Looking at the Wikipedia article for fish, it looks like a reasonable definition would be:

* Everything in the subphylum vertebrata (i.e. vertibrates)

* Except tetrapoda (tetrapods: amphibians, reptiles, mammals and the like).

It's not perfect because tetrapoda does fit within vertebrata in a biological / genetic sense (as a sibling comment put it: fish is not a monophyletic group). But it's a precise enough definition that I don't think we need to claim that we're all fish or that there's no such thing as a fish (as the QI elves would say).

▲

dillydogg 6 days ago | parent | next [-]

But what about our precious friends the coelacanths?

Edit: foolish me coelacanths are not tetrapods

But a better question may have been regarding the lungfishes

▲

quietbritishjim 6 days ago | parent | next [-]

First of all: I think it's ok if the definition of fish is a bit blurry around the edges.

But actually I think coelacanths are quite a fun example. I hadn't heard of these before, thank you!

Yes, they're not tetrapods, but (I've just discovered) they're not even vertebrates (no spine). According to my definition, they shouldn't be fish, but they do seem quite fish like.

They are chordates (they have a spinal cord, just no backbone for it), so I could expand my definition to any chordate that isn't a tetrapod. But there are some rather non-fishy chordates [1] so that doesn't work either.

[1] https://en.m.wikipedia.org/wiki/Tunicate

(For those that don't know, the top level subclassification of animals is phylum. There are a lot of phyla but a common ones are chordates (all vertebrates plus a few odd animals like discussed above), arthropods (insects and insect-like things like spiders and crabs), and molluscs (like slugs and clams). When I was at school, animals were just vertebrates or invertebrates but the reality is more interesting. I went down that rabbit hole when I found out that, weirdly, octopuses are molluscs.)

	▲	IAmBroom 6 days ago \| parent [-]
		> They are chordates (they have a spinal cord, just no backbone for it) None of the cartiligenous fish have backbones. Nor any other bones. Coelacanths have backbone-functioning cartilige.

▲

goodmatt 6 days ago | parent | prev [-]

[dead]

▲

daedrdev 6 days ago | parent | prev [-]

Mammals include orcas and whales

▲

SideburnsOfDoom 6 days ago | parent [-]

And orcas and whales are not fish.

▲

hinkley 6 days ago | parent | next [-]

Orcas and whales are flip floppers (no pun intended).

We left the water and they went back. (I have a theory that given enough time, Labrador retrievers would form a new branch of marine mammals with similar morphology to seals).

▲

shawn_w 6 days ago | parent | prev | next [-]

Whales are fish that spout and have horizontal tail fins. (Currently re-reading Moby-Dick and that's the definition Ishmael comes up with.)

▲

quietbritishjim 6 days ago | parent [-]

I think it's ok for there to be two meanings of "fish": a biologically formal (but not perfect) definition like I gave in my comment above, and a more informal meaning of "animal thing in the sea" that includes whales and even "starfish". It's very common for words to face more than one meaning. But that doesn't mean you can invalidate one by referring to the other.

	▲	SideburnsOfDoom 3 days ago \| parent [-]
		I have multiple meanings of "fish", and that's ok. But none of them include marine mammals such as seals, dolphins and whales. And none of them include penguins, even though penguins flying through the water on their little wings are impressively graceful and fast. None of these animals are fish to me under any meaning of the word.

▲

emmelaich 6 days ago | parent | prev [-]

But literarily (not literally) they can be.