Chondrichthyes

Skate Brain Regions

White shark (Duane Raver)

Myliobatis aquila sasrája

Chimaera monstrosa1

CR IUCN 3 1

Chondrichthyes or cartilaginous fishes are a diverse group of fish that have skeletons primarily made of cartilage, rather than bone. This group includes over 1,000 species, such as sharks, rays, and skates, which are found in both freshwater and marine environments around the world. Unlike bony fishes, members of the Chondrichthyes lack a swim bladder and have several other distinctive features, including gill slits, pectoral fins that are not fused to the head, and a unique skin covering made of dermal denticles.

Evolution and Classification

The Chondrichthyes are believed to have first appeared in the oceans of the late Silurian to early Devonian period, approximately 420 million years ago. They are divided into two subclasses: Elasmobranchii, which includes sharks, rays, and skates; and Holocephali, which comprises the chimaeras, sometimes known as ghost sharks.

Elasmobranchii

The Elasmobranchii are characterized by their leathery skin covered with small, tooth-like scales called denticles, which help to reduce friction while swimming. They also have five to seven gill slits on each side of their head, and most species are predatory.

Holocephali

Holocephali, or chimaeras, differ from their elasmobranch relatives in having a single external gill opening and a long, whip-like tail. They are generally deep-sea dwellers and have a more sedentary lifestyle compared to the often highly mobile sharks and rays.

Anatomy and Physiology

Chondrichthyes have several anatomical and physiological adaptations that distinguish them from other fish. Their skeletons are made of cartilage, which is lighter and more flexible than bone. This allows for more efficient swimming and maneuverability, especially important for the predatory lifestyle of many sharks and rays.

Their skin is covered in dermal denticles, which provides protection and assists in streamlining their bodies to reduce drag while swimming. Chondrichthyes also have a highly developed sense of smell and, in many species, an electrosensory system that allows them to detect the electrical fields generated by the muscle contractions of their prey.

Reproduction among Chondrichthyes can vary significantly between species. Some sharks lay eggs (oviparous), while others give birth to live young (viviparous). Rays and skates typically lay eggs encased in a protective capsule known as a "mermaid's purse."

Conservation

Many species of Chondrichthyes are currently threatened by human activities, including overfishing, habitat destruction, and pollution. Sharks, in particular, are targeted for their fins, which are highly valued in certain cultures for making shark fin soup. This has led to a significant decline in shark populations worldwide. Conservation efforts are underway to protect these ancient and ecologically important creatures, including the establishment of marine protected areas and the regulation of fishing practices.