The cyprinid fish (8mm) and the whale shark (12m) are some of the smallest and largest fish in the sea.

At first, this might seem like an easy question to answer, but it is difficult to define what makes a fish a fish because there is so much diversity among animals that we consider to be fishes. There are more than 27,900 species of fishes alive today, living in marine and freshwaters, in environments as hot as 104°F/40°C and as cold as 28°F/-2°C, and ranging in length from 0.3 inches/8 mm to 39 feet/12 m. What characteristics unite such a diverse group of animals?

All fishes …

• have a brain protected by a braincase and an obvious head region with eyes, teeth, and other sensory organs

Most fishes …

• are vertebrates with vertebrae protecting the spinal cord
• live in water
• breathe primarily with gills rather than lungs
• have paired limbs, in the form of fins that aid in locomotion
• are unable to regulate their own internal body temperatures
• are covered with scales that protect their bodies

There are many exceptions to these guidelines. For example, hagfish aren’t vertebrates and don’t have scales; mudskippers can live outside the water; lungfish use lungs to breathe; lampreys don’t have paired fins; and tuna are warm blooded!

A few exceptions aside, a fish has the following basic body plan:

The term fish is a convenient term used to refer to diverse aquatic organisms, such as lampreys, sharks, coelacanths (SEE-luh-kanths), and ray-finned fishes — but it is not a taxonomic group that would be used in a phylogenetic classification scheme, as “vertebrates” or “hominids” is. That’s because phylogenetic taxonomic groups must be clades. A clade is a group that includes all the descendents of a common ancestor and that ancestor, and all the different organisms that we think of as fish don’t form a clade. Look at the phylogeny here. Almost everything you see on this phylogeny is a fish — with one exception. The lobe-finned lineage (technically called the Sarcopterygii, sar-KOP-tuh-RIJ-ee-eye) includes both the lobe-finned fish and four-legged vertebrates, like frogs, dinosaurs, bats, and us humans! Because this non-fish lineage is nested within a bunch of fish on the tree of life, the fish do not form a clade.