There is no such thing as a fish
Cladists conclude that biologically there is no such thing as a fish.
For centuries we have seen a great deal of misunderstanding and controversy regarding a natural group Cetacea (whales and dolphins), that concerns its place in the natural classification.
These animals were always misclassified, even though, dozens of naturalists, since Aristotle, have studied and dissected them. This is why this group proves an interesting case study for intellectual inertia in the history of science.
So how come so many scientists misplaced this group in the natural classification, even though, they themselves had gathered data to support the relationship these animals shared with what we now know as mammals?
Firstly, let me summarise the definition of fish and what falls under this definition:
“The term fish most precisely describes any non-tetrapod craniate that has gills and whose limbs, if any, are in the shape of fins.”
This is the term taken from Wikipedia, which goes on to say that fish, unlike birds or mammals, are not a single clade. They are a paraphyletic collection of taxa, and as paraphyletic groups are no longer recognised in systematic biology, the term “fish” as a biological group must be avoided.
Examples of aquatic animals identified as fish centuries ago are shellfish, cuttlefish, starfish, crayfish and jellyfish.
However, a few others included seals, whales, amphibians, crocodiles. Animals such as whales and dolphins are now categorised as mammals and for a good reason.
A little about Aristotle
Aristotle is the first natural historian from who we have any extensive work. His work in zoology, although not without errors, was the grandest biological synthesis of the time, remaining the ultimate authority for many centuries after his death.
Distinguishing between homology and analogy, he was able to recognise Cetaceans as a natural group with many similarities with other mammals.
He wrote that:
“all creatures that have a blow-hole respire and inspire, for they are provided with lungs. The dolphin has been seen asleep with its nose above water as he snores. All animals have breasts that are internally or externally viviparous, as for instance all animals that have hair, as man and the horse; and the cetaceans, as the dolphin, porpoise and the whale -for these animals have breasts and are supplied with milk”
Thus, he was the first person to separate whales and dolphins from fish.
So where has the conclusion that there is no such thing as a fish come from?
Starting from the beginning, a clade is a term for all of and only the modern species descended via evolution from a specific common ancestor.
Since many studies suggest that all life started from the formation of Oceans, which would be the Abiogenesis (origin of life), the first living creatures could be what we now call “fish”. From there life evolved and moved to land over centuries.
There is so much diversity among what we call ‘fish’, that they spread far beyond the subgroups of Fish, reptiles, birds, amphibians and mammals.
If you try and categorise each animal into only those 5 main subgroups, you will find how occasionally there will be an animal that cannot be categorised correctly. The diversity is too great in this case.
So for cladists: “either there is no fish at all, or we are fish”.
“Wait, what?” — You.
So yes, although ‘fish’ is biologically a word that’s rather controversial between scientists and cladists, I think you and I will find that we will continue to use the term ‘fish’ to describe most aquatic animals.
Simply because, to us, it makes no difference and it’s what we're used to.
