specifically choose to eat on a Friday in Lent falls under an entirely separate and, for Jews, prohibitive classification, illustrating how two traditions can build very different dietary systems from different starting principles even while both restricting food for religious reasons. Between the Torah's own text and modern kosher certification stands Maimonides' twelfth-century law code, the Mishneh Torah, whose section on forbidden foods, Hilchot Ma'akhalot Assurot, gives the fins-and-scales rule its classical, systematic legal statement, the layer later rabbinic authorities and certifying agencies build directly upon.