Sharks, Rays, and Cartilaginous Fishes

Sharks, rays, skates, and chimaeras belong to the class Chondrichthyes—vertebrates with skeletons made of cartilage rather than bone. This class includes approximately 1,200 species ranging from the 20-cm dwarf lantern shark to the 12-meter whale shark (the largest fish). Cartilage is lighter than bone, reducing energy costs of maintaining neutral buoyancy, but is less rigid—sharks compensate with thick dermal denticles (tiny tooth-like scales) that provide structural support and reduce drag. Unlike bony fish, sharks lack a swim bladder and must swim continuously or rest on the bottom to avoid sinking. Their liver, comprising up to 25% of body weight and filled with low-density squalene oil, provides partial buoyancy compensation.