Different animals adapt to the climate of their environment (eg thick fur, different coats for summer/winter, etc). But for the most part, fish look fundamentally the same (scales, fin placement, etc) across cold and warm water species whether salt, brackish or fresh water.
In: Biology
Warm water fish have sweat glands
Cold water fish can boil saltwater in their stomachs to keep warm
You can only live within a temperature range, too. You’ll die in 50⁰ water and in 100⁰ water.
Also, water is so much denser than air, it transfers heat so much more. And species that live in only cold or only warm water might have minmaxxed and adapted tightly to a particular temperature.
Edit: I think the adaptations to temperature aren’t visible because there’s no way to fight the temperature of the water. Aquatic mammals have tons of insulation, but fish are almost all cold-blooded and just accept the temperature around them. Different adaptations would be internal chemistry stuff to operate at a different rpm
a cold water fish like trout excel in colder temperatures w/ efficient oxygen uptake & streamlined bodies while warm water fish, such as tilapia, are adapted to warmer temperatures with broader bodies & unique metabolic processes, helping them thrive in their environments.
All animals have a different temperature range. It’s mostly just the temperature best suited to their environment.
Unfortunately, the short answer is “a lot of different ways”. Fish are wildly diverse and sometimes distantly related, and that means that lots of them have evolved different solutions to their environmental challenges.
Fish in cold water can have slower metabolisms, more fat, proteins that function like antifreeze, or even different blood chemistry to allow them to survive cold temperatures. These generally cause those fish to overheat in warmer water, or even just don’t function as well when it’s warmer. Fish in warmer water generally lack these necessary adaptations to function in colder environments, so they need to stay in warmer ones. Some species, like tilapia, face wide ranges of temperature and so are able to survive in both very cold and very warm water with ease.
“Fish” is not really a biological category so much as one of convenience. So the range between fish is like the range between land animals; it’s like asking why are polar bears and giraffes so different in their climate requirements when they both have four legs and fur and lungs.
The adaptations you see in mammals (thick fur, etc) happen precisely because these animals mostly _don’t_ have adaptations to live at different temperatures. Mammals have to keep their core body temperature within a pretty narrow range, so if they live in cold climates they need lots of insulation to make this possible, and if they live in warm climates they don’t need heavy insulation.
Fish, on the other hand, aren’t warmblooded (with a few exceptions). Instead of generating heat on the inside and using insulation to try to stay a different temperature from their environment, they are just directly adapted to live at various different temperatures. Some live better at warm temps, others at cold temps.
Fish have adaptations for different water temperatures, but they aren’t the sort of adaptations you can see. Instead, it’s things like having altered proteins, or different cell membrane chemistry, etc. Biochemical differences that make sure the stuff in the cell works well at…well…at whatever temperature the fish lives at. And since their cells are adapted to the right temperature, they don’t need any visible insulation to help them keep their insides warmer.
Reformed fish biologist here
Think about when you’re thawing something frozen – It goes a lot faster in water than it does in air.
As terrestrial animals, we have air around us that provides insulation from the elements, and we have our skin that forms a barrier between the cold and also other things like salt or chemicals. Mammals like us are also warm blooded, so while we also have a very tight thermal range (36-37°C) we can create the temperature we need in whatever environment we happen to be in.
Animals in the water don’t have the ability to control their body temperature because they’re kind of cold blooded (they’re technically alcohol blooded in some climates – yes drunk), and because water provides zero insulation, so their bodies quickly become the same temperature as their surroundings.
Fish have similarly tight tolerance for salt concentration, oxygen concentration (related to temperature but not for everything)
So now let’s pretend that the whole ocean exists as it is today, but with only one species of fish. Like Goldilocks, these fish like it warm, but not too warm. But all of a sudden, some Goldilocks notice that where the water is colder (in another bay perhaps) there’s better food. Lots of fish go over to the new food source, but a lot of them die. Eventually these fish have babies in two different places, and they change just enough over time to adapt to their new environment. They’re now distinct from the original fish – two species, with different tolerances for hot and cold water, driven by food availability.
A good living example of this would be African Chichlid fishes – many species that came from a common ancestor living in the same lake.
If you keep expanding this over the globe, you’ll see more and more specialized niches filled by more and more specialized fishes – each wjth biology that’s quite specific to the temperature, pH, and salinity of the water they’re used to.
Some good comments so far, but also worth bearing in mind that oceans have much less temperature variation than land.
Typically seas vary from -2C to something like 30C (excluding some temporary extreme local temperatures at the upper end).
It’s still plenty big enough that fish need adaptation of course, but not to the same extent as land dwellers would, all else equal.
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