Random Fish Generator

What the fish generator does

You want a fish. For a tank you are planning, a drawing, a story, a quiz, a kid who is deep in a fish phase, or because you saw Goblin Shark on the list and needed to know.

This gives you one. Pull a batch and you will get a spread from the goldfish on a shelf to the hammerhead to things you have never heard of.

What follows is a genuinely strange fact that most people never learn. According to modern biology, the word fish does not describe a real group of animals at all. It is one of the most useful words we have and it points at nothing coherent, and that is not a trick of language, it is a fact about the tree of life.

How to use it

1. Choose how many you want. One for a subject, a handful to browse.
2. Press Generate. A fresh set every time.
3. Press Copy to take the set with you.

A biologist once proved there is no such thing as a fish

Stephen Jay Gould, one of the most respected evolutionary biologists of the last century, wrote a sentence that sounds like a joke and is not. There is, he said, surely no such thing as a fish.

He meant it precisely, and once you see why, you cannot unsee it.

Start with what a proper biological group is. Modern classification only recognises groups that contain an ancestor and every one of its descendants. Everything descended from the first bird is birds. Everything descended from the first mammal is mammals. These are real branches you can cut cleanly off the tree, and each one holds together because everything inside it shares one origin.

Now try to draw the branch that contains all the fish.

You cannot, and here is the exact reason. Some fish are more closely related to you than they are to other fish. A lungfish shares a more recent ancestor with a cow, with a lizard, with you, than it does with a salmon. So if you draw a branch big enough to hold both the lungfish and the salmon, that branch has to include the ancestor they share, and every descendant of that ancestor comes with it. Which means the branch that holds all the fish also holds every amphibian, every reptile, every bird and every mammal. It holds you.

So there is no branch that contains salmon and lungfish but leaves out cows. To make fish a real group, you would have to call yourself a fish too.

That is what Gould meant. There are about thirty thousand animals with fins and scales that live in water, and they do not form a group. They are scattered across the base of the vertebrate tree, and the things we call land animals are just one twig that crawled out and kept going. A fish is not a kind of animal. It is everything on the tree that stayed in the water and never became us, which is a description of a lifestyle, not a lineage.

The word is still enormously useful. Your fishmonger is not confused. But it is a word like tree, or weed, or bug, that groups things by how they look and how they live rather than by what they actually are, and biology quietly does not recognise it.

Why a lungfish is your cousin and a salmon is not

That claim above is worth slowing down on, because it is the strangest true thing on this page.

Somewhere around 400 million years ago, one group of fish had lungs, or something like them, alongside gills. Most fish today do not. The lungfish still does, which is where the name comes from, and it can gulp air at the surface and survive its pond drying out, buried in mud, breathing, for months.

That air-breathing lineage is the one that produced land animals. A fish with lungs crawled far enough out of the water, over an enormous span of time, to become amphibians, then everything that followed. So the lungfish is not just similar to our ancestors. It sits on our side of the family, on the branch that grew lungs and eventually grew us.

The salmon never grew lungs. It stayed on the other branch, the ray-finned fish, which is a completely different and much larger part of the tree.

So run the family tree honestly and you get a sentence that sounds insane and is correct. A lungfish is more closely related to a human being than it is to a salmon. You and the lungfish are both on the lung-growing branch. The salmon is on the other one. The fact that the lungfish looks like a fish and you do not is irrelevant to the genealogy, because looking like a fish is the ancestral, default condition, and looking like you is the weird recent exception.

The coelacanth, that famous living fossil hauled up in 1938 after being thought extinct for tens of millions of years, is on the same lung-side branch as the lungfish, which is exactly why its discovery mattered so much. It was a living look at the kind of animal we came from.

So on this list, quietly, are a couple of your distant cousins, sitting in the same word as several thousand animals you are far less related to. The word flattens all of that. The tree does not.

The shark on this list is older than trees

There are a lot of sharks on this list, and sharks deserve a moment because of what they are made of and how long they have been at it.

First, the age. Sharks are older than trees. That is not a figure of speech. Sharks in essentially their modern form have been swimming for well over 400 million years, and the first true trees appeared later. Sharks predate Saturn's rings, predate most of the continents being where they are, and have survived several of the mass extinctions that wiped out nearly everything else, including the one that took the dinosaurs. A shark is one of evolution's oldest and least improvable designs.

Second, what they are made of. A shark has no bones. Its entire skeleton is cartilage, the same flexible stuff as the tip of your nose and the top of your ear, which is lighter than bone and never fully hardens. That is why shark fossils are mostly just teeth. The teeth mineralise and last, but the cartilage skeleton usually rots away, so a hundred million years of sharks is known largely from drawers of loose teeth, and a shark grows and sheds thousands of those in a lifetime, replacing them on a conveyor from behind.

