system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do most deep sea creatures look like they're hell-spawns?
gpt
They evolved to deal with conditions that are completely alien to us. As a result, they share very few traits that are recognisable to us.

The animals that we find the most appealing, are usually the ones most similar to what we find cute. Usually, other mammals that are warm, soft, have eyes and expressions that we can imagine we see emotions in.

Deep-sea fish are not that. For instance, there's extremely little light down there. Not enough to really see by as we do. Deep-sea fish tend to have enormous bulbous eyes. Those eyes don't really let them see things but those huge eyes might just be enough to see other fish or animals as they pass overhead and block what extremely little light filters down from the surface.

Noticing that might make the difference between going hungry and catching prey, or spotting a predator and avoiding becoming prey yourself.

The lack of light also means the colour works very differently down there. There's not much point in being colourful if nobody can see you. So many deep-sea fish just have that pallid corpselike absence of colour.

Others are transparent to avoid casting a shadow against the faint glow of light from above. Some fish are bright red as a form of camouflage. Low energy frequencies of light are filtered out first as you descend underwater. That's why things quickly start to look blue-greenish.

Red is the lowest energy frequency of light. So red part of the light spectrum is the most absent in the deep. Without red light to reflect, the colour red is basically invisible. Most deep-sea fish have completely lost the ability to see red.

Speaking of prey. Without sunlight, nothing grows down there. Small animals can survive on marine snow. Basically tiny flakes of organic detritus that constantly come down from above. Slightly larger animals can eat the filter feeders that survive on marine snow. But larger animals like deep-sea fish only have one food source available. Each other.

The deep-sea is essentially a freezing cold, pitch-black desert. Everything is scarce down there, including opportunities to eat. So most deep-sea fish have maws full of long needle-like teeth. Essentially they can't afford to let prey escape. So when they manage to grab prey, those maws full of needles make sure that prey can't slip their grasp.

Deep-sea fish have many other adaptations to deal with the scarcity of food down there. Many fish have enormous mouths so they don't have to pass up on larger prey. Some have incredibly stretchy mouths and stomachs to the point where they can swallow prey that is bigger than they are.

Others develop some truly alien adaptations like little bioluminescent lights to attract prey. Remember how red is essentially invisible down there because there's no red light? There's one predator that has its own bioluminescent red spotlight that allows it to easily find red fish in the dark. And the best part is that since most deep-sea fish can't see red, his spotlight is invisible to other fish.

The weirdness just keeps going. Energy preservation is very important down there so most deep-sea fish are ambush predators that don't swim around much. As such, they don't have the neat hydrodynamic torpedo shapes we usually associate with fish. Many are just sort of shapeless and blobby to help them deal with the pressure.

And they developed many other strange adaptations. When a male angler fish finds a female, he never lets her go. It's very hard to find a mate in the dark emptiness after all. Instead, he bites down on her body and slowly fuses with her to become part of her. The male is nourished by connecting to her bloodstream and slowly wastes away until the male is little more than a pair of gonads attached to the female to provide sperm for her eggs.

Simply put, the deep-sea is a completely alien environment that places very unusual demands on its denizens. They look frightening and alien to us because nothing about them makes sense to us in the context we're used to. Their proportions are bizarre, their eyes are disturbing, their teeth look like a nightmare.

But for them, those are just the tools they need to survive the world they live in.