Jawed fish include cartilaginous and bony fish.
Cartilaginous fish include sharks, rays and, skates. Cartilaginous fishes have a skeleton made of cartilage, a material that is lighter and more flexible than bone. These type of fishes have movable jaws that are usually armed with well developed teeth. The mouth is often located underneath the head. Cartilaginous fish have rough, sandpaper-like skin because of the presence of tiny scales, which have the same composition as teeth.
Sharks are often referred to as living fossils because many are similar to species that swam the seas more than 100 million years ago. Sharks are found throughout the oceans at all depths. However, they are more prevalent in tropical coastal waters. A few species are known to travel far up rivers. For example, the bull shark has been established in some rivers and lakes in the tropics.
Rays and Skates
Rays and skates have flattened bodies with the gill slits underneath. Their pectoral fins are flat, look like wings, and are typically fused with the head. The eyes are usually on top of the head.
There are about 300 species of rays and skates worldwide and most live on ocean floor.
Stingrays have a whip-like tail usually equipped with stinging spines for defense. Poison glands produce venom that can cause serious wounds to anyone who steps or falls on them. Stingrays cover themselves with sand, becoming nearly invisible. They feed on clams, crabs, small fishes, and other small animals that live in sediment.
Electric rays are rays that have special organs on each side of the head that produce electricity. They can deliver shocks of up to 200 volts that can stun the fishes they eat and discourage predators.
Some rays do not spend their lives on the bottom. Eagle rays, manta rays, and devil rays fly through the water, using their pectoral fins like wings. Eagle rays return to the bottom to feed while mantas feed in midwater on plankton.
The majority of fishes are bony fishes. As the name implies, they have a skeleton made at least partially of bone. There are about 21,500 species of bony fishes—about 98% of all fishes and almost half of all vertebrates. A little more than half of all bony fishes live in the ocean, where they are by far the dominant vertebrates.
Bony fish have thin, flexible, overlapping scales that develop from bone. The scales are covered by a thin layer of tissue, as well as protective mucus. A flap of bony plates and tissues known as the gill cover protects the gills. In addition to their bony skeleton, they have highly maneuverable fins, protrusible jaws, and a swim bladder.