Air bladder

[Sarah]

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

[exinilu]

a small bag of air or gas inside a fishs body, that helps it to float or breathe