Rays are members of the class Chondrichthyes, the cartilaginous fish, that includes sharks, skates, and chimeras. The flattened shape of rays makes them unique among fish. Their pectoral fins are much larger than those of other fish, and are attached the length of the body, from the head to the posterior.
Rays, and their relatives the skates, comprise the order Rajiformes, which includes 318 species in 50 genera and seven families. These families include the eagle rays (Myliobatidae, 20 species in three genera); the electric rays (Torpedinidae, 30 species in six genera); the mantas (Mobulidae, eight species in two genera); and the stingrays (Dasyatida, 100 species in 19 genera).
Rays are found in all of the world's oceans, in tropical, subtropical, and temperate waters. Some species, such as the great manta ray, are pelagic, spending their lives swimming; they take in water through the mouth, unlike bottom-dwelling species, which draw water though two holes (called spiracles) on their back. In all species of rays, the gills are on the underside of the body.
Like their relatives the sharks, rays have a well-developed lower jaw and an upper jaw which is separate from the skull. In many species of rays, the teeth have fused into strong bony plates. In Myliobatidae, these plates are strong enough to crush the shells of the clams and other mollusks on which the rays feed.
Many species of eaglerays have multiple rows of tooth plates, up to nine in some species of cow-nosed rays. They are generally free-swimming rays, often found in large groups. These rays are shaped like diamonds, their whip-like tails can be nearly twice the length of their bodies. Their skin is soft, and they "fly" gracefully through the water by moving their pectoral "wings" up and down.
The most remarkable feature of the electric ray is its ability to generate an electric field of considerable punch. Although an output of 75–80 volts is the norm, jolts of 200 volts have been recorded. The electric rays use this ability to stun prey and dissuade attackers. Most electric rays live in shallow water, spending their time on the bottom. They are generally more rounded than other rays, and are slow swimmers. They range in size from the lesser electric ray, which grows to about 1 ft (30.5 cm) in length, to the Atlantic torpedo, which grows to over 6 ft (1.8 m) long and can weigh more than 200 lb (91 kg). Unlike other rays, electric rays lack the venomous tail spine.
The venomous tail spine gives the stingray its common name. The venom is rarely fatal to humans, but the spine is barbed and thus difficult to remove if it is inserted. More swimmers and divers are injured by stingrays annually than by all other species of fish combined. In large specimens the spine can be up to 1 ft (30.5 cm) long, and human swimmers jabbed in the chest or stomach have died. Stingrays are primarily tropical marine bottom dwellers, though two genera in South America have adapted to life in freshwater.
Like the stingray, the manta ray has a fearsome reputation among humans. For centuries it was considered a monster with the power to crush boats. Other common names for the manta ray include "devilfish" and "devil ray," derived, in part, from the hornlike projections on their heads at the sides of their mouths, which actually serve to scoop prey into the mouth. Like many of the sea's other giants, manta rays feed on plankton. These are the largest of the rays, growing up to 17 ft (5.2 m) long and 22 ft (6.7m) wide, and weighing up to 3,500 lb (1,590 kg), as is the case with the Pacific manta.
Rays eat a diverse diet, ranging from plankton to mollusks and crustaceans to fish. The bottom-dwelling species are also noted scavengers, using their ability to sense electrical fields to find prey buried in the sand.
Rays produce eggs, which are either released into the environment in a protective egg case (sometimes called a mermaid's purse), or brooded inside the mother until the young rays are sufficiently developed to live on their own. Rays reproduce slowly; the manta ray, for example, produces just one offspring at a time.
Rays are edible, though they are generally considered "trash fish" by commercial fishermen, who often throw them back as bycatch (some fishermen prefer to use the flesh from the pectoral wings to bait lobster traps). A net full of schooling species, such as the cownosed ray, can outweigh the winches' ability to haul it up. Shell fishermen wage war against rays, which have a taste for clams and oysters. In Chesapeake Bay, fishermen drive pointed wooden stakes into the mud surrounding their shellfish beds; any ray that attempts to eat the shellfish is impaled upon the sticks. Despite these instances, rays remain quite numerous.
Science EncyclopediaScience & Philosophy: Quantum electronics to Reasoning