We’ve all heard that a shark must swim continuously or sink. Do you know why? It’s because sharks do not have a swim bladder. “This gas-filled sac provides buoyancy and helps to keep the fish afloat by keeping it in a neutrally buoyant state.” When in this state, the fish is neither rising nor sinking in the water.
Many bony fish – those having skeletons of bone rather than cartilage like our shark friends – have a swim bladder. It is located in the dorsal portion of the fish as noted in the anatomical sketch included in this post. The swim bladder can expand or contract by filling with or emptying itself of gas through the use of the gas gland. (In some less developed types of fish, the fish fills or empties the swim bladder with gulps of air.) The gas never passes through the wall of the swim bladder. You can think of the swim bladder as a sort of internal helium balloon.