You probably think you know what a fish is. It's a scaly thing that swims in the water and breathes through gills. Simple, right? My toddler can point to a goldfish and say "fishy." But if you sat down with a group of hardcore cladistic taxonomists, they’d tell you something that sounds like a flat-earth conspiracy: why fish don't exist.
It sounds like a prank. It isn't.
Evolutionary biology has a massive problem with the word "fish." It’s a category that makes sense to our eyes but fails miserably when we look at DNA and the tree of life. If we want to be scientifically honest about what a fish is, we either have to include humans in that definition or admit that "fish" is just a convenient lie we tell ourselves to make the ocean less confusing.
The problem with the word fish
Taxonomy is the science of naming things. For a long time, we named things based on how they looked. If it had fur and gave milk, it was a mammal. If it had feathers, it was a bird. This worked fine until we started looking at ancestry. In modern biology, a group is only "real" if it includes a common ancestor and all of its descendants. We call this a monophyletic group or a clade.
Here is where the wheels fall off the wagon.
If you take a lungfish and a salmon, you’d say they are both fish. They look similar. They live in the same neighborhood. But a lungfish is actually more closely related to a cow, a hawk, or you than it is to a salmon. Seriously. The common ancestor that the lungfish shares with us lived more recently than the ancestor it shares with that salmon.
So, if you want to group the lungfish and the salmon together and call them "fish," you logically have to include humans in that group too. Otherwise, you’re just picking and choosing based on aesthetic vibes rather than actual lineage.
Stephen Jay Gould and the no-fish theory
The legendary paleontologist Stephen Jay Gould is often credited with popularizing the idea of why fish don't exist. He spent years studying the intricacies of evolution and eventually came to the realization that the category of "fish" is paraphyletic.
That’s a fancy way of saying it’s a broken group.
Imagine you’re organizing a family reunion. You invite everyone descended from your great-grandfather. That’s a monophyletic group. Now imagine you invite everyone descended from him except for the people who moved to Ohio. That’s what "fish" is. We’ve taken a huge branch of the tree of life and arbitrarily sliced off the parts that crawled onto land.
It’s messy.
Biologists like Gould argued that if we want our language to reflect reality, we have to stop using "fish" as a formal biological rank. It doesn't mean anything in the way that "Mammalia" or "Aves" means something. It’s a "trashcan taxon"—a place where we throw everything with fins because we don't know what else to do with them.
Salmon, Lungfish, and the Cow problem
Let’s get specific. Most of what we call fish today are "ray-finned fishes" (Actinopterygii). These are your tuna, trout, and goldfish. Then you have "lobe-finned fishes" (Sarcopterygii). This group includes the coelacanth and the lungfish.
The coelacanth is a famous "living fossil." People thought it went extinct 65 million years ago until one showed up in a fishing net in 1938. It has fleshy, limb-like fins. Those fins are the precursors to our arms and legs.
- A salmon is a ray-finned fish.
- A lungfish is a lobe-finned fish.
- A cow is a tetrapod (four-limbed animal).
Genetically and evolutionarily, the lungfish and the cow are siblings. The salmon is a distant cousin. If you put the salmon and the lungfish in a bucket and label it "fish," you are ignoring the fact that the lungfish has more in common with the person holding the bucket than the other animal inside it.
Honestly, it’s kind of insulting to the lungfish.
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Why our brains hate this
Human beings are obsessed with patterns. We like categories because they help us navigate the world without having to think too hard. We see a wet, scaly thing and our brain shortcuts to "fish." It’s an easy label.
But nature doesn't care about our labels. Evolution is a messy, continuous flow. There was never a moment where a fish gave birth to something that wasn't a fish. It was a billion tiny transitions over hundreds of millions of years.
When we say "fish don't exist," we are acknowledging that the category is a human invention, not a biological boundary. It’s like the word "vegetable." In a kitchen, a carrot is a vegetable. In a lab, a carrot is a root. There is no such thing as a "vegetable" in botany. It’s a culinary term, just like "fish" is basically a culinary or recreational term.
