Which organ is absent in cartilaginous fish?

Many animals naturally float at the surface and have to force their way down below the surface. Fish however, have a very convenient adaptation. Two ways to animals in the ocean control their buoyancy, or the upward force exerted by a fluid, are with a swim bladder and an oily liver. Osteichthyes (bony fish) use swim bladders that are filled with oxygen taken in by their gills. The more air in the swim bladder the more buoyant the fish and the less air in the swim bladder the less buoyant the fish. The swim bladder is similar to human lungs in the way that it expands and deflates.

Chondrichthyes (cartilaginous fish) use an oil filled liver to control their buoyancy instead of the swim bladder. The oil lightens the shark’s heavy body to keep it from sinking and saves the sharks energy when using its fins to keep itself moving. The oily liver is also used for other daily functions such as digestion. The name for this oil is squalene. Without these organs they would not be able to control their location in the water column. These organs can help them stay neutrally buoyant.

Chondrichthyes is a class that contains the cartilaginous fishes: they are jawed vertebrates with paired fins, paired nares, scales, a heart with its chambers in series, and skeletons made of cartilage rather than bone. The class is divided into two subclasses: Elasmobranchii and Holocephali.