Swim bladder

The swim bladder (also air bladder or gas bladder) is an internal organ that contributes to the ability of a fish to control its buoyancy despite changes in external water pressure. It also allows the animal to ascend or descend without having to waste energy in swimming.

The swim bladder is an air-filled sac located in the dorsal portion of the fish. It has flexible walls that contract or expand according to the ambient pressure. The walls of the bladder contain very few blood vessels and are lined with guanine crystals, which make them impermeable to gases. The bladder has a gas gland that can introduce gases to the bladder to increase buoyancy. To reduce buoyancy, gases are released from the bladder into the blood stream and then expelled into the water via the gills.

Not all varieties of fish have a swim bladder. Some varieties of shark, for example, can control their depth only by swimming.