We’ve built our entire understanding of the ocean on a word that lacks a scientific foundation.
The impact of cladistics
Cladistics changed everything in the 1960s and 70s. Before that, we used the Linnaean system, which was all about ranks: Kingdom, Phylum, Class, Order, Family, Genus, Species. It was very tidy. Very Victorian.
But Willi Hennig, the father of cladistics, pushed for a system based purely on common descent. This shift is why your old school textbooks are mostly wrong now. It's why birds are now technically considered dinosaurs. If you want to be a "bird," you have to belong to the group that includes the T-Rex.
Applying this logic to the water, the group Pisces (the old Latin name for fish) was dismantled. It was revealed to be a collection of unrelated lineages that just happened to solve the problem of "living in water" in similar ways. It's convergent evolution at its most deceptive.
Sharks aren't even close
If you think the lungfish-cow connection is weird, look at sharks. Sharks are cartilaginous fish (Chondrichthyes). They don't even have bones. They diverged from the ancestors of bony fish (like salmon) over 400 million years ago.
To put that in perspective, a shark is more distantly related to a goldfish than a human is to a lizard.
When you go to an aquarium, you’re looking at creatures from completely different evolutionary universes. Calling them all "fish" is like calling everything with wings a "bird," including bats, butterflies, and Boeing 747s. It’s a massive oversimplification that hides the incredible diversity of life on Earth.
What should we call them instead?
If "fish" isn't a thing, how do we talk about the stuff in the sea? Biologists use more specific terms. They talk about Actinopterygii (ray-finned), Chondrichthyes (sharks and rays), and Myxini (hagfish).
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It’s less catchy. It doesn't work well on a menu. "I’ll have the grilled Actinopterygii, please" just sounds pretentious.
But accuracy matters. When we use the word "fish," we are reinforcing an old-fashioned view of the world where humans are "above" or "separate" from the rest of life. Recognizing why fish don't exist forces us to see our own place in the lineage. We are just very specialized, dry-land versions of those lobe-finned ancestors. We are, in a very real sense, highly modified "fish."
The cultural legacy of a non-existent group
Lulu Miller wrote a fantastic book called Why Fish Don’t Exist. It’s part biography of David Starr Jordan (a famous taxonomist) and part philosophical exploration of this very topic. She explores how Jordan spent his life trying to bring order to the chaos of the ocean, only to have his work literally shattered by the 1906 San Francisco earthquake.
The story serves as a metaphor for the human condition. We want the world to be organized. We want "fish" to be a real thing because it makes the world feel manageable. Accepting that they don't exist is a lesson in humility. It’s an admission that our language is too small for the complexity of the universe.
Moving forward with the "No Fish" mindset
So, where does this leave you? Does it mean you can't go fishing or eat fish and chips? Of course not. In common parlance, the word still works. It's a functional term for a specific lifestyle: aquatic, vertebrate, non-tetrapod.
But if you want to understand the deep history of your own body, you have to let go of the label. You have to see the connection between your own arm bones and the fins of a coelacanth. You have to appreciate that the ocean isn't filled with "fish"—it’s filled with a dozen different lineages that have been doing their own thing for hundreds of millions of years.
Actionable steps for the curious:
- Check out a Cladogram: Look up a phylogenetic tree of vertebrates. Trace the line from humans back to the first jawed vertebrates. Notice where the "fish" branches fall off.
- Re-examine the Aquarium: Next time you see a shark and a tropical fish together, remember they are further apart evolutionarily than you are from a turtle.
- Read the Source Material: If you want the deep dive, look into Stephen Jay Gould’s essays or Lulu Miller’s work. They explain the philosophical fallout of this biological reality much better than a textbook ever could.
- Stop Grouping by Appearance: Try to identify animals by their ancestry rather than their traits. It’ll change the way you look at every living thing, from the pigeon in the park to the tuna in the can.
The world is a lot more chaotic than we like to admit. Sometimes, the most basic words we use to describe it are the ones that are the most broken. Fish are the perfect example. They are a ghost in the machine of biology—a category that exists in our minds but vanishes the moment we look at the DNA.