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—simply 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.
The paleness of fish flesh results from water’s buoyancy and its resistance to movement. Continuous cruising requires long-term stamina and is therefore performed by slow-twitch red fibers, well supplied with the oxygen-storing pigment myoglobin and fat for fuel. Since cruising in buoyant water is relatively effortless, fish devote between a tenth and a third of their muscle to that task, usually a thin dark layer just under the skin. But water’s resistance to movement increases exponentially with the fish’s speed. This means that fish must develop very high power very quickly when accelerating. And so they devote most of their muscle mass to an emergency powerpack of fast-twitch white cells that are used only for occasional bursts of rapid movement.