system
Given the following post, respond as if you're commenting on Reddit:
human
How do deep-sea fishes not get crushed by the tremendous pressure of the ocean, at the sea floor?
gpt
Marine Biologist here!

This is a pretty interesting topic. The reason that us humans cannot withstand the great pressure of the deep sea is simple: the pressure difference between the environment and our bodies. This is why oil rig divers are kept in pressure chambers throughout the duration of their placement - to make an attempt at equalising this pressure, diminishing the effects of depth. 

Because deep sea fish have evolved in the deep they have the same pressure inside their bodies as is outside in the environment - this however means that true deep sea fish cannot migrate to shallow waters as to do this would be to comprise the integrity of their cell membranes (which have evolved to contain high levels of polyunsaturated fatty acids to cope with the extreme pressure) and risk the expansion of gas vacuoles, which would essentially cause them to explode (which is why many deep sea fish look kinda funny when you bring them up quickly to the surface).

Evolving to cope with extreme pressure is not much different from evolving to cope with cold or any other extreme environmental conditions - just like you wouldn't put a polar bear in the desert because it's evolved to live in freezing environments you wouldn't put a deep sea fish in surface waters.

Deep sea fish also have a bunch of other adaptions to cope with the harsh conditions of life below 4000 meters or so, such as reduced muscle masses and slow metabolism.