Cartilaginous Fishes

Sharks, skates, and rays belong to the class Chondrichthyes. Because the fishes in this class have skeletons composed of cartilage, they are also called cartilaginous fishes. Cartilage is a flexible, lightweight material made of cells surrounded by tough fibers of protein. Sharks, skates, rays, and ratfishes differ from lampreys and hagfishes in that they have movable jaws, skeletons, and paired fins. Almost all of the approximately 800 species of sharks, skates, rays, and ratfishes live in salt water. All species are carnivores, and some are scavengers. The skin of cartilaginous fishes is covered with placoid (PLAK-oyd) scales—small, toothlike spines that feel like sandpaper. Placoid scales, shown in Figure 39-6, probably reduce turbulence of the water flow and thus increase swimming efficiency.


Sharks have smooth, torpedo-shaped bodies that reduce turbulence when swimming. This shape is called a fusiform body shape. Figure 39-7 shows the smooth, fusiform body of a shark.

The largest sharks, the whale shark (up to 18 m, or 59 ft, long) and the basking shark (up to 15 m, or 49 ft, long), feed on plankton, floating plants and animals. Like other filter-feeding fishes, the whale and basking sharks filter the water with slender projections on the inner surface of the gills, called gill rakers.

The mouth of a typical shark has 6 to 20 rows of teeth that point inward. When a tooth in one of the front rows breaks or wears down, a replacement moves forward to take its place. One shark may use more than 20,000 teeth over its lifetime. The structure of each species' teeth is adapted to that species' feeding habits. Sharks that feed primarily on large fish or mammals have big, triangular teeth with sawlike edges that hook and tear flesh.

Pharynx Fish

figure 39-7

Rays and Skates

Rays and skates have flattened bodies with paired winglike pectoral fins and, in some species, whiplike tails. Rays have diamond- or disk-shaped bodies, but most skates have triangular bodies. Most rays and skates are less than 1 m (3.3 ft) long. Rays and skates are primarily bottom dwellers. Their flat shape and coloration camouflage them against the floor of their habitat. Most rays and skates feed on mollusks and crustaceans. Figure 39-8 shows an example of a ray.


There are about 25 species of strange-looking, mostly deep-water cartilaginous fishes that are grouped separately because of their unique features. The ratfishes, or chimaeras, have gill slits covered by a flap of skin. Some have a long, ratlike tail. They feed on crustaceans and mollusks.

Adaptations of Cartilaginous Fishes

In cartilaginous fishes, gas exchange occurs in the gills, which lie behind the head. Efficient gas exchange requires a continuous flow of water across the gills. Some fast-swimming sharks are able to push water through their mouth, over their gills, and out of their gill slits by swimming. However, most cartilaginous fishes pump water over their gills by expanding and contracting their mouth cavity and pharynx. When lying on the bottom, rays and skates cannot bring in water through their mouth, which is located on the ventral surface of their body; instead, they draw water in through their spiracles, which are two large openings on the top of the head, just behind the eyes.

figure 39-7

A fusiform body shape allows sharks to slip through the water with little resistance. The dorsal fins provide stability. The pectoral fins provide lift similar to the way airplane wings function in the air. The shark's internal anatomy includes organs for digestion, reproduction, and maintaining homeostasis.

figure 39-8

This blue-spotted stingray, Taeniura lymna, is an example of a bottom dweller. This stingray was photographed in the Red Sea near Egypt.

This blue-spotted stingray, Taeniura lymna, is an example of a bottom dweller. This stingray was photographed in the Red Sea near Egypt.

Instead of releasing ammonia, cartilaginous fishes use energy to convert ammonia into a compound called urea, which is much less toxic. Sharks retain large amounts of urea in their blood and tissues, thus raising the concentration of solutes in their body to at least the same level as that found in sea water. Because the concentration of sodium and chloride in the body of a shark is less than the concentration found in sea water, these ions still diffuse into the body across the gills and are absorbed with food. The rectal gland, located in the posterior portion of the intestine, removes excess sodium and chloride ions from the blood and releases them into the rectum for elimination. Sharks that periodically migrate to fresh water excrete sodium, chloride, and urea along with any excess water that enters their bodies.

Cartilaginous fishes maintain their position in the water in two ways. First, because the caudal and pectoral fins generate lift, or upward force, as a fish swims, it can remain at the same level in the water, counteracting the tendency to sink, as long as it keeps moving. Second, many cartilaginous fishes store large amounts of low-density lipids, usually in the liver. A shark's lipid-filled liver may account for 25 percent of its mass. Lipids give these sharks buoyancy by reducing the overall density of the body.

Reproduction in Cartilaginous Fishes

Cartilaginous fishes differ from jawless fishes in that fertilization occurs inside the body of the female. This type of fertilization is called internal fertilization. During mating, the male transfers sperm into the female's body using modified pelvic fins called claspers. In a few species of sharks and rays, the females lay large yolky eggs right after fertilization. The young develop within the egg, are nourished by the yolk, and hatch as miniature versions of the adults. The eggs of many species develop in the female's body, and the young are born live. In some of these species, the mother nourishes the developing sharks while they are in her body. No cartilaginous fishes provide parental care for their young after birth or hatching.

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