Why are crabs and other animals present in the extreme depths of the ocean not affected by deep ocean pressures while steel submarines can implode if they exceed certain depths?

Because the animals were made to live in these conditions, whereas a steel submarine is a man-made object and not designed to live in these extreme depths. More Crabs and other deep-sea creatures have no air spaces in their bodies. What causes a submarine to implode at great depth is the difference in pressure on the inside and outside of the structure. Even a typical ocean fish (not a man-made object) would die at great depth because its swim bladder would be compressed. A creature with no air spaces in its body has incompressible water inside, and so it is not compressed by the pressure of the deep sea. Note that scuba equipment allows the pressure inside a diver's lungs to equal that of the water outside, but this has its limits: * At great pressure, a mammal's blood (including that of a human) absorbs far more gas than it does at sea level, and the gases we normally breathe nitrogen and oxygen become toxic in such high concentrations. Deep-sea animals do not have this problem because they are not breathing air, but only the oxygen dissolved in sea water, which is sparse at great depths. * At extreme depths, the pressure of the water would be greater even than the air pressure inside a typical scuba tank.