And how can animals live in the depths of the oceans then:

But there are many animals, who can live 3 km deep and even deeper. Certainly they must be especially adapted for being able to survive under these extremely heavy masses of water.

Those aninamls must not have cavities filled with air.

Thus those creatures must consist of a lot of water just as the jellyfish.

They cannot have ear canals, lung, and swim bladder. They regulate their swimming depth with the help of oily liquid substances.

Why do those animals, who are living in the depths of the oceans, not have any swim bladder?

a) Due to the fact that they never go up to breathe air, they could never fill their swim bladder with air.

b) The temperature is too low there. The air in the swim bladder would freeze.

c) The water pressure is much too high. The swim bladder would be compressed.