Unless you have exactly the same density as seawater, it takes effort to float or sink. Our bodies have a density of ..., just a little more than water so that it's hard to float without extra help. But we're not so dense that it's easy to dive without the addition of weights when you go scuba diving. Fish have roughly the same density as humans so what's a fish to do?
Many fishes have an internal sack of air called a swim bladder ... In fishes such as the Bichirs and ..., lungs ...
Fishes that live exclusively in the deep ocean don't have swim bladders ...
Sharks don't have swim bladders so what's a shark to do? They have large livers and count on lift ...
