Fish reproduction and cartilaginous fishes

Variety in habitat, food and enemies has made fish structure varied: bottom fishes are slow and flat, reef fishes colourful, deep-sea fishes have light-producing organs, and open-water fishes have streamlined bodies and strong tail fins. By skeleton, fishes divide into cartilaginous and bony classes. The skeleton of cartilaginous fishes is cartilage; they lack gill covers and a swim bladder and have 5–7 pairs of gill slits; they consist of the sharks and rays. Sharks are large with a torpedo-shaped body and strong tail fin; the skin has thick plate-like scales whose enamel-covered toothlike spines became teeth in the jaw. Most are predators and bear live young. There are over 500 shark species: the smallest are only 20–30 cm, the largest is the whale shark (over 12 m); yet the biggest sharks feed on plankton and small crustaceans. Rays have a flat, diamond- or disc-shaped body and a whip-like tail fin; they swim by waving their pectoral fins. Some have a venomous spine or an electric organ; most bear live young, a few at a time.