Dasyatidae

Diversity

The family Dasyatidae includes stingrays, or whiprays, and river stingrays, encompassing nine genera and about 70 species. Like other rays, they have enlarged pectoral fins that form a disc. In this family the disc stretches forward to include the head, and ranges from less than 30 cm to over 2 m in diameter. Stingrays can be found in all tropical and subtropical seas. River rays form a freshwater subfamily of Dasyatidae, and live only in fresh water in parts of South America and Africa. Most stingrays are benthic, burying themselves partially under sand or mud in relatively shallow water. This habit makes them easy to accidentally step on, and the sting they deliver in defense has made stingrays famous. They appear in the writings of Pliny, Homer, and Captain John Smith, and aboriginal peoples from various parts of the world have used stingray spines for spear tips and other weapons. Members of Dasyatidae are viviparous (bear live young), and invest a lot of energy in relatively few young over a lifetime. This reproductive strategy renders them potentially vulnerable to human activity.

Geographic Range

Stingrays of the subfamily Dasyatinae can be found in all tropical and subtropical seas. Members of the subfamily Potamotrygoninae are freshwater stingrays that occur only in the Atlantic and Caribbean watersheds of northern and central South America, and in rivers in West Africa.

• Biogeographic Regions
• nearctic nearctic
  • native native
• palearctic palearctic
  • native native
• oriental oriental
  • native native
• ethiopian ethiopian
  • native native
• neotropical neotropical
  • native native
• australian australian
  • native native
• oceanic islands oceanic islands
  • native native
• indian ocean
  • native native
• atlantic ocean atlantic ocean
  • native native
• pacific ocean pacific ocean
  • native native
• mediterranean sea
  • native native

• Other Geographic Terms
• cosmopolitan cosmopolitan

Habitat

Dasyatinae is primarily a marine subfamily, although some members live in brackish or fresh water. They are most common in shallow tropical waters but can be found in temperate regions as well. For the most part they live on the bottom, usually partially buried in sand or mud, sometimes near coral reefs. They may occupy turbulent intertidal waters, their flat bodies enabling them to hug the bottom, or live demersally (at the bottom) on continental shelves. Some are common in mangrove swamps. Others venture into the open ocean, with one species, the pelagic stingray , living entirely in the open ocean, away from the bottom. The subfamily Potamotrygoninae lives only in fresh water, sometimes found more than 1600 km away from the ocean. They lie buried in sand or mud in backwaters and shallows of rivers. Members of this group only occur in West Africa and the Atlantic drainages of South America. They do not appear in all South American Atlantic-draining river systems, however, and some, like Potamotrygon leopoldi , are only found in a single river. Their restricted habitat renders the group vulnerable to human activities (see Conservation).

• Aquatic Biomes
• pelagic pelagic
• benthic benthic
• reef reef
• lakes and ponds
• rivers and streams
• coastal coastal
• brackish water brackish water

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

Systematic and Taxonomic History

Compagno (in Hamlett, 1999) notes that “there are at least as many classifications…of cartilaginous fishes as authors who have published them,” and that “the higher classification of chondrichthyans is currently in a state of flux.” Although there is less disagreement regarding the classification of lower taxa (familial, generic and specific), there are still a number of divergences in opinion at the familial level. Compagno (in Hamlett, 1999) maintains his earlier (1973) classification, in which Dasyatidae (whiptailed stingrays), and Potamotrygonidae (river stingrays) each stand as a separate family. Here we follow Nelson (1994), who acknowledges Compagno’s work but places the two groups, as subfamilies, into the single family Dasyatidae. The two subfamilies are Dasyatinae , with six genera and about 50 species, and Potamotrygoninae , with three genera and about 20 species.

Physical Description

Stingrays of the family Dasyatidae have expanded pectoral fins that form a circular, oval, or rhomboidal disc. These fins extend forward to the snout, such that the head appears enclosed by the disc. The pectoral disc is no more than 1.3 times as wide as it is long. From the side the ray is relatively flat, and the head is even with the body. The eyes are located on the sides of the top of the head, with the spiracles (respiratory openings) close behind the eyes. Like all rays, they have ventral gill openings. These form five small pairs and the internal gill arches do not have filter plates. Their mouths, which contain fleshy papillae on the floor, are small and located under the end of the snout. Since their mouths are directed downward and often placed against the sand, stingrays use their spiracles rather than their mouths for water intake, and, if the gills are covered with sand, the spiracles are also used for expelling water. Stingrays have small to medium-sized teeth that do not form flat crushing plates. Teeth are arranged in rows, with some members of Potamotrygoninae having over 60 rows of teeth in each jaw, arranged in groups of five. Like other rays, stingrays have a spiral valve in their intestine that increases food absorption, and lack a swim bladder. Along with eagle rays ( Myliobatidae ), stingrays reportedly have the most complex brains of all elasmobranch fishes.

Their dorsal skin may be smooth, or covered with denticles or thorns. They do not have a dorsal fin. Some also lack a caudal (tail) fin, while in others the caudal fin is reduced to long dorsal and ventral fin folds that may or may not extend to the tip of the tail. The tail is usually longer than the disc and bears one or more long, serrated spines behind the pelvic fins. The spines, which are used only in defense, are modified placoid scales, tipped with barbs. Each spine has grooves on its underside that contain venom-producing soft tissue. Stingrays have been reported to whip their tails with such force that they can drive their spines, which may reach 40 cm long, through the wooden bottom of a boat. The stings are constantly being shed and replaced. Members of the subfamily Dasyatinae range from less than 1 m long to more than 4 m long. In at least one species, Dasyatis centraura , females are reported to be larger than males. Stingrays of the subfamily Potamotrygonidae (river stingrays) tend to be smaller, usually less than 30 cm in diameter and less than 1 m long, although a few may attain 2 m. A unique aspect of river stingrays is their chemical adaptation to fresh water; their blood contains very low concentrations of urea, and their rectal gland (used by fishes for salt secretion) is reduced. Some male river stingrays have more prominent cusps on their teeth than females do. Stingrays tend to have drab coloration, but river stingrays in particular often have various patterns and markings over the brown or gray background.

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

• Sexual Dimorphism
• sexes alike
• female larger

Development

Members of the family Dasyatidae, 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, stingrays have only one litter per year, usually bearing two to six 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 stingrays, 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. Embryos are so well nourished in the uterus that in Dasyatis americana , for example, the young ray’s net weight increases by 3750% from egg to birth. Development in the uterus usually takes about two to four months. At birth the ray is fully developed and looks like a small adult.

Reproduction

Only a few species of elasmobranch (subclass including all sharks and rays) fishes have been observed during courtship and mating. However, 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.

Rays bear young on a yearly cycle, although pregnancy usually lasts only several months, generally spanning some period in the spring, summer, and fall. River rays (subfamily Potamotrygoninae) begin breeding in September or October. The young are usually born in February, but the duration of pregnancy depends on the specific geographic region and altitude. Gestation may take up to 12 months. 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. Stingrays usually bear between two and six 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 Dasyatidae 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

Little specific information regarding lifespans in Dasyatidae was found, but in general rays, like their relatives the sharks, grow and mature slowly and are long-lived. Some researchers estimate that the largest sharks and rays may not reach maturity until 20 to 30 years of age, and that they may live to maximum ages of 70 to 100 years or more. The family Dasyatidae does not include the largest rays and may not reach such extremes of longevity.

Behavior

Marine stingrays of