What is Class Chondrichthyes?

Class Chondrichthyes takes its name from two Greek words. "Chondros"; means cartilage, and "ichthys"; refers to fish, therefore the name "cartilaginous fish." The skeletons of sharks, skates, and rays are made of flexible cartilage instead of bone. Until recently, the cartilaginous skeleton was thought to reflect a more primitive evolutionary stage relative to the bony skeleton. It is now thought that the lightweight and elastic cartilaginous condition may have evolved from the bony skeleton, especially in light of new evidence of vestigial (left over evolutionary structures) bone material in the spinal column of sharks.

Sharks have several other features that make them quite different from the bony fishes. They are flattened dorsoventrally (top to bottom) instead of laterally, and they don't use swim bladders for buoyancy, but rather depend to a large extent on their vast reserves of liver oil. Most sharks must swim constantly to maintain a stream of water past their gills, whereas bony fish can remain stationary in the water column, pumping and circulating water for gas exchange.

The tough, outer skin of Chondrichthyes is rough like sandpaper because of small, teethlike scales ("placoid scales") embedded within the dermis. In fact, placoid scales, also called denticles, are thought to represent mini versions of the shark's main teeth. Teeth in a shark replace themselves in rows that move from the inside of the mouth out as older teeth break or fall off. Contrary to popular belief, not all sharks and rays are ferocious predators and scavengers. The largest, the whale shark, is in fact a filter (or suspension) feeder, straining the water for food.

Sharks have often been described as being the "perfect killing machines," largely because of their streamlined, torpedo shape, their power and swiftness, and above all, their huge jaws and teeth.

A shark attribute usually mentioned only in passing, because it is not quite so "in your face" as flashing teeth, is their sensory arsenal. Sharks not only have very keen visual and olfactory (sense of smell) apparatus, but they also have electrical sensors and a lateral line system that detects vibrations in the water.

Reproduction in sharks varies among the different species. Fertilization of eggs is internal, but the development of the embryo can occur in one of three ways, depending on the organism. Fertilized eggs can be laid outside the mother to hatch by themselves; in some species the eggs are nourished and hatched within the uterus; and in still other species, the young offspring develop within the uterus, much like mammals do.