Chondrichthyans

Chondrichthyans are a class of jawed vertebrates whose skeletons are made of cartilage rather than bone. This group includes sharks, rays, and skates (Elasmobranchii) and the chimaeras or ghost sharks (Holocephali). Together they constitute the cartilaginous fishes, among the earliest jawed vertebrates with durable cartilage-based skeletons.

Chondrichthyans are characterized by a cartilaginous skeleton, placoid dermal scales (also known as tooth-like scales), paired fins, and a variety of body forms adapted to different habitats. They exhibit a wide range of feeding strategies, from streamlined predators to bottom-dwelling ambush feeders. Gas exchange occurs through gills, and many species rely on continuous movement to ventilate water over the gills, while others use spiracles or buccal pumping. Jaws bear replaceable teeth arranged in multiple rows in most sharks.

In reproduction, males use claspers for internal fertilization. Reproductive modes vary among groups and species: some are oviparous (egg-laying), others are ovoviviparous (eggs hatch inside the mother), and others are viviparous (live birth). Lifespans can be long and growth and maturity slow, which can contribute to vulnerability under intense fishing pressure.

Ecology and conservation: Chondrichthyans occupy marine environments from coastal shallows to the deep sea and often occupy important ecological roles as predators or mesopredators. Many species are threatened by overfishing, bycatch, and finning, driven by demand in fisheries and trade. Conservation measures include quotas, protected areas, and international trade controls, aimed at sustaining populations and ecosystem balance.