Urolophidae

Diversity

The family Urolophidae, also known as stingarees, consists of two genera and about 35 species. They are bottom-dwelling rays in warm seas, usually lying partially buried under the sand. Their rounded pectoral discs are colored to blend in with the sand, mud, or rocks on which they live. Urolophids are relatively small rays, and feed on a variety of invertebrates, small fishes, and crustaceans. Their tails, distinguished by the presence of a well-developed caudal fin, are equipped with one or more serrated stinging spines. Like other rays they are viviparous; urolophids give birth to between two and four young each year, or in some cases, every two years. Because of their low birth rates and sometimes restricted range, urolophids are susceptible to human activity, although only one species is currently known to be threatened.

Geographic Range

Urolophids can be found in the eastern Indian Ocean, the western Pacific, the eastern Pacific from California to Chile, and the western Atlantic, including the Caribbean. They are not known in the western Indian Ocean, the Mediterranean, or the eastern Atlantic.

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

Habitat

Urolophidae is a marine family, although some members enter estuaries. Restricted to tropical and warm temperate waters, urolophids are bottom-dwellers along coastlines and along the continental shelf. Most live in relatively shallow water but some occupy depths of at least 700 m down the continental slope. They generally prefer sandy bottoms in which they can bury themselves, but a few species live on rocky substrates (bottoms) or in association with sea vegetation such as kelp. Urolophids tend to have patterns and coloring that blend in with their environment.

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

• Aquatic Biomes
• benthic benthic
• coastal coastal
• brackish water brackish water

• Other Habitat Features
• estuarine estuarine
• intertidal or littoral intertidal or littoral

Physical Description

Urolophids, or stingarees, are rays with a rounded, oval, or rhomboidal disc created by the pectoral fins. The disc is less than 1.3 times as broad as it is long. Their snouts are confluent with the rest of the disc. From the side they appear relatively flat, with the head not elevated. The spiracles (respiratory openings) are close behind the eyes, which are dorsolateral (above and to either side) on the head. The mouth is small and located on the underside of the snout, and often has several papillae on its floor. Teeth are small and do not form flat crushing plates as in some other rays. There are five pairs of small gill openings, and the internal gill arches do not have filter plates or ridges. Some stingarees lack a dorsal fin; in others the fin is small, located just in front of the sting and behind the pelvic fins. The serrated stinging spine, located about halfway down the tail, is large and functional. A distinguishing feature of these rays is the presence of a moderately large, elongated caudal fin that extends to the tip of the tail. In the genus Urolophus the caudal fin lobes are confluent, while in the genus Urotrygon they are separate. The tail is slender but not whip-like, and shorter than in stingrays . In coloration stingarees range from uniform grayish, yellowish, or brownish, to patterns of spots, reticulations, or dark mask-like bands. Their discs may be smooth or covered with small denticles. These rays tend to be small, not more than 76 cm in length.

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

Development

Members of the family Urolophidae, like other rays and their shark relatives, employ a reproductive strategy that involves investing large amounts of energy into relatively few young over a lifetime. Once sexually mature, stingarees have only one litter per year, 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 elasmobranchs have been observed during courtship and mating. However, stingarees 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 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.

Pregnancy in at least some urolophids 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) as in the case of Urolophus halleri . 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. 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 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 is modified 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 post-birth parental care in Urolophidae 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
• female parental care female parental care

Lifespan/Longevity

Little specific information regarding lifespans in Urolophidae was found, but in general rays, like their relatives the sharks, grow and mature slowly and are long-lived.

Behavior

Urolophids (stingarees) appear to spend most of their time partially buried in the sand, or, for a few species, lying on rocks or among sea vegetation. They flap their pectoral fins to bury themselves with sand, and also to uncover the benthic (bottom-dwelling) organisms on which they feed. If stepped on they can thrust their flexible tails upward to deliver a sting with their tail spines.

• Key Behaviors
• natatorial natatorial
• motile motile
• sedentary sedentary
• social social

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 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 Urolophidae cannot. They are able, however, to inflict a venomous sting with their tail spine in defense.

• Communication Channels
• visual visual
• tactile tactile
• chemical chemical

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

Food Habits

Many stingarees feed on fishes, worms, shrimps, and other small organisms they uncover when they flap their pectoral fins along the bottom. Some are able to eat hard-shelled mollusks and crustaceans.

