A shark's skeleton is composed of cartilage and a network of collagen fibers. This differs from most other vertebrates and allows for the especially efficient attachment of muscles, which results in little waste of mechanical energy. The cartilaginous skeleton of the shark is also lightweight compared with bone.
The lightweight skeleton allows a shark to expend less energy when staying afloat, as all shark species lack a swimmer bladder. Due to a lack of bone and bone marrow, a shark must produce red blood cells in its spleen and its epigonal organ, a collection of specialized tissue around the gonads. A primitive backbone called the notochord is present in young sharks and is eventually replaced with cartilage.