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It is inspired by this furry trout.

I mean fish (not necessary to be a trout), not aquatic mammals, and real fur, not from fungi or mold (or looks like fur because of the dead cell, like Mirapinna esau).

What kind of organs are needed, so a fish can have/grow fur (I assumed the fur probably is waterproof, but correct me if such thing will hinder the fish) either combine with/on the fish scale? Can this hinder their hydrodynamic or gills to breathe?

Since it grows hair, I assume the fish probably live in a cold climate, but I am also curious if this kind of fish can live in other climates or not.

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    $\begingroup$ both hair and feathers are basically just modified scales. $\endgroup$ – Kilisi Sep 23 at 10:10
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    $\begingroup$ Insulation used for mammals like whales or otters wouldn't work for fish, because the idea behind it is to keep the cold and the water out. Through it's gills, a fish's blood is always in contact with water (and therefore cold). Not saying your hairy fish can't be hairy, just that it isn't hairy to stay warm. $\endgroup$ – aherocalledFrog Sep 23 at 21:09
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    $\begingroup$ Hair causes drag, so evolutionary pressure pretty much rules out hair on cold-blooded animals. OTOH a very deep-sea fish might grow hairlike extensions to use as vibration or current flow sensors. $\endgroup$ – Carl Witthoft Sep 24 at 12:19
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    $\begingroup$ Could you specify exactly what you mean by "fish"? Many things in the sea that you'd naturally class as "fish" and that look similar; have very little genetically in common with each other. See here for more: "Fish are a paraphyletic group: that is, any clade containing all fish also contains the tetrapods, which are not fish" $\endgroup$ – Bilkokuya Sep 25 at 10:09
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    $\begingroup$ There's no reason a fish can't have hair. You would have to modify the laws of fluid dynamics to make hairy fish survivable. $\endgroup$ – Selemat Dogon IV Sep 25 at 11:44
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Hair/fur is basically just modified scales, so it doesn't take any new organs; just modified expression of existing ones.

The bigger problem is that hair seems to be a bad idea for aquatic animals. While some semi-aquatic mammals still have hair (e.g. otters), the trend among the most well adapted aquatic mammals is to lose their fur and become smooth skinned with a subcutaneous fat layer. This has occurred in at least three separately evolved clades - whales/dolphins, walruses, and manatee - and so shows convergent evolution. Where convergent evolution occurs it is good evidence that a feature is beneficial. So it seems likely that being hairless is beneficial for an aquatic organism.

I'd also note that in semi-aquatic mammals which use fur as an insulator, the insulation effect is primarily produced by air trapped between the hairs and so it can only really function in an organism that spends time out of the water in order to fill the gaps between the hairs with air.

I'd suggest therefore that while in principle hairy fish could be produced with relatively little change, in reality they would never be favoured by evolution, and so are unlikely to ever occur in nature.

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    $\begingroup$ Swimmers and cyclist (humans) make a large effort to go the other direction - loose all hair possible to gain that extra bit of slimness... So we see it from both sides - natural selection and explicit manual effort for creatures that need to move fast. May help to design hairy slow near-surface fish. $\endgroup$ – Alexei Levenkov Sep 23 at 22:09
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    $\begingroup$ Fiction can do anything it wants, but consider how to keep it plausible: Hairy fishes will be not hydrodynamic, so, not fast and agile like most fishes. A real-life comparison can be made to Betta splendens fishes (Siamese fighting fish) or goldfish, both of which have been bred by people to have unwieldy, impractical showy, flowing long fins. I can't think of any wild fishes which have developed a similar trait, but if they were to do so, it would have to be the result of selection pressure and adaptation. Without a real survival and reproduction aid, hairy fishes would die fast in the wild. $\endgroup$ – Beanluc Sep 23 at 22:10
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    $\begingroup$ Plenty of fish are not hydrodynamicaly efficient, but that is usually for good reasons, like being a bottom dwelling ambush predator or because other pressures outweighed speed like in deep-sea anglers. I can see a hairy fish if your local seaweed is hair-like. Or well, you coudn't see said hairy fish... $\endgroup$ – Borgh Sep 24 at 7:20
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    $\begingroup$ @Beanluc Sexual selection can do some wild things, but it'd require the fish in question to have a different reproductive strategy than most fish do. $\endgroup$ – nick012000 Sep 24 at 12:26
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    $\begingroup$ About convergent evolution: I don't know if it has been already refuted, but some antropologists thought that humans evolved in a similar way: we are decent swimmers and we have subcutaneous fat, while the rest of apes don't. Maybe our ancestors lived fot long time in beaches, eating crabs and other seafood $\endgroup$ – Pablo Lozano Sep 25 at 10:01
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First "fur" appeared long before mammals - it was the amphibian mammals' ancestors. Yes, "hairy tritons", if you like.

To cut a long story short: first it was scales all along. Then mammal's ancestors "turned" scales into hair and later dinosaurs "turned" scales into feathers (mammals are older than dinosaurs). First hairs were tactile organs - vibrissa.

