Please don’t assume from the title of this post that it’s about how fish don’t actually exist, and/or that they are some kind of spy robot developed by the government, which is a popular meme about birds that I’m pretty sure is a joke (although nowadays you never know). Fish definitely exist, if by that you mean things that swim and have scales and gills and so forth. So what does the title mean? I came across it because it’s the name of a podcast in which the hosts dive into unusual or little-known facts: There’s No Such Thing As A Fish. It’s also a statement reportedly made by the paleontologist, evolutionary biologist, and historian of science Stephen Jay Gould, and elaborated on by Richard Dawkins in his book “The Ancestor’s Tale.”
So what did Gould mean? In essence, his point is that the category of “fish” is way too broad, and that not all the members of that group share things in common. For example, the lungfish and an ancient fish known as the coelacanth have more in common with a camel than they do with a salmon, even though all of them live in the water and swim. There are more than 32,000 species that qualify to be considered “fish,” a far greater number than any other group of vertebrates, and there are some wide differences — the hagfish (known for its prodigious production of slime, which expands in volume by 10,000 percent in a fraction of a second) has a rudimentary skull but doesn’t have a spine, and takes in nutrients through its skin.
As a commenter on this blog post pointed out, the problem comes from the intersection between colloquial language and the scientific categorization of different species. And the problem with the category of “fish” is that if you include anything that has fish-like ancestors, it pretty much includes all mammals, including human beings, since we are ultimately descended from fish. But within that broad category there are discrete groups of fishy things, like the ray-finned fishes, which includes everything from goldfish to tuna, or the cartilaginous group, which includes sharks and rays.
Here’s what the Oxford Encyclopedia of Underwater Life (yes there is such a thing) says about the term fish:
“Incredible as it may sound, there is no such thing as a fish. The concept is merely a convenient umbrella term to describe an aquatic vertebrate that is not a mammal, a turtle, or anything else. There are five quite separate groups (classes) of fishes now alive, plus three extinct ones, not at all closely related to one another. Lumping these together under the term fishes is like lumping all flying vertebrates — namely, bats (mammals), birds, and even the flying lizard — under the single heading birds, just because they all fly.”