Why Do Land Animals Have Denser Bones Than Sea Animals?

[Hereformore]

Given the greater pressure underwater, i.e. at the bottom of the ocean, to survive there youd think that those animals would have stronger/denser bones, but we find that land animals have denser bones to withstand the higher net weight (gravity).

But that seems counter intuitive because underwater the pressure is much higher at lower depths (like in the movies when submarines start to creak and get crushed under the pressure).

So what's the deal? I know its a pressure vs force distinction but I am having trouble seeing it.

 

[russ_watters]

Submarines are filled with air at low pressure. Bones and fish bodies aren't. So there is nothing for the pressure to push against to crush. If there were, human divers wouldn't be able to dive more than a few feet!

So the fluids are continuous and the pressures are equal, so nothing to push against.

[edit] Actually, even worse, when lying in bed it would feel like you'd have 10,000 lb of weights stacked on top of you due to the air pressure!

 

[Drakkith]

Bones are used to support an animal against uneven forces (among other things), like having to stand up against gravity. Water pressure is evenly distributed so bones don't have to do any work against it.

 

[A.T.]

Given the greater pressure underwater, i.e. at the bottom of the ocean, to survive there youd think that those animals would have stronger/denser bones

You do realize that there are aquatic animals, which don't have any skeletons? In fact, as life developed in water none of it had skeletons. The musculo-skelettal system developed mainly to enable certain types of movement.