This represents a big problem to humans as our bodies would be crushed at such high pressures. This is because we have large air spaces inside our bodies. In order to be able to venture further pressurised submersible vehicles have been developed that protect our bodies from being crushed. For animals that have bodies that are completely filled with water an increase in pressure has little effect. For this reason jellies, salps, squid etc. have no difficulty when moving even quite large distances through the water column.
Many fish have a swim bladder that contains gases.
The volume of gas in the bladder can be adjusted as they move up and down, in much the same way that a diver alters their buoyancy using an inflatable vest. This would be unlikely to occur as animals move up and down in the water column on their own. Surviving in the Zone of Darkness From about 1000m to the bottom of the ocean is completely dark. Many of the animals that have been described in the section dealing with the twilight zone can also inhabit this zone which is on average about 4000m deep with a number of trenches and basins that can be as deep as 7000m. In this zone the water is also extremely cold with temperatures that are usually down to 1 o C or less at 4000m.