What organ helps many bony fish control buoyancy?

Many bony fish use a swim bladder to control buoyancy. This internal, gas-filled organ lets a fish remain at a preferred depth without constantly swimming to generate lift.

By adding or removing gas, the swim bladder changes the fish’s overall density. In some species, the bladder connects to the digestive tract, allowing the fish to gulp air or release it. In others, blood vessels and a specialized gas gland move gases into the bladder, while an oval window helps remove them.

The organ has other roles too. Its gas-filled surface can reflect sound, making swim bladders important to sonar surveys, and in some species it helps amplify or detect sounds. Evolutionarily, the swim bladder is homologous to the lungs of lungfish and land vertebrates.

Not every fish has one. Sharks and rays lack swim bladders and instead rely on oily livers, buoyant cartilage, or lift generated by swimming. Fast-swimming bony fish such as tuna may also lack or greatly reduce the organ.