Sharks, rays, and skates (Elasmobranchii) form a group of cartilaginous fishes also known as the elasmobranchs. There are about 800 species of elasmobranchs, over half of which are skates and rays. The remainder of the group are sharks.
Elasmobranchs differ from their cousins the ray-finned fishes in several ways. Instead of having a skeleton that consists of bone, the elasmobranchs have a body frame constructed of cartilage. Elasmobranchs also lack swim bladders (gas-filled sacs that contract or expand to moderate its bouyancy) that are characteristic of ray-finned fishes. Elasmobranchs have between five and seven pairs of gill slits that are exposed, while ray-finned fish have a bony plate called an operculum that covers their gills.
Sharks, when compared to rays and skates, are designed more for speed and power. Many species rely on agility to capture prey. Their bodies are torpedo-like in shape, streamlined for quick movement through the water. The thickest part of their body is about one third of the way back from the head, and it tapers off from there in a form that reduces drag when swimming. Sharks propel themselves through the water by moving their caudal (tail) fins laterally back and forth.
The bodies of skates and rays, in contrast to sharks, are flattened into disc or diamond shapes. They have reduced caudal fins and swim by undulating their pectoral fins up and down in a wave-like manner. Many species of rays and skates are either bottom feeders or filter feeders.