Final answer:
The swim bladder is a gas-filled organ used by bony fishes to control buoyancy, allowing them to conserve energy and remain at desired depths without constant swimming. Not all fish have it—cartilaginous fish lack swim bladders and use different buoyancy mechanisms.
Step-by-step explanation:
The swim bladder is a gas-filled organ located in the abdominal cavity of bony fishes, designed for controlling buoyancy. The swim bladder allows fish to maintain their position in the water without wasting energy. It is homologous to the lungs of lungfish and terrestrial vertebrates, functioning by gas exchange with the blood to regulate its volume and density.
Not all fish have swim bladders. Cartilaginous fishes, such as sharks and rays, lack this organ. Instead, they may use other mechanisms, like oil-filled livers, for buoyancy. Bony fishes have swim bladders and are categorized into two groups: Actinopterygii (ray-finned fishes) and Sarcopterygii (lobe-finned fishes), which includes species that are ancestors to terrestrial vertebrates.
The swim bladder is critical for fish as it allows them to ascend or descend in the water column with ease, thereby conserving energy and optimizing conditions for feeding and avoiding predators. By altering the gas volume within, fish can achieve neutral buoyancy and stay at desired depths without constant swimming.