You Have Been Lied To, Fish Don’t Really Exist

What is a fish? This seems like a simple enough question. They are scaled, finned creatures with gills that let them live in water. We look at them in aquariums, we eat them, and sometimes are our pets. A kindergartener could tell you what a fish is. At least that is what most people think, but the term “fish” is one of the biggest lies involving taxonomy.

A little background before we get into the deep end. In taxonomy, there is a thing called a clade. In simple terms, clades are descendants from a common ancestor. Where this interferes with “fish” being an accurate classification is that life on land’s common ancestor came from the water. If it were around today, we would call it a fish.

This would mean if we decided that fish was a true classification, we would have to say that everything that evolved from that common ancestor must also be a fish. This would include humans, birds, turtles, and pretty much every familiar living vertebrate.

For argument’s sake ignore all land-dwelling vertebrates. From that point of view, you could say that everything in the water with fins and gills is the group “fish”. That argument is one often made to hold onto the taxonomic lie. The argument makes a fatal mistake though. It completely underestimates just how diverse that group would be.

That group would be the largest vertebrate group. For example, it would include 33,000 different species. More than all other vertebrates combined. It would put sunfish, hagfish, lungfish, sturgeon, tuna, sharks, and many others closely together. At first glance, you might think well what’s the problem with that? The problem is that most of those species are not even closely related!

One example being tuna and sharks. They both look sort of similar, but both are in different taxonomic classes. Tuna is a part of the class Actinopterygii which is the ray-finned bony fish. The class is characterized by the bony spines that make their fins. Sharks on the other hand are in the class Chondrichthyes. This class is best known for its cartilaginous skeleton. A cartilaginous skeleton means they lack bones but instead have cartilage just like humans’ ears and noses.

This might not seem like a drastic difference, but it means in evolutionary history tuna and sharks are separate from each other by thousands of branching divergences. So much so in fact tuna are closer related to humans than they are to sharks!

If you said fins were the cut-off point then you’d have to deal with the different types of fish fins. Bony fish have fins closer related to the human hand than the fin of a shark. If you said gills make a fish then would the many species of salamanders with gills be a fish? The point is nowhere in the tree of life can you make a group, we call fish that wouldn’t include something such as a mouse, cow, amphibian, or human that you would say isn’t a fish. They just contain such diversity that the blanket term of “fish” becomes scientifically inaccurate.

The idea that fish don’t exist is weird. It seems like a crackpot theory, but it really isn’t. What the idea shows is that nature will not and does not fit into the little groups that we create to help explain the world. There are always things that don’t exactly fit, and that is a good thing. The lesson we should take from the knowledge of fish not being real is that sometimes we must look past what we think we know to see the true beauty of how things relate.