Evolutionary biologist Stephen Jay Gould has observed, after a lifetime of study, concluded that there is fundamentally no such thing as a fish.
""Nothing in biology makes sense except in the light of evolution."
—Theodoseus Dobzhansky.
Look away now, creationists—What he meant is that, from an evolutionary standpoint, there is no genus of "fish", that a shark and (say) a hagfish do not share a universal common ancestor. It's almost as though no one species of fish is connected to other "fish", like humanity has looked at things living in water and just called them all "fish". Sharks…? Fish. Trout…? Fish.
Perch…? Fish. Eels…‽ Fish. Clams…? well no, they are
shellfish and of course Gould did not consider them. The only thing fish have in common is living in water. There are
bony fish,
cartilagenous fish… Presented with this seemingly infinite variety, our mental and verbal map is overwhelmed and we simply dump them into the same set of "things that live in water and have broadly the same shape". But then seahorses…? Also actually fish. This was why Gould came to his conclusion, and why the wonderful
QI "comedy quiz show" alluded to his analysis in the pithy phrase of the title.
This may not be a direct quotation from Gould (I bagged this line from the brilliant QI British TV show and I encourage you to look it up on YouTube!). It's also the name of an excellent podcast from the question-setters of the show. The closest I can come to explaining the latter is that it's an audible equivalent of E2 in that the connections and deep rabbit hole themselves are surprising and fascinating.
So at this point I'm done, other than to encourage you to look up both the podcast and the show (the show is more than "quite interesting", it's often genius-level fascinating, funny, wide-ranging, thought-provoking and addictive. Both the show and the podcast are easily found on YouTube, so go out and look them up; you will either thank or curse me, but you will (I'm confident) be informed and entertained.
For Tem42, especially.
Hazelnut says re No Such Thing As A Fish: So what you're saying is that fish are a conspiracy theory by Big Oceanography, yes?
Yes, it's the Eeluminati!
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