Plesiobatidae

Diversity

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.

Geographic Range

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.

• Biogeographic Regions
• indian ocean
  • native native
• pacific ocean pacific ocean
  • native native

Habitat

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.

• Habitat Regions
• temperate temperate
• tropical tropical
• saltwater or marine saltwater or marine

• Aquatic Biomes
• pelagic pelagic
• benthic benthic

Physical Description

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.

• Other Physical Features
• ectothermic ectothermic
• bilateral symmetry bilateral symmetry
• venomous venomous

Development

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.

Reproduction

Only a few species of elasmobranch (subclass including all sharks and rays) fishes have been observed during courtship and mating. However, deepwater stingrays have a system that involves internal fertilization, so it can logically be inferred that mating communication between male and female must happen to an extent that allows the male to insert at least one of his two claspers (male reproductive organs that are modifications of the pelvic fins) into the female’s cloaca to deposit sperm. Elasmobranch fishes have relatively complex endocrine (hormonal) systems; based on knowledge of other vertebrates with similar systems, it is likely that females signal to males through chemical or behavioral cues to indicate when their hormonal state is appropriate for mating. In Urolophus jamaicensis , a close relative of deepwater stingrays researchers found that gland secretions seal the open groove on males’ claspers into a closed tube that protects semen from being diluted before it passes into the female. These secretions coagulate on contact with sea water, help transport sperm into the female, and provide lubrication for clasper insertion.

No reproductive information mentioning Plesiobatidae (deepwater stingrays) in particular was found. However, 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 reproductive characteristics. A general idea of reproduction in Plesiobatidae can be achieved on the basis of information known about its former family, Urolophidae . Pregnancy in at least some urolophids (stingarees, the close relatives of deepwater stingrays) lasts about three months, generally spanning some period in the spring, summer, and fall. It may take up to two years, however, for the egg follicle to accumulate enough yolk for ovulation (release of an egg to be fertilized). This means that at least some stingarees may have litters only once every two years, but it is likely that other groups within the family give birth on a yearly cycle. Information indicating which group Plesiobatidae falls into was not found. In general, within any given group of rays, individuals appear to go through mating, gestation, and parturition (birth) at the same time as all the other females in the group. Stingarees (and likely deepwater stingrays as well) usually bear between two and four young at a time, after nourishing the embryos with milky fluid (histotroph) secreted by the uterus (see Development for a description of this system, called aplacental uterine viviparity). In some groups the epithelium, or wall, of the uterus has evolved to form trophonemata, elongated villi that extend into the uterine cavity to provide greater surface area for respiratory exchange and histotroph excretion. This advanced system of nourishing young inside the uterus can produce offspring that are relatively large at birth (see Development). According to one investigator, a young ray is rolled up like a cigar during birth, which, along with the lubricating histotroph, facilitates the birth of such proportionally large young. The young ray then unrolls and swims away. Likewise, sting-bearing young are able to pass out of the mother’s body without stinging her because their stings are encased in a pliable sheath that sloughs off after birth.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
  • internal internal
• viviparous viviparous

No reported evidence of parental care in Plesiobatidae was found. After such extended nurturing inside their mothers’ bodies, young rays come into the sea quite able to feed and fend for themselves (see Development and Reproduction).

• Parental Investment
• no parental involvement

Lifespan/Longevity

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.

Behavior

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.

• Key Behaviors
• natatorial natatorial
• motile motile

Communication and Perception

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.

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical
• electric electric
• magnetic magnetic

Food Habits

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.

• Primary Diet
• carnivore carnivore
  • piscivore piscivore
  • eats non-insect arthropods

Predation

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.

Ecosystem Roles

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.

Economic Importance for Humans: Positive

Deepwater stingrays are not reported to be of use to humans.

Economic Importance for Humans: Negative

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.

• Negative Impacts
• injures humans
  • venomous venomous

Conservation Status

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

Additional Links

Encyclopedia of Life

Contributors

Monica Weinheimer (author), Animal Diversity Web.

References

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