Chondrichthyes

Chondrichthyes, or cartilaginous fishes, are a class of jawed vertebrates whose skeletons are made primarily of cartilage rather than bone. The group includes sharks, rays, and skates (Elasmobranchii) and the chimaeras or ghost sharks (Holocephali). They contrast with bony fishes (Osteichthyes) in features such as a cartilaginous endoskeleton, tooth-like placoid scales, and gill arches without an operculum.

Most species have five to seven gill slits on each side and lack a bony operculum; spiracles are present in some sharks and rays. The skin is covered with placoid scales, giving a rough texture. They lack a swim bladder and rely on a large liver rich in oil and body design for buoyancy. Sensory systems include the ampullae of Lorenzini for electroreception and a lateral line for detecting vibrations.

Reproduction is primarily internal with various modes: oviparous egg laying, ovoviviparous development (eggs hatch inside the mother), and viviparous birth. Fertilization is typically internal. Teeth are not fused and are replaced throughout life. Many species undertake seasonal migrations for feeding or breeding.

Chondrichthyans are found in oceans worldwide, from shallow coastal zones to the deep sea; a few inhabit freshwater. They occupy diverse ecological roles as predators and scavengers and influence marine community structure. The two major lineages diverged early in their evolutionary history, with sharks and rays representing the Elasmobranchii and chimaeras the Holocephali.

Because of slow growth, late maturity, and high value of products such as fins, many chondrichthyan species are vulnerable to overfishing and habitat degradation. Conservation and management strategies include catch limits, bycatch reduction, and protection of critical habitats, coordinated at regional and international levels.