The term "fish" is used to refer to almost any animal that lives in water, including actual fish, shellfish, crustaceans, and marine mammals. In most cases manuscripts describe several species of fish individually.

The variety of species of fish is uncountable, as is the variety in habits of feeding and reproduction. Some lay eggs, while other give birth to live young. Male fish do not commit adultery with fish of other species. Fish (pices) get their name, like cattle (pecus), from feeding (pascendo) says Isidore of Seville.

Some manuscripts include "fish" that are the aquatic versions of land animals (sea-pig, sea-ass, sea-hare, etc.). The reasoning was that in a complete creation there should exist a marine equivalent for most land animals.