No, it’s not.
Before you ask why, think about why it might not be a fish. Look at it: does it look like a fish? No. Why not? Well, it doesn’t have… fins. And it doesn’t have… gills! And… well look at it! It’s not a fish.
OK, but then: what is a fish?
Yes, there are the fins and the gills, but what else really defines a fish? There are plenty of organisms that live under the sea that we don’t really think of as fish, like jellyfish, or sea cucumbers, or aquatic mammals like dolphins and whales (and did you know that killer whales are dolphins, not whales?). Why aren’t they fish?
Well, basically, you weren’t too far off with the fins and gills. A fish is defined by the Oxford English Dictionary as a limbless cold-blooded vertebrate animal with gills and fins living wholly in water. So yeah, a fish, basically. But what might surprise you to learn is that there’s no such thing as a fish.
Well, of course there are. All those fish you’re thinking of right now? They’re all fish, obviously. But look back at that definition of a fish. A bit vague, isn’t it?
That’s because, scientifically, there isn’t really such a thing as a fish. There’s no class or phylum known as fish. It’s just a general descriptive term to refer to animals with common features, but which may all have evolved separately. Salmon and sharks might not really share anything genetically: they developed similar features due to natural selection. If you’re living underwater, you’re going to survive better if you’ve got fins and gills.
But does that mean you’re wrong if you call a trout or a pollock a fish? No, of course not! They’re obviously fish. Sure, we can trust science in classifying different organisms. But the rest of us, what do we know? We look at all these animals that all look the same, and say, Yeah, those things are all the same, let’s call them fish. That jelly-looking thing? Well, OK… that star thing?? Well, it doesn’t really have gills or fins, but, OK I suppose…
Consensus. That’s how language develops. And even if there are some funny jelly- and star-shaped outliers, we’re all pretty sure what a fish is. And language also develops to fulfil a function. And for most of us, that function was simply to lump all these similar animals together. Distinguishing between different types of aquatic animals has never really been important for most of us.
So no, a jellyfish isn’t a fish. But if you want to think of it as a fish, then why not? But a dolphin!? Forget about it!