Question

How can organisms living in the Marina trench live in the highest water pressure?

Answer 1

Most of the deep sea organisms present here do not have air filled cavities that could get squished by the high pressure. Thus, their bodies are unaffected by the pressure.

Now, having taken air out of consideration, water itself is virtually incompressible. Hence, the water outside the organism's body doesn't affect the water within the body too much.

Scuba divers take advantage of a similar principle: when they dive, their regulators feed them air that is equalized to the same pressure as is at the depth of the water they are at. As a result, there's an equilibrium between the air and water pressure. Scuba divers need to allow air in their sinuses and ears to equalize with the outside pressure as they go deeper. However, the rest of the diver's bodies are incompressible.

Another factor to consider is that much of the life present here mostly composes of jelly. No bones and relatively less muscles that could get affected by the pressure, means that these organisms can spend most of their lives in these deep trenches.

Some animals, such as whales, which visit these depths (but don't live there all the time) have other adaptations, such as more flexible, cartilaginous rib cages that can collapse under pressure.