Leviticus 11:9, commentary and pictures

Of all that live in the waters, in seas and in rivers, only those having both fins and scales may be eaten, and all other water creatures are forbidden as unclean. This is one of the restrictions on animal food given to Israel, belonging to the law given through Moses that distinguished clean foods from unclean foods. Under that law swine's flesh and the flesh of certain other animals were forbidden to the Hebrews.

It was part of health and sanitary instruction by which Israel were trained in the wilderness to leave behind unhealthful habits before entering Canaan, where the food restrictions applied. The distinction was wise and not arbitrary, for the forbidden creatures were not the best articles of food but were unfit and injurious to health. Such food fills and pollutes the blood with impurities and shortens life, while the permitted foods were the least objectionable; the restrictions were intended to preserve life and health, to keep the blood pure, and to lessen the harmful results of eating animal food. If Israel had submitted to this prohibition together with the rest of the food laws, everything injurious to health would have been removed from them and no sickness would have been allowed among them.

The rule was perverted by unwarranted extremes beyond what was required, adding minute regulations and traditions about clean and unclean that made the law burdensome and kept people constantly anxious. Notably, requiring the straining of all water used and of drinking water to remove the smallest insect that might be classed with unclean animals was never commanded. Daniel and his companions illustrate its application by refusing foods at the king's table that the law pronounced unclean, in order to preserve their health and clear minds for service.