Primarily because whatever form of life you are thinking of, will have evolved in that specific pressure environment - so one answer could be: evolution. read more
There are many animals which does not have brain. corals and jellyfish (Cridarians) are just tow animals that have no brain. In adition echinoderms such as starfish, sea urchins, sand dollars, etc. are also have no brain. read more
Pressure-The pressure at the bottom of an ocean is many folds compared to the pressure on the surface. This pressure keeps on increasing as one move deeper. The pressure is greater in oceans because the water is pressed down heavily. At 10m below the water, the pressure doubles, At 20, the greatness is thrice than on the surface. read more