There’s no such thing as a fish

Stephen Jay Gould was an American geologist, paleontologist, biologist and popular-science author who spent most of his career teaching at Harvard University and working at the American Museum of Natural History in New York. At some point (it’s not clear exactly when) Gould — who had spent a lifetime studying evolutionary biology — declared that “there’s no such thing as a fish.” This comment became the name of a popular podcast spun off from the QI TV series, in which the hosts discuss interesting facts. But what did Gould mean? Obviously there are things called fish. Was this some attempt to be funny, like the guy who tried to convince people that birds aren’t real?

Not exactly. What Gould meant was that the term “fish” doesn’t really have any scientific or categorical meaning per se. In other words, lots of things that are defined as fish — many of which even have the term “fish” in their name, like the hagfish — are not really similar enough to be considered part of the same category of living things. As the Wikipedia entry for the podcast notes, a salmon is more closely related to a camel than it is to a hagfish, for example. All the things that we might believe to be common to fish — living underwater, having gills, fins, giving birth via eggs, etc. — are not universally true for everything that is usually thought of as a fish (also, there are lots of things called fish that aren’t, including the cuttlefish, starfish, crayfish, and jellyfish).

The Oxford Encyclopedia of Underwater Life also states that “there is no such thing as a fish.” It explains that the term “fish” is a convenient umbrella term to describe an aquatic vertebrate that is not a mammal, a turtle, or anything else. Lumping all the different groups and classes of fishes now alive under the term “fish,” it says, is like lumping all flying vertebrates –- including bats (which are mammals), birds, and even the flying lizard –- under the single heading “birds,” just because they all fly. The relationship between a lamprey and a shark, the encyclopedia says, “is no closer than that between a salamander and a camel.”

As a commenter on a blog post about Gould’s statement at TYWKIWDBI (which stands for “Things You Wouldn’t Know If We Didn’t Blog Intermittently”) put it:

The “no such thing as fish” is the kind of mess that you get trying to reconcile colloquial speech with scientific knowledge. Taxonomy wants to name entire groups — one common ancestor and all its descendants. These are “monophyletic groups,” as opposed to paraphyletic groups (groups that have an ancestor and some but not all of its descendants). From that perspective, all the things we colloquially call “fish” are not a proper group — if we went back to the common ancestor of all fish, and then looked at that common ancestor’s descendants, we’d get quite a lot of things that aren’t “fish,” including humans.

He goes on to explain that most of the fishy things we think of, from goldfish to tuna belong to the Actinopterygii, the ray-finned fishes (speaking of goldfish, did you know there’s a cousin of the goldfish that is the size of a school bus?) Sharks, rays and other fishes whose bodies are made up mostly of cartilage are part of another group, Chondrichthyes. And then, as he puts it “there are a couple weirdos like hagfish and lampreys,” and of course the coelocanth, which is part of Sarcopterygii, the lobe-finned fishes, a category that also includes humans (the aptly-named lungfish can breathe air). In other words, when you are talking scientifically, you have to add some qualifiers to the term “fish,” or accept that essentially all vertebrates are fish.

On a related note, the hagfish is a very strange thing: it is the only living thing that has a skull but no spine or skeleton. It also has no jaw, which makes it more like an eel than a fish, and it is fairly closely related to the lamprey (which does have a jaw). The main thing the hagfish is known for is its ability to produce slime, which it does when threatened — hagfish slime is one of the fastest-expanding materials known to man: it expands to 10,000 times its original size in 0.4 seconds (the hagfish can also tie itself in a knot in order to escape a predator). In 2017, a container truck filled with hagfish — which are a delicacy in some countries — flipped over and cars were literally covered with slime in a matter of seconds. A scientist described the easiest way to tell whether something is a hagfish: ““Look at the hand holding the fish,” the marine biologist Andrew Thaler once noted. “Is it completely covered in slime? Then, it’s a hagfish.”

Related: The Vatican at one point declared that from a religious perspective, the capybara — a giant rodent — was a fish, so that Venezuelans could eat them during Lent, when Catholics weren’t allowed to eat meat

Also related: There’s no such thing as a “tree” either, and for much the same reason. Avocado and cinnamon are both from fairly closely-related tree species, even though they are nothing alike, and it’s possible that the last common ancestor between an apple and a peach was not even a tree at all. Also, cactuses, aloe vera, jade plants, snake plants and other common plants known as succulents are not even remotely related to each other.

Update: Here’s a court case in California between the Almond Alliance of California and the Fish and Game Commission, in which the question before the court was whether “the bumble bee, a terrestrial invertebrate, falls within the definition of fish, as that term is used in the definitions of endangered species in section 2062” of the California Endangered Species Act. The court decided that it did, because the act defined the term “fish” to include all invertebrates.

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