kashrut kosher - *Asher Bamayim* (אשר במים) in kashrus halacha

To find the kashrus status of an organism, we first need to correctly classify it as plant, animal, bird, or asher bamayim*, as each group has its own distinct requirements. E.g., animals need to chew cud and have cloven hooves, while asher bamayim need (fins and) scales.

What is the halachic definition of asher bamayim (אשר במים)?

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^{* This is how the Torah calls water-living creatures. Rabbinical writings use the word dag (דָג).}