Swim bladder
A gas-filled sac inside many bony fish that lets them float at a chosen depth without swimming.
Also: air bladder, gas bladder, maw
The swim bladder is a balloon of gas in the body cavity, below the backbone. By adding or removing gas, a fish matches its overall density to the water around it, so it can hover without effort. Some fish fill it by gulping air; others release gas from the blood through a gland. In carps and catfish it also links to the ear and helps them hear. Sharks have none and rely on oily livers instead, and many deep-sea and bottom fish have lost it. Dried swim bladder, called fish maw, is traded as a food in East Asia, a demand that has driven the near-collapse of the totoaba and croakers.