So it is plausible that some fish would develop hairs in the form of vibrissa. But since hairs are not good for swimming, it is not plausible to have all-hairy fish (only with some "genetics games")

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    $\begingroup$ Genes for hair are the only issue, hair came from scales. $\endgroup$ – Kilisi Sep 23 at 10:21
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    $\begingroup$ Catfish have whiskers, though people wouldn't call that hair, so hair for vibrissa is probably unlikely. $\endgroup$ – 458 Sep 23 at 15:41
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They can use it to capture food

While a fish could theoretically have hair (hair is just modified scales), hair on aquatic animals is usually a disadvantage, because it slows the creature down. Even aquatic mammals lose their hair as they adapt for a fully-aquatic existence.

But there are a few exceptions to this: The orangutan crab and the yeti crab are two unrelated, furry crustaceans that have evolved hair for similar reasons. The yeti crab cultures bacterial colonies in its fur, which it "harvests" for food. The orangutan crab is a filter-feeder that captures bits of food particles with its hair, as well as sand and bits of rocks which help it camouflage.

It is not impossible for a fish to evolve hair for similar reasons. However, it bears mentioning that both crabs have a relatively low-energy lifestyle compared to most fish. They don't need to eat very much and since they aren't moving quickly anyway the hair doesn't slow them down much. So, if you have a hairy fish it probably isn't going to be built for speed.

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    $\begingroup$ Maybe it could culture algae on its hair? Perhaps the hair shelters the algae enough from the water currents that it grows abundantly, and the fish swims near the surface to allow the algae to get ample sunlight. The fish's migratory patterns also grant the algal colonies greater motility and allow them to spread further. The fish would have to be a social species, as they likely wouldn't be able to eat the algae on their own hair, and would thus feed off one another's algae in a school. $\endgroup$ – Doktor J Sep 24 at 15:32
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It sure won't be a trout

The obvious con to hair in water is more drag, so more energy required, so more food needed. That is an evolutionary disadvantage, because more food means more going out there to find it, and means the same supply feeds less individuals. One outcome is the hairy fishes are gradually phased out of evolution for lack of surviving, and that's no fun. Another outcome is they evolve in a different direction.

To offset the disadvantage, your fish need to move as little as possible. That makes it an ambush predator, capable to pounce at incredible speed unto unsuspecting prey, but overall below par as a swimmer. It doesn't need to swim fast and far anyways, and it won't try to eat anything that can escape the initial attack. This is already a thing, so it's not that outlandish to imagine. If you want to increase the weirdness factor, you could imagine a fish that hunts with its tongue like a chameleon. Although I don't know how well that'd work underwater, that would be pretty weird.

It would use its hair to camouflage in the environment, to surprise smaller fry and to hide from bigger fishes who would outpace it. It would of course help to live in a hairy environment, for instance if you had hairy coral or hairy seaweed, or at least if those pass as hairy in the eyes of a fish. It would possibly develop a slower metabolism to decrease the energy input and the amount of time it needs to feed, though that means doubling down on the "moving as little as possible" lifestyle. They might prefer warm waters, that much less energy to spend in heating.

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As stated in other answers, hair can't be used to keep a fish warm and so a hairy fish would have to use the hair as camouflage to retain this feature. There are better ways of doing this than using hair, for example, the "Hairy Frogfish" is covered in dermal spinules to resemble the plant-life in coral reefs and ambush prey. The spinules produce less drag than hair enabling the Frogfish to lunge quickly.

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When does an otter become a fish?

It doesn't. It's a mammal, but you know what I mean.

If you want fur you need a mammal, and a mammal somewhere on the evolutionary line between an otter and a dolphin might still have fur. Otters range from the Arctic to the tropics, so there isn't any problem there. The biggest problem is that they still give birth on land.

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    $\begingroup$ I think a better example would be a platypus. Platypuses (-pi?) lay eggs like fish so are closer to "fish". I get your point that it is all just semantics, though. $\endgroup$ – Captain Man Sep 24 at 15:15
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They're not done evolving their hair away

After all, how often are normally-hairy land mammals genetically hairless?

In an environment where life had a stronger hold on the land than in the water, it's possible that hairy land animals are evolving into aquatic species (such as the whale examples from our world), but have not yet evolved to be hairless.

This could be due to a disaster/population die-off in the water, or perhaps even due to the fish on this planet being a relatively new adaptation for life that started on land.

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You don't need different organs; as has been stated, it is quite possible for modified fish scales to produce hair if needed. What you need is evolutionary pressure, a niche so that hair makes sense despite the obvious disadvantages.

Hair might be used as camouflage, or to cultivate algae, as mentioned.

Hair might also be used to trap gas. Your fish might live in oxygen-poor waters, and surface to capture air in its fur. Oxygen from the mass of tiny air bubbles diffuses into the water, giving the hairy fish a turbo-boost.

Or thick fur might have a protective function, eg against the dangerous toxic spines of a particular threat species.

Or it might simply be the peacock's tail effect: sexual signalling that the individuals can carry around a gigantic, useless mass of hair and still function.

Or -given that it is biologically possible - maybe someone just bred weird furry fish and released them for their own bizarre porpoises. (sorry)

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