Bony fish have a swim bladder, a gas filled sac. The swim bladder originates as part of the belly. The swim bladder aids in creating an equilibrium with what is going on inside the fish and the surrounding water.
Some fish have a bladder that is part of the respiratory system, called an air bladder. It even creates a drumming sound. Croaker, red drum and black drum are part of the group.
Fish that have a skeleton made of cartilage do not have a swim bladder.
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