Chondrichthyes

Chondrichthyes, a class of jawed fish, includes all cartilaginous fish—animals whose skeletons are made mostly of cartilage, not bone. These aquatic vertebrates have paired fins, paired nostrils, placoid scales, and a conus arteriosus in the heart. They belong to the infraphylum Gnathostomata and are distinct from all other jawed vertebrates. The class splits into two subclasses: Elasmobranchii (sharks, rays, skates, and sawfish), which lack opercula, and Holocephali (chimaeras, sometimes called ghost sharks), which are occasionally placed in their own class. Living chondrichthyans range from the 10 cm finless sleeper ray to the over 10 m whale shark.

Their skeleton is cartilaginous. During development, the notochord is gradually replaced by a vertebral column, except in Holocephali, where it remains intact. Some deepwater sharks have a reduced column. Without bone marrow, red blood cells form in the spleen and the epigonal organ (tissue around the gonads that may also aid immunity), as well as in the Leydig’s organ, found only in certain cartilaginous fish. Holocephali lack both the Leydig’s and epigonal organs.

Except for electric rays, which have thick, flabby bodies and soft, loose skin, chondrichthyans have tough skin covered in dermal teeth (also called placoid scales or dermal denticles), giving it a sandpaper-like feel. In most species, these denticles point in one direction, so the skin feels smooth one way and rough the other. Holocephali lose these teeth in adults, keeping them only on the male’s clasping organ near the tail. Originally, the pectoral and pelvic girdles had no dermal elements and were not connected. Later, each pair of fins became ventrally linked when scapulocoracoid and puboischiadic bars evolved. In rays, the pectoral fins connect to the head and are highly flexible. Most sharks have a heterocercal tail that aids movement.

Chondrichthyans have tooth-like dermal denticles that usually provide protection and, in many cases, streamlining. Some species also have mucous glands. It is thought that oral teeth evolved from dermal denticles that moved into the mouth, but the reverse is possible—for example, the bony fish *Denticeps clupeoides* has most of its head covered in dermal teeth, as likely does *Atherion elymus*. This is probably a secondary trait, meaning no direct link between teeth and original scales is certain. Old placoderms had no teeth, only sharp bony plates in the mouth, so it is unknown whether dermal or oral teeth came first. Some suggest that all original bony plates are gone and that present scales are modified teeth, though there is no evidence for this.

All chondrichthyans breathe through five to seven pairs of gills, depending on the species. Pelagic species generally must keep swimming to move oxygenated water over their gills, while demersal species can actively pump water in through spiracles and out through gills—though this is a general rule with many exceptions. A spiracle is a small hole behind each eye, ranging from tiny and circular (as in the nurse shark) to extended and slit-like (as in wobbegongs). Many larger pelagic species, like mackerel sharks and thresher sharks, no longer have spiracles.

The nervous system includes a small brain, 8–10 pairs of cranial nerves, and a spinal cord with spinal nerves. Sensory organs include ampullae of Lorenzini—jelly-filled pores that detect electric fields, aiding in prey location, navigation, and temperature sensing. The lateral line system has modified epithelial cells that sense motion, vibration, and pressure. Most species have large, well-developed eyes, powerful nostrils, and olfactory organs. Their inner ears have three large semicircular canals for balance and orientation, and their sound-detecting apparatus works best at lower frequencies. Some species have electric organs for defense and predation. Brains are relatively simple, with the forebrain not greatly enlarged. The structure and formation of myelin in their nervous systems closely resemble that of tetrapods, leading evolutionary biologists to view Chondrichthyes as a key group in myelin’s evolutionary history.

Like all jawed vertebrates, chondrichthyans have an adaptive immune system.

Fertilization is internal. Development is usually live birth (ovoviviparous) but can be through eggs (oviparous); a few rare species are viviparous. There is no parental care after birth, though some species guard their eggs.