They belong to a small, ancient side of the fish world, the cartilaginous fish, with the rays and one other oddity on this list, the ratfish, and that whole group split off very early from the bony fish that make up almost everything else in the sea and every animal on land, you included. So a shark and a salmon are not close relatives having a slight disagreement about diet. They are separated by one of the deepest splits in the whole vertebrate story, cartilage on one side and bone on the other, and that split is far older than the split between you and the salmon.

The list treats them all as fish, in one bucket. In terms of the actual tree, a shark is about as distantly related to a cod as a fin-and-scale animal can be.

Where these come from

The fish here come from a list our team put together and checked by hand. Press the button and one is drawn out of it at random.

That is the whole thing, and it runs on your own device.

The tank ones and the ocean ones

If you came here to plan an aquarium rather than to think about phylogenetics, this list is quietly split down the middle and it is worth knowing which half you are pulling from.

Some of these are ordinary aquarium fish, animals bred for tanks for generations, hardy and small and used to captivity. Guppies, mollies, tetras, danios, most of the barbs and many of the cichlids. If you are new, this is your half. They forgive mistakes, they breed readily, and a beginner can keep them alive while learning.

Some are wild ocean animals that have no business in a living room. A Great White is on this list and cannot be kept in an aquarium at all, full stop, because they do not survive captivity and every serious attempt has ended with a dead shark. A tuna is a warm-blooded open-ocean athlete that needs to swim kilometres a day. A whale shark is the size of a bus.

And a dangerous middle group looks like the first half and behaves like the second. A common pleco is sold as a cute algae-eater the size of a finger and grows to nearly half a metre. A red-tailed catfish stays small and adorable for exactly as long as it takes you to get attached, then reaches well over a metre and needs a tank the size of a room. These are the fish that end up dumped in rivers, where the warm-water ones sometimes survive and become an invasive disaster, which is its own quiet tragedy that starts on a shop shelf.

So a random pull is a mix of pets, impossibilities and traps. If it is for a real tank, the rule is boring and correct. Look up the adult size and the care needs of the specific fish before you buy anything, because half the animals on this list outgrow, outlive or outfight the tank a beginner would put them in.

Questions people ask

Is it fish or fishes?

Both, and the difference is genuinely useful once you know it. Fish is the plural when you are talking about a number of them, even a lot, of the same or unspecified kind. Fishes is the plural biologists use when they mean several different species. So a net can hold a thousand fish, but coral reefs are home to thousands of fishes. Given the section above, fishes is almost the more honest word, because it quietly admits they are not all one thing.

Do fish drink water?

Some do and some must not, and it depends which side of a salt problem they live on, which is a nice bit of hidden physics. Saltwater fish are constantly losing water to the sea through their skin, so they drink constantly to keep up. Freshwater fish have the opposite problem, water floods into them, so they barely drink at all and instead pee almost constantly to bail it out. It is one reason you cannot move most fish between fresh and salt water. Their whole plumbing is tuned to one side.

Do fish sleep?

Mostly yes, though not like you. Many fish drop into a rest state, less active, slower, lower metabolism, sometimes wedged in a crevice or hovering in place, but they usually cannot close their eyes because most of them have no eyelids. Some sharks have to keep swimming to breathe and seem to rest parts of the brain while still moving. So the goldfish is probably sleeping when it goes still and dull at night, with its eyes open, which is a slightly unsettling thing to picture.

Do goldfish really have a three-second memory?

No, and it is one of the most thoroughly disproven myths about any animal. Goldfish have been trained to push levers, run mazes, tell colours and shapes apart, and remember the routine for months. They learn feeding times and recognise the person who feeds them. The three-second line is pure folklore, possibly invented to make people feel less bad about a tiny bowl, and the truth is that the tiny bowl is a worse idea precisely because the fish is bored in it and remembers being bored.

I want to start a tank. Which of these should I pick?

Start from the beginner half described above and ignore everything glamorous. A small group of guppies, platies, danios or tetras will teach you how to keep water healthy, which is the actual skill, with animals that forgive the mistakes you are certain to make. Avoid anything on this list that is a shark, a large catfish or a big cichlid until you know what you are doing, and always check the adult size before you buy, because that is the number that turns a cute purchase into a crisis six months later.

References

1. Gould, S. J. (1981). What, if anything, is a zebra? Natural History, 90. Reprinted in Hen's Teeth and Horse's Toes (1983). New York: W. W. Norton. https://archive.org/details/hensteethhorseto0000goul
2. University of California Museum of Paleontology. Understanding Evolution: What is a fish? https://evolution.berkeley.edu/fisheye-view-tree-of-life/what-is-a-fish/
3. Amemiya, C. T. et al. (2013). The African coelacanth genome provides insights into tetrapod evolution. Nature, 496(7445), 311 to 316. https://doi.org/10.1038/nature12027