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).

Isidore of Seville [7th century CE] (Etymologies, Book 12, 6:1-6): Fish (pisces) have their name, as do cattle (pecus), from feeding (pascendo). Some fish are amphibious, able to swim and to walk on land. Beasts and birds were named first, because they are easily seen; when fish were named, the names were often related to similar land animals, by shape or color or habit. According to David the Psalmist, the sea contains fish without number.