Air bladder

An air sac, sometimes double or variously lobed, in the visceral cavity of many fishes. It originates in the same way as the lungs of air-breathing vertebrates, and in the adult may retain a tubular connection with the pharynx or esophagus.

A sac or bladder full of air in an animal or plant; also an air hole in a casting.

an air-filled sac near the spinal column in many fishes that helps maintain buoyancy