Animalia Life

Most of the more than three hundred living species of the order Batoidea, the rays and skates, are adapted for living on the bottom. In body form they are strongly depressed (flattened), with enlarged pectoral fins extending forward to the head region. Their teeth are usually pavementlike, for crushing their hard-shelled invertebrate prey. Most species give birth to live young, except the skates (family Rajidae), in which the eggs develop in a leathery egg case (the "mermaid's purse" that beach visitors often find in the sand). Several ray families include members with a venomous spine on the tail, including the Dasyatidae (stingrays), the Potamotrygonidae (river stingrays), and the Myliobatidae (eagle rays). The largest species among rays and skates is the giant manta ray or devilfish (Manta birostis), which may attain a width of over six meters between the tips of its pectoral fins and a weight in excess of 1,300 kilograms. Like two other cartilaginous fish giants mentioned earlier (the whale shark and basking shark), the giant manta ray is a plankton feeder. It directs plankton into its mouth as it swims by means of large scooplike extensions on its head-the "horns" responsible for the name "devilfish." It then filters out the plankton with its comblike gill rakers. Rays and skates swim by means of flapping movements of their winglike pectoral fins. Some species, including eagle rays and manta rays, can make spectacular leaps from the water. Among the more remarkable rays are the electric rays (family Torpedinidae). These sluggish fishes use electrical discharges of up to two hundred volts, produced by a pair of disk-shaped electric organs on the sides of the head, to stun their prey and perhaps to repel predators. Another specialized group among the rays is the sawfish family (Pristidae). Asawfish resembles a somewhat flattened shark in body form but has a long, flat, toothed extension (the "saw") on the end of its snout. This is used to slash through a school of prey fish.

The Place of Rays in Evolution
Cartilaginous fish represent an early line in the evolution of vertebrates. Understanding their interrelationships is crucial to an understanding of the ancestry of other fishes and of tetrapods (amphibians, reptiles, birds, and mammals) and thus, ultimately, of humans. Even though humankind's ancestors split from the ancestors of sharks and rays more than 400 million years ago, many anatomical and physiological features are shared. A prime example is the eye, which is extraordinarily similar in all vertebrates. The same system of eye movement, involving six muscles innervated by the same three cranial nerves, has remained unchanged throughout vertebrate evolutionary history. Thus, the study of shark eyes, or any other aspect of shark biology, deepens the understanding of the evolution of higher animals. Much remains to be learned about rays and other cartilaginous fishes. Studying their ecology, behavior, and evolutionary relationships is important for further understanding of their basic biological nature. It is also essential for maximizing the benefit of commercially important species and minimizing the risk posed by dangerous species.

Ray Facts

Classification:
Kingdom: Animalia
Subkingdom: Bilateria
Phylum: Chordata
Subphylum: Vertebrata
Superclass: Gnathostomata
Class: Chondrichthyes
Subclass: Elasmobranchii (sharks, skates, and rays)
Order: Batoidea
Family: Rajidae
Geographical location: Worldwide
Habitat: Bottom of the oceans, seas and freshwater reservoirs
Gestational period: Unknown
Life span: Unknown

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