The class Chondrichthyes consists of the cartilaginous fishes, including sharks, batoids (rays, skates, guitarfish, and sawfishes), and chimaeras, or ratfishes. A diverse group comprising more than 700 species, Chondrichthyans are found throughout the world's oceans and in some freshwater environments. The group as a whole is characterized not by mineralized bone but by a skeleton of soft, flexible cartilage lined with hard tissue. Chondrichthyans lack the air-filled swim bladder found in most bony fish, and therefore must swim continuously to stay afloat. Buoyancy is assisted by light oils in the liver, which can comprise up to 25 percent of a shark's total body weight.

Chondrichthyan males have a pelvic clasper, a specialized organ used in mating. Unlike most bony fishes, all chondrichthyans have internal fertilization. Reproduction can be oviparous (laying eggs, notably the "mermaid's purses" found on beaches), viviparous (live-bearing), or ovoviviparous (eggs carried within the mother).

The physiology of...