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“cold-blooded” , allowing their body temperatures to vary as ambient temperatures change, though some of the large active swimmers like white shark and tuna can hold a higher core temperature.

Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams (e.g., char and gudgeon) to the abyssal and even hadal depths of the deepest oceans (e.g., gulpers and anglerfish).

At 32 000 species, fish exhibit greater species diversity than any other group of vertebrates. Fish are an important resource worldwide, especially as food.

Commercial and subsistence fishers hunt fish in wild fisheries or farm them in ponds or in cages in the ocean.

They are also caught by recreational fishers, kept as pets, raised by fish keepers, and exhibited in public aquaria.

Fish have had a role in culture through the ages, serving as deities, religious symbols, and as the subjects of art, books and movies.

Because the term “fish” is defined negatively, and excludes the tetrapods (i.e., the amphibians, reptiles, birds and mammals) which descend from within the same ancestry, it is paraphyletic, and is not considered a proper grouping in systematic biology.

The traditional term pisces is considered a typological, but not a phylogenetic classification.

The earliest organisms that can be classified as fish were soft-bodied chordates that first appeared during the Cambrian period. Although they lacked a true spine, they possessed notochords which allowed them to be more agile than their invertebrate counterparts. — www.wikipedia.com