About fish
What kind of fish does Torah forbid?
For living beings living in water, the Torah gives a clear criterion for "kosher": only those who have fins and scales in the water are suitable for food.
Why does the Torah emphasize this? Because some fish (mackerel, for example) shed their scales when they are pulled out of the water.
This means that fish such as sturgeon or catfish are "unclean", since they have no scales, and "seafood" like crabs, shrimps and oysters are forbidden for food because they have neither scales nor fins.
Note that regarding the latter, the Torah is expressed in the following words: "They are an abomination to you, and they should be vile to you - do not eat their flesh and abhor their dead" (Vayikra, 11: 12-13).