Family Plesiobatidae
deepwater stingrays

The family Plesiobatidae, or deepwater stingrays, contains only one species, formerly included in the genus Urotrygon but placed in its own family in 1990. Deepwater stingrays are found on soft bottoms at depths between 44 m and 680 m. They are large, dark rays with a rounded disc that ends in an angular pointed snout. They are viviparous and likely share reproductive characteristics with Urolophidae (see Development and Reproduction). They feed on a variety of fishes and invertebrates (see Food Habits). They are not of interest to fisheries, and despite their venomous defensive sting, do not pose any threat to humans. (Compagno, 1999; Froese, Pauly, and Woodland, 2003; Hamlett and Koob, 1999; Hamlett, 1999; Last and Stevens, 1994; Nelson, 1994)

Deepwater stingrays can be found in marine waters near South Africa and Mozambique, Australia, and in the western Indian Ocean and the west-central Pacific Ocean from Japan to the Phillippines and Hawaiian Islands. (Froese, Pauly, and Woodland, 2003; Last and Stevens, 1994; Nelson, 1994)

Deepwater stingrays are bathydemersal (they live and feed on the bottom below 200 m). They occupy only marine habitats, at depths ranging from 44 to 680 m. They live on soft bottoms on the outer continental shelf and its upper slope. (Froese, Pauly, and Woodland, 2003; Last and Stevens, 1994)

Plesiobatidae is a family containing a single genus and species: Plesiobatis daviesi. Formerly this species was considered a member of the genus Urotrygon, family Urolophidae, but Nishida (1990) placed it in its own family. (Compagno, 1999; Nelson, 1994)

The family Plesiobatidae consists of one species (Plesiobatis daviesi) of large, deepwater stingrays. Their pectoral disc is rounded and confluent with a broadly angular, pointed snout. The disc is slightly longer than it is wide, is grayish brown to black, and is covered with small denticles. The snout is long, with a lobe on its thin tip. The head is not elevated above the rest of the disc. The two spiracles (respiratory openings) are located close behind the ray’s small eyes. Their teeth are small and do not form flat crushing plates as do the teeth of some other rays. There are five pairs of small gill openings, and the internal gill arches lack ridges or filter plates. Deepwater stingrays have no dorsal fin, but they do possess pelvic fins and a moderately large caudal fin that extends to the tip of the tail. The tail is slender and almost as long as the ray’s disc, but is not whip-like. The large, functional sting is located about halfway down the tail. Deepwater stingrays measuring 2.7 m long have been reported. (Compagno, 1999; Froese, Pauly, and Woodland, 2003; Last and Stevens, 1994; Nelson, 1994)

Although no information was found pertaining specifically to development in Plesiobatidae (deepwater stingrays), the one species in this family was formerly included in the family Urolophidae, and was not, based on texts used for this report, separated on the basis of any developmental characteristics. Therefore, a general idea of development in Plesiobatidae can be achieved on the basis of information known about its former family, Urolophidae. Deepwater stingrays, like other rays and their shark relatives, employ a reproductive strategy that involves putting a great investment of energy into relatively few young over a lifetime. Once sexually mature, these rays have only one litter per year, or less often, usually bearing two to four young. Since few young are produced, it is important that they survive, and to this end rays are born at a large size, able to feed and fend for themselves much like an adult. Rays develop from egg to juvenile inside the mother’s uterus, sometimes to almost half their adult size. In this system, called aplacental uterine viviparity, developing embryos receive most of their nutriment from a milky, organically rich substance secreted by the mother’s uterine lining. An embryo absorbs this substance, called histotroph, by ingestion, or through its skin or other specialized structures. Researchers have found that in some rays, the stomach and spiral intestine are among the first organs to develop and function, so that the embryo can digest the uterine “milk.” Rays’ eggs are small and insufficient to support the embryos until they are born, although the first stage of development does happen inside tertiary egg envelopes that enclose each egg along with egg jelly. The embryo eventually absorbs the yolk sac and stalk and the histotroph provides it with nutrition. Development in the uterus usually takes about three months. (Hamlett and Koob, 1999; Last and Stevens, 1994; Moyle and Cech, 2000)

No specific information regarding lifespans in Plesiobatidae was found, but in general rays, like their relatives the sharks, grow and mature slowly and are long-lived. (Last and Stevens, 1994; Moyle and Cech, 2000)

No information was found on the behavior of deepwater stingrays. They are known to live on the bottom in deep water. It is possible (although not certain) that they share some of the behaviors of their close relatives in the family Urolophidae, such as flapping their pectoral wings to uncover benthic (bottom-dwelling) organisms or to bury themselves in sand or mud. (Wheeler, 1985)

Rays perceive and interact with their environment using sensory channels common to many vertebrates: sight, hearing, smell, taste and touch. Rays also belong to a group of fishes, the elasmobranchs, whose electrical sensitivity seems to exceed that of all other animals. Elasmobranch fishes are equipped with ampullae of Lorenzini, electroreceptor organs that contain receptor cells and canals leading to pores in the animal’s skin. Sharks and rays can detect the electrical patterns created by nerve conduction, muscular contraction, and even the ionic difference between a body (i.e. of prey) and water. In lab experiments, members of the family Urolophidae (which formerly included Plesiobatidae) changed their feeding location according to artificially induced changes in the electrical field around them. Other experiments have demonstrated that cartilaginous fishes use electrosensory information not only to locate prey, but also for orientation and navigation based on the electrical fields created by the interaction between water currents and the earth’s magnetic field. Although some rays can produce an electric shock to defend themselves or stun prey, members of the family Plesiobatidae cannot. They are able, however, to inflict a venomous sting with their tail spine in defense. (Allen, 1996; Bleckmann and Hofmann, 1999)

Deepwater stingrays are carnivorous and feed on a variety of prey. Their diet includes polychaete worms, cephalopods, small deepwater fishes, lobsters, crabs, shrimps, and eels. (Froese, Pauly, and Woodland, 2003)

Ray spines have been found embedded in the mouths of many sharks. The great hammerhead Sphyrna mokarran, in particular, appears to specialize in eating stingrays. It uses its hammer head to knock a ray to the bottom, and then pins the ray, once again with its head, pivoting around to bite the ray’s disc until the ray succumbs and can be eaten. In addition to their defensive venomous sting, deepwater stingrays have dark coloring that blends in with their murky deepwater environment. (Helfman, Collete, and Facey, 1997; Last and Stevens, 1994)

In their benthic (on the bottom), pelagic (deepwater) habitat, deepwater stingrays affect the populations of prey animals such as invertebrates and small fishes. They in turn are eaten by larger fish. (Froese, Pauly, and Woodland, 2003; Helfman, Collete, and Facey, 1997)

No information was found regarding any negative impact on humans. Deepwater stingrays do possess a functional defensive sting, but they live at depths that remove them from any danger of being stepped on by people. (Compagno, 1999; Froese, Pauly, and Woodland, 2003)

Deepwater stingrays are not reported to be of use to humans. (Froese, Pauly, and Woodland, 2003)

There is no known conservation threat to Plesiobatidae at this time.