Family Urolophidae

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. (Böhlke and Chaplin, 1968; Compagno, 1999; Hamlett and Koob, 1999; Last and Stevens, 1994; Moyle and Cech, 2000; Nelson, 1994; The World Conservation Union, 2002; Wheeler, 1985; Wourms and Demski, 1993)

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. (Böhlke and Chaplin, 1968; Last and Stevens, 1994; Nelson, 1994; Wheeler, 1985)

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. (Böhlke and Chaplin, 1968; Last and Stevens, 1994; Nelson, 1994; Wheeler, 1985)

The family Urolophidae consists of two genera, Urolophus (also known as Trygonoptera) and Urotrygon, with about 35 species. (Nelson, 1994)

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. (Böhlke and Chaplin, 1968; Compagno, 1999; Last and Stevens, 1994; Nelson, 1994; Wheeler, 1985)

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. (Böhlke and Chaplin, 1968; Hamlett and Koob, 1999; Last and Stevens, 1994; Moyle and Cech, 2000; Wheeler, 1985)

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. (Last and Stevens, 1994; Moyle and Cech, 2000)

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. (Böhlke and Chaplin, 1968; 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 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. (Allen, 1996; Bleckmann and Hofmann, 1999; Helfman, Collete, and Facey, 1997)

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. (Wheeler, 1985)

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. (Böhlke and Chaplin, 1968; Helfman, Collete, and Facey, 1997; Last and Stevens, 1994)

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. (Helfman, Collete, and Facey, 1997; Last and Stevens, 1994; Wheeler, 1985)

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. (Böhlke and Chaplin, 1968; Last and Stevens, 1994; Wheeler, 1985)

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. (Last and Stevens, 1994)

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. (The World Conservation Union, 2002)