Cartilaginous fish such as sharks, skates, and rays are vertebrates whose internal skeleton is made entirely of cartilage and contains no ossified bone. Cartilaginous fish are also known as Chondrichthyes and have one or two dorsal fins, a caudal fin, an anal fin, and ventral fins which are supported by girdles of the internal skeleton.
Placoid scales, or dermal teeth, are characteristic of the skin of both sharks and rays. The touch of shark's skin is similar to the feel of sandpaper and was used as such for many years. The tiny teeth that protrude from the skin vary in each species of shark. The tooth tip is dentine with an overlay of dental enamel, while the lower part of the tooth is made of bone, which anchors the tooth to the skin.
The skin of rays is naked in places, that is, without dermal teeth, but on the back of or upper tail surface, the dermal teeth have developed large, strong spines.
The jaw teeth of both sharks and rays are in fact modified dermal teeth, which are lost when they become worn, and are replaced by rows of new teeth from the space behind. In some species of sharks, the jaw looks like an assembly line, with new teeth filling spaces immediately.
Both sharks and rays breathe through gills and have an opening called a spiracle on both sides of the head behind the eye. The spiracle enables the rays, which often bury in the sand, and seabed-resting sharks to take in water, pump it through the gill chamber, and release it through the gill slits without taking in large amounts of mud and sand. These fish usually take respiratory water in through the mouth, extract the oxygen from the water in the gills, and pass it out through the gill slits.
Cartilaginous fish are divided into two subclasses on the basis of gill slits and other characteristics. The first is the Elasmobranchs, which have at least five gill slits and gills on each side, one spiracle behind each eye, dermal teeth on the upper body surface, a tooth jaw, and an upper jaw not firmly attached to the skull. Sharks (Selachii), rays, and skates (Rajiformes) belong to this group. The chimeras (Holocephali) have one gill opening on each side, tooth plates, and a skull with a firmly attached upper jaw.
Cartilaginous fish do not have swim bladders, so a swimming motion must be maintained continuously, even when sleeping, or they will sink to the bottom. The caudal fin of the shark provides the propellant force in swimming, the dorsal fin provides balance, and the pectoral fins are used for upward force and depth rudders.
The flattened body and the rear spine of the rays makes their swimming motion unique and completely different than that of sharks. The large flattened body of the rays has become fused with the pectoral fins, which produces vertical waves from front to rear, similar to that of a bird in flight.
The chimeras utilize their pectoral fins when swimming, beating these fins simultaneously for propulsion, or alternately, to change direction. This method is highly effective for this group of cartilaginous fish, but is seen most often in bony fish.
The pectoral fins in the male cartilaginous fish are also used for mating. The rear part of the pectoral fin is modified as a copulatory organ. All cartilaginous fish have internal fertilization. Some species are oviparous, or egglayers, and some are ovoviviparous, hatching the eggs within the female and giving birth to live young. Still others may be viviparous, with the young developing in utero, similar to mammals, with the yolk sac developing into a yolk placenta providing nutrients to the embryo.
Only true rays, species of sharks which live near the sea bed, and the chimeras lay eggs. The eggs are often encased in a leathery shell with twisted tendrils which anchor the egg case to rocks or weeds. These leathery shells are known as the "mermaid's purse."
Cartilaginous fish are predatory, meaning that they feed on other animals, from zoo plankton to shellfish to whales. Cartilaginous fish themselves are sought after by humans as a food source. Shark meat, once marketed under the pseudonyms of "flake" and "steakfish" is now popular worldwide. Shark fins have long been popular in the Orient. Rays are considered delicacies in Great Britain and France, and thornback rays and flapper skates are often sold as sea trout.
F. C. Nicholson