Bony fishes have a specialized organ called a gas bladder, or swim bladder, whose interior can be filled with gas liberated from gas glands. The swim bladder works as a hydrostatic organ since it varies the relative density of the body regulating buoyancy and the depth of the animal in water.
Chondrichthyes do not have swim bladders and thus they must continuously agitate their body to keep swimming and maintain their depth in water. As an additional swimming aid, in chondrichthyes the liver is big and oily; this feature helps to reduce their body density relative to water.
