What Constitutes A Fish

Let’s be honest—when you picture a fish, you probably see a slimy, scaly thing that swims around going “glub glub.” But defining a fish is like trying to nail jelly to a tree: messy, slippery, and you’ll probably get it wrong. The truth is, “fish” isn’t even a neat biological category—it’s more like a club where the bouncer has terrible eyesight.

The Three Big Lies About Fish

First myth: All fish have scales. Nope. Meet the eel, which is basically a snake that lied on its resume. And the catfish? That whiskered goofball has skin so smooth it feels like a wet balloon. Evolution didn’t get the memo about scales.

Second myth: Fish have fins. Sure, most do, but the seahorse has a teeny, useless fin that’s basically a fashion accessory. And the mudskipper uses its front fins like little arms to walk on land—like a fish that got tired of swimming and started a hiking club.

Third myth: All fish swim. Wrong again. The ocean sunfish is so lazy it just drifts like a cosmic pimple. And the stonefish is a master of sitting still—so still that other fish try to date it, thinking it’s a very ugly rock.

Gills, the Ultimate Fish Superpower

The one thing most fish share is gills. These are like tiny, underwater vacuum cleaners that pull oxygen out of water while the fish just vibes. But even this rule has exceptions: the lungfish has actual lungs and can live for months in mud. It’s the Houdini of the aquatic world.

Most jawless fish, like lampreys, have gills too—but they also have a sucker mouth with teeth. Imagine a fish that looks like a sock puppet with a drinking problem. That’s a lamprey.

Fish That Make You Question Everything

There’s a fish called the blobfish that looks like a grumpy grandpa made of sad gelatin. It lives so deep underwater that it would explode in your hand if you brought it to the surface. Pro tip: don’t hug a blobfish.

Then there’s the parrotfish, which produces coral sand—every day, a parrotfish poops out a cloud of beach. So when you’re lying on a tropical shore, you’re basically relaxing on fish butt. You’re welcome.

And the archerfish hunts by shooting a jet of water at bugs. It’s the only fish that has perfect aim—which means it’s secretly better at darts than your uncle Stan.

Are There Even “Real” Fish?

Scientists now say “fish” is a polyphyletic mess, which is fancy talk for “we have no idea.” Technically, a lobe-finned fish like the coelacanth is closer to you and me than it is to a salmon. Yes, you share a great-great-great-great-grandfish with a coelacanth. Try that fact at a dinner party.

Even cartilaginous fish—sharks, rays, and skates—are weird uncles in this family. Shark skeletons are made of cartilage, like your ear. So a shark is basically a giant, toothy ear that ate too many seals.

The Final Truth About Fishiness

So what qualifies as a fish? Honestly, it’s a vibe. If it lives in water, has gills (mostly), and isn’t a whale (which is a mammal that forgot IMAX tickets were a thing), society calls it a fish.

But remember: jellyfish are not fish. They are floating blobs of “nope.” Neither are crayfish (they’re tiny lobsters with attitude). And starfish are echinoderms, which means they’re space aliens that decided to stay.

So next time someone asks “what constitutes a fish?” just shrug and say, “If it looks like a fish, swims like a fish, and tastes good with lemon, who cares?” Then eat a tuna sandwich. The fish won’t judge you—it’s dead. Probably.