Fish owe their small, light bones, delicate connective tissue, and large, pale muscle masses to the fact that water is much denser than air. Fish can attain a neutral buoyancy—can be almost weightless—sim-ply by storing some lighter-than-water oils or gas in their bodies. This means that they don’t need the heavy skeletons or the tough connective tissues that land animals have developed in order to support themselves against the force of gravity.