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

Predation

Ray spines, some of them likely belonging to urolophids, have been found embedded in the mouths of many sharks. The great hammerhead Sphyrna mokarran , in particular, appears to specialize in eating stingrays. They use their hammer-shaped heads to knock a ray to the bottom, and then pin 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, most stingarees have cryptic coloring that blends in with the sandy or rocky bottom. Some researchers describe stingarees as almost impossible to find unless they move.

• Anti-predator Adaptations
• cryptic cryptic

Ecosystem Roles

In their benthic (on the bottom), warm, usually shallow-water habitat, stingarees affect the populations of prey animals such as invertebrates and small fishes. They in turn are eaten by larger fish and humans.

Economic Importance for Humans: Positive

Stingarees are seldom used commercially even though large numbers are frequently caught in nets, but several species have edible flesh. Some are reported to be “chewy unless prepared properly.” Native peoples in many parts of the family’s range have used ray spines for spear tips, daggers, or whips.

• Positive Impacts
• food food

Economic Importance for Humans: Negative

Stingarees can cause serious wounds with their tail spines. The serrated spine tip can be difficult to remove without surgery if it breaks off in the wound. Because they tend to occupy shallow water and are often colored to blend in with the bottom, they are a hazard to waders. Some fishermen walk with a “stingray shuffle” to make the rays swim away without stepping on them and getting stung.

• Negative Impacts
• injures humans
  • bites or stings
  • venomous venomous

Conservation Status

One species in the Australian genus Urolophus is listed as near threatened. It lives in an area of intense fishing pressure, and females often abort embryos when captured. These factors, along with low fecundity (they bear only two young at a time), making it vulnerable to human activity. Other urolophids, sharing these characteristics, may become threatened in the future.

Additional Links

Encyclopedia of Life

Contributors

Monica Weinheimer (author), Animal Diversity Web.

References

Allen, T. 1996. Shadows in the Sea: The Sharks, Skates, and Rays . New York, NY: Lyons and Buford.

Bleckmann, H., M. Hofmann. 1999. Special Senses. Pp. 300-328 in Sharks, Skates, and Rays . Baltimore, MD: The Johns Hopkins University Press.

Böhlke, J., C. Chaplin. 1968. Fishes of the Bahamas and Adjacent Tropical Waters . Wynnewood, PA: Published for the Academy of Natural Sciences of Philadelphia by Livingston.

Compagno, L. 1999. Systematics and Body Form. Pp. 1-42 in Sharks, Skates, and Rays . Baltimore, MD: The Johns Hopkins University Press.

Hamlett, W., T. Koob. 1999. Female Reproductive System. Pp. 398-443 in Sharks, Skates, and Rays . Baltimore, MD: The Johns Hopkins University Press.

Hamlett, W. 1999. Male Reproductive System. Pp. 444-470 in Sharks, Skates, and Rays . Baltimore, MD: The Johns Hopkins University Press.

Helfman, G., B. Collete, D. Facey. 1997. The Diversity of Fishes . Malden, MA: Blackwell.

Last, P., J. Stevens. 1994. Sharks and Rays of Australia . Australia: CSIRO.

Liem, K., A. Summers. 1999. Muscular System: Gross Anatomy and Functional Morphology of Muscles. Pp. 93-114 in Sharks, Skates, and Rays . Baltimore, MD: The Johns Hopkins University Press.

Moyle, P., J. Cech. 2000. Fishes: An introduction to ichthyology – fourth edition . Upper Saddle River, NJ: Prentice-Hall.

Nelson, J. 1994. Fishes of the World – third edition . New York, NY: John Wiley and Sons.

The World Conservation Union, 2002. "IUCN 2002" (On-line). 2002 IUCN Red List of Threatened Species. Accessed December 03, 2003 at http://www.redlist.org/ .

Wheeler, A. 1985. The World Encyclopedia of Fishes . London: Macdonald.

Wourms, J., L. Demski. 1993. The reproduction and development of sharks, skates, rays, and ratfishes: introduction, history, overview, and future prospects. Pp. 7-19 in The Reproduction and Development of Sharks, Skates, Rays, and Ratfishes . Dordrecht, The Netherlands: Kluwer Academic Publishers.