Is there really no such thing as a fish?
In days gone by, the word ‘fish’ was pinned to virtually any creature that lurked beneath the waterline. Gradually, as people paid more attention to the biology of those animals, it became clear that some ‘fish’ belonged to other groups, such as reptiles or mammals. This left a motley collection of aquatic vertebrates, including sharks, stingrays, hagfish, sturgeon, lungfish, goldfish and tuna. They are all animals that (usually) breathe through gills, have bodies covered in scales (though not always) and limbs in the form of fins.
The problem is that birds, frogs, lizards, turtles, mammals and all the other vertebrates evolved from fish ancestors. So, strictly speaking, those land-dwelling vertebrates should all be called fish, too. Generally, though, most ichthyologists are content to refer to the closely related gathering of aquatic animals as fish – as long as we don’t go forgetting our own fishy ancestry.