There is no such thing as a fish. You might not believe me right now, but by the end of the episode I’m confident you will understand, and maybe even agree, with my opinion. Let’s learn some biological English vocabulary while trying to answer the question… Do fish actually exist?
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Vertebrate (n) – an animal that has a spine
Cows, frogs, and ostriches are all vertebrates
cold-blooded (adj) – cold-blooded animals can only control their body heat by taking in heat from the outside or by being very active
Snakes and lizards are cold-blooded animals
Limbless (adj) – having no limbs (no arms, legs, or wings)
Snakes are limbless reptiles
Common ancestor (n) – one species which is the ancestor of two or more species later in time
Humans and gorillas share a common ancestor
To evolve (v) – to develop gradually
Dogs evolved from wolves
Descendant (n) – an animal that lives after and is related to another animal that lived in the past
Lemurs are descendants of early primates
To distinguish (v) – to notice or understand the difference between two things
It is difficult to distinguish between these two species
Characteristic (n) – a typical or noticeable quality of someone or something
A squashed face is an unfortunate characteristic of pugs
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Mammals exist – they are the warm-blooded vertebrate animals that usually feed their young with milk and give birth to live babies. We are mammals – so are elephants, mice, dolphins, and tigers.
I’m sure you have seen a bird before – they are warm-blooded vertebrate animals that lay eggs, have feathers and a beak, and often can fly. Ostriches, canaries, pigeons, and eagles – they are definitely birds, and they definitely exist.
How about amphibians? These are cold-blooded vertebrate animals that live fully in water when young, and then undergo a transformation as adults where they live mainly on land and breath through lungs. Frogs, newts, toads, and salamanders – and they all certainly exist.
Mammals exist, birds exist, amphibians exist, and I’m pretty sure other types of animals like reptiles exist… but what about fish? Do fish exist?
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Do Fish Exist?
I’m sure you read the title of today’s episode and thought I must have made a mistake or a typo. “There is no such things as a fish” … that must be wrong, right? Well, according to a certain group of scientists, it is a factual statement. There is actually no such thing as a fish.
The definitions I gave earlier for mammals, amphibians, and birds were just the first result when I searched those words on google, so I’ll do the same for fish. According to the google defintion, which I think comes from the Oxford dictionary, a fish is a “limbless cold-blooded vertebrate animal with gills and fins living wholly in water.” In other words, a cold-blooded animal with bones that lives and breathes under water.
This sounds like a nice defintion of a fish to me. In this category, we could include sharks, tuna, salmon, angler fish, lungfish, and clown fish. They are all cold blooded, they all live under water, and they all have bones. So, they must all be fish – where is the problem.
Well, the problem is that these animals are not actually that closely related. In fact, the lungfish is more closely related to a cow than it is to salmon. You heard me right… the lungfish (which lives underwater and has cold blood) is closer to the warm-blooded land-living cow than to the salmon.
Let me try to explain. We are going to need to have a brief lesson in taxonomy to understand this – I’m not a scientist, and I’m sure there are a few scientists listening right now, so I apologise if I make a mistake!
Taxonomy, in a nutshell, is how we categorize all living things. It is sometimes described as the tree of life! It is based on the idea of a common ancestor – the concept that a wide range of diverse animals today are descended from one living creature that lived millions of years ago. All mammals, for example, are thought to have one common ancestor.
This tree of life is separated into different categories: Kingdom, phylum, class, order, family, genus, and species. There are a lot of complicated words here, so I’ll try to include a picture or image on the blog to help you understand. But let’s try to understand it with our own species – the human.
Our kingdom in animalia (like every single animal on the planet), our class is mammalia (like other mammals), our order is primates (along with monkeys, apes, and lemurs), and our species is Homo Sapien. The reason these classifications are accepted by scientist is that they indicate where animals evolved from.
Fish, however, do not all have the same common ancestors. There is an incredible amount of diversity in our oceans, lakes, and rivers. And animals have been living in the oceans for significantly longer than life has existed outside of the ocean.
One amazing fact to illustrate this point is that sharks are older than trees. The earliest shark fossils we have found date from around 450 million years ago. The oldest evidence of trees dates back to around 360 million years ago. Sharks, or the descendants of modern sharks, are probably 90 million years older than trees and 200 million years older than dinosaurs. These numbers are so large it is hard for us to understand.
Life has existed in the oceans for an incredibly long time. And during this time, there have been various different evolutions and splits in the tree of life! Mammals, birds, reptiles – all of these evolved from something that we would call a fish. All life started in the water. We are often able to distinguish between these types of animals because of their appearance and physical characterisitcs, but under the ocean there is just as much diversity.
Like I mentioned earlier, the example often highlighted is the lungfish. Lungfish are a type of animal that live in freshwater, and if you look at them, they appear to fit the characteristics of fish. However, lungfish can breathe air and have lungs. The lungfish are quite an important “fish” for us – they are the closest living relative to all amphibians, reptiles, birds, and mammals. We, humans, share a common ancestor with lungfish around 390 million years ago.
This is why we say that lungfish are closer to cows than salmon. Both cows and lungfish evolved from an animal which had lungs. Salmon never had lungs. Therefore, lungfish and cows have a common ancestor closer than lungfish and salmon do.
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Does it matter?
Classifying and categorising animals is something that humans love to do. When I said that “there is no such thing as a fish,” I wasn’t suggesting that sharks, salmon, and lungfish don’t exist. Instead, I used the phrase to highlight how limited, perhaps even pointless, the term fish is. If we consider lungfish and salmon to both be fish, then from an evolutionary perspective humans, frogs, ostriches, and lizards are also fish… and of course we are not fish!
Actually, the term fish is only useless to scientists. It is probably quite useful for fisherman and fishmongers. They probably care more about the appearance and taste of the animal, than where it evolved from or which other animals it is related to.
But organising animals based on appearance is not necessarily the best system – in fact, it is a terrible system. It is not based on science or evidence, but on human judgement. Just because a tuna, a shark, a clownfish, and a lungfish resemble each other, doesn’t mean they are the same thing!
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One of the reasons I wanted to make this episode is to demonstrate the limitations of language. Our word fish, which I’m pretty sure has an equivalent in every language, seems like a perfectly acceptable word. But it completely ignores that incredible diversity of life in the ocean.
The category fish, from an evolutionary perspective, is useless. Not all fish are closely related – millions of years of evolution has created different branches of the “tree of life.” Actually, “tree of life” is a strange choice here considering that fish are millions of years older than trees.
I’m not suggesting you stop using the word fish – it is pretty useful in restaurants and the supermarket. But I do suggest you think twice about the categories and definitions we use in our daily lives!
What do you think? Do we need some new words to describe fish? Are you surprised that a lungfish is more related to you than it is to a salmon? What is an unbelievable fact that you know?
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