How many species of fish are there?

How to get close to this number

Anchor on the vertebrate groups you know. This site puts birds at around eleven thousand and mammals at around six and a half thousand; the explanation says fish exceed all vertebrate groups combined. Adding birds, mammals, reptiles and amphibians and then exceeding the sum puts fish in the tens of thousands.

The reason is that "fish" is not a coherent evolutionary group. It bundles together lineages separated by hundreds of millions of years — more distant from each other than a bird is from a mammal — under a single word that means "aquatic vertebrate that is not a tetrapod". A paraphyletic category will always look more diverse than a real one.

The unfinished part is the interesting one: around two hundred and fifty new species are named every year and the deep ocean is assumed to hold many thousands unseen. Most of the planet's habitable volume is deep water, and it is the least surveyed environment on Earth — so this count will keep rising for decades.