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You’re probably thinking this is some kind of flat-earth style conspiracy theory. It’s not. I’m not saying there aren't scaly things swimming in the ocean or that your tuna sandwich is a hologram. They're real. But "fish"? As a biological category? That’s a total myth.
If you look at a cow and a lungfish, and then you look at a lungfish and a salmon, you’d naturally assume the two swimmers are more closely related. They both have scales. They both breathe underwater. They both look, well, fishy. But evolutionary biology tells a different story. The lungfish is actually more closely related to you, me, and that cow than it is to the salmon. This is the core reason why fish don’t exist in the way we were taught in third-grade science.
The term "fish" is what scientists call a paraphyletic group. It’s a bucket we throw things into because they happen to live in the water and look somewhat similar, not because they share a unique common ancestor that other animals don't. It’s kinda like grouping all "things with wheels" together—bicycles, Toyotas, and office chairs—and pretending they belong to the same biological family. It doesn't work.
Cladistics and the death of the "Fish" label
Back in the day, we classified animals based on superficial traits. If it had fur, it was a mammal. If it had scales and lived in a pond, it was a fish. This was the Linnaean system. It was simple. It was clean. It was also mostly wrong.
Modern biology uses cladistics. This is all about the family tree. In a "valid" biological group (a clade), you have to include the common ancestor and all of its descendants. Here’s where the "fish" problem gets messy. If you want to create a group that includes a salmon and a lungfish, you have to include everything that evolved from their common ancestor. Since land-dwelling vertebrates (tetrapods) evolved from a branch of lobe-finned fish, you can’t have a group called "fish" unless you also include humans, dogs, eagles, and crocodiles.
Basically, you are either a fish, or fish don't exist.
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Stephen Jay Gould, the legendary paleontologist and evolutionary biologist, famously championed this idea. He argued that after you've categorized all the distinct lineages—the hagfish, the sharks, the ray-finned fish—there’s nothing left over that defines a "fish" without being so broad it includes us. It’s a realization that feels dizzying. We’ve spent centuries naming a group that doesn't actually exist on the Tree of Life.
The weird world of Lungfish and Coelacanths
Let’s get specific. Look at the Coelacanth. For a long time, we thought they were extinct. Then one turned up in a fishing net off the coast of South Africa in 1938. It was a massive shock to the scientific community.
These creatures are "lobe-finned." Their fins are attached to fleshy, lobelike stalks supported by bones. These aren't just flimsy rays; they are the precursors to arms and legs. Genetically and structurally, a Coelacanth has more in common with a lizard than it does with a goldfish.
Then there’s the Lungfish. These guys can breathe air. They have actual lungs. When their ponds dry up, they bury themselves in the mud and wait it out. If we are being intellectually honest about DNA and ancestry, putting a Lungfish and a Trout in the same category while leaving out a Frog is just bad science. It’s picking and choosing traits based on where the animal lives rather than what it actually is.
Taxonomy is supposed to reflect reality. If the goal of biology is to map out the history of life, using a word like "fish" is like using a map where "the North" is just "anywhere it’s cold." It’s a vibe, not a location.
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Why we still use the word anyway
If the science is so clear, why hasn't the world changed? Why do we still have fish markets and fish sticks?
Language is stubborn.
We use "fish" as a functional term. It’s a "grade," not a "clade." In everyday life, we care more about what an animal does than who its great-great-great-times-a-million grandfather was. If it swims and tastes good with tartar sauce, we call it a fish. It’s convenient. But convenience isn't truth.
There’s also the "No Such Thing as a Fish" phenomenon, popularized by the researchers behind the show QI. They’ve spent years hammering home the point that the diversity among "fish" is actually greater than the diversity among all land-dwelling vertebrates. A shark is more evolutionarily distant from a salmon than a camel is from a cat. When you realize that, the word "fish" starts to feel incredibly small and useless. It hides the spectacular diversity of the underwater world.
The impact of the "No Fish" realization
So, what do we do with this? Does it matter?
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It matters because how we categorize the world changes how we protect it. When we lump thousands of vastly different species into one bucket, we tend to manage them as one monolithic resource. We talk about "global fish stocks" as if we're talking about a single crop like corn or wheat.
In reality, we are dealing with lineages that have been diverging for hundreds of millions of years. A shark’s biology is nothing like a tuna’s. Their reproductive cycles, their nervous systems, and their ecological roles are worlds apart. By abandoning the "fish" umbrella, we start to see the ocean for what it really is: a complex tapestry of distinct, unrelated, and highly specialized lives.
Moving toward a more accurate vocabulary
If you want to sound like an expert—or just annoy your friends at dinner—start using more specific terms.
- Chondrichthyes: These are the cartilaginous guys. Sharks, rays, and skates. They don't even have "true" bones.
- Actinopterygii: The ray-finned fish. This is most of what you think of as fish—tuna, bass, goldfish.
- Sarcopterygii: The lobe-finned fish. This includes the coelacanth and the lungfish. And, technically, you.
It’s a bit of a mouthful. But it’s real.
Actionable insights for the curious mind
Knowing that why fish don’t exist is rooted in evolutionary lineage gives you a different lens on the natural world. Here is how you can apply this perspective:
- Audit your culinary labels: Next time you’re at a grocery store, look at the "seafood" section. Notice how we group biological outliers like shrimp (arthropods), clams (mollusks), and salmon (vertebrates) simply because they share an environment. It’s a commercial category, not a natural one.
- Study Cladistics: If you want to understand how life is actually organized, look into "phylogenetic systematics." It’s the method of grouping organisms by shared ancestry. It’ll ruin how you look at "reptiles" and "birds" too (spoiler: birds are technically dinosaurs).
- Read "Why Fish Don't Exist" by Lulu Miller: If you want the human side of this story, this book is a masterpiece. It follows the life of David Starr Jordan, a taxonomist who spent his life naming fish, only for his work to be literally shattered by the 1906 San Francisco earthquake. It explores the psychological need humans have to categorize chaos, even when those categories are fake.
- Observe Anatomy: The next time you see a "fish," look at the fins. Are they thin and fan-like, or do they have a meaty base? You might be looking at a cousin closer than you think.
The world is much more chaotic and interconnected than our neat little labels suggest. Letting go of the word "fish" isn't about losing something; it's about gaining a more accurate, more wondrous view of the massive variety of life that navigated the waters long before we ever crawled out of them. We are just one branch of a tree that we've been mislabeling for far too long.