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I'm designing alien creatures for illustrations, and I'm just sketching some that might live in areas with a lot of geothermal activity. So basically, animals that live around geysers and hot steam. I know that land animals have some pretty amazing coping mechanisms when it comes to heat, but I couldn't really think of any when it comes to water creatures.

I was wondering, how I could (more or less) justify fish living in almost boiling hot water? Does anyone have an idea how fish or fish-like creatures could live in such hot water, given that the atmosphere contains a lot more O2 and there's still some in the water for the fish to breathe? I'm not a scientist but an artist, so please forgive me if I overlook something important here. :>

The big problem is heat rejection. Cellular biology has similar constraints as heat engines, so for energetic creatures there needs to be a way to dispose of waste heat. At the temperatures of boiling water, proteins actually start changing form or denaturing, which is why a raw egg looks and behaves differently from a hard boiled egg.

For a fish, this might be dealt with by having a small surface to volume area, so the fish does not absorb too much heat, and having some sort of organ or structure which acts like a heat sink. When the fish approaches the maximum heat load it swims away from the volcanic vent and radiates heat into the cooler waters. Such a fish might look more like a beach ball or rugby ball than what we think if as a fish.

This could provide some interesting adaptations; the fish might have extensible structures like fins which act as radiators in the cooler water, shedding heat faster. Predators might lurk near the thermoclines, searching for mobile hot spots which allow them to zero in on prey. Creatures might even evolve like giant squids, with tentacles or similar structures to "fish" for prey or edible materials from the cooler areas outside the vents.

So in terms of illustration and design, you need to think of how the environment will affect the creature, and what sorts of adaptations will be needed to deal with them. A plump, football or rugby ball shape provides minimal surface to volume, limiting heat absorption. Large size actually helps since the interior volume increases as the cube of the size, while the exterior area only increases as the square of the size. The large interior mass could act as the heat sink. Once it swims out of the hot zone, it spreads large fins to radiate the heat (or maybe elaborate structures like a lionfish), but will have to contend with potential predators evolved to seek out hot spots in the water. Spines, poison, powerful jaws or even sheer size are all common defense mechanisms in the wild. Poisonous creatures often have very bright and distinctive colour schemes to announce to potential predators that they are poisonous.

Square Cube law

Large size vs predator

Lionfish. Large extensible fins, poison spines and bright colours all in one

A giant squid. This creature could lurk around the edges of the hot water vent and reach in to grab prey

So think about the environment, and have some fun with it.

There are a lot of such organisms right here on Earth. Undersea volcanic activity produces hydro-thermal vents. These are places where water is heated and then rises out of the sea floor. Typically this also brings with it a wide variety of chemicals that are ordinarily not available in such quantity in the ocean water.

And there are organisms that find these chemicals tasty, and the temperature in the vented water quite comfortable. There is a huge variety of life, and a surprising total biomass. Though mostly it's not fish as such. For example there are tube worms that do a kind of filter feeding.

This picture is from here on Wikipedia.

Mostly the life forms in these vents don't wander very far from the vent. They are adapted to the temperatures and feed on the chemicals that come out of them. If an entire ocean were as hot as these vents then likely there would be life forms more adapted to travelling around.

Have you looked at the creatures that live on and around the black smokers on the mid-Atlantic Ridge? Many of the creatures there thrive in water that would flash to steam if it was at sea-level. They freeze to death if they get too far away from the 900°C water pouring at of the volcanic rocks. Most of these creatures are crustaceans though I'm not sure about the presence of bony fish in these ecosystems.

Might I suggest you consider armour-plating. The armor plating would enable the fish to swim into the super-heated water to feed off the bacterial matting/crustaceans/tube-worms then move-out to the slightly colder water to cool-down again. The plates themselves would be the equivalent of Ganoid Scales - a bony substance that continues to grow in plates or scales throughout the lifetime of the creatures.

The placoderms in the link above were primitive by the standards of modern fish, but then again the creatures living next to Geo-thermal vents may well have been there since the times of the origin of life.

^{Attribution: Zhi-Kun Gai et al. / Dimitris Siskopoulos 2019 Sci-news.com}

The above picture, a jaw-less primitive armored fish Rhegmaspis xiphoidea was recently discovered (as a fossil) in China.

They came in all sorts of forms, big and small, jaw-less and jawed.

^{Attribution: First found, deviant art Oct 2013, artist "Hontor".}

There are still examples today, but the adaptation would be for other reasons than protection from heat:

^{Attribution: SWNS.com, Daily Telegraph 18 July 2019.}

For example the above picture of an armored sucker-mouth North-American catfish, found in a waterway in the UK.

Or another example, the "Robust armored-gurnard", found in the waters off Sumatra:

^{Attribution: duniabaru.com 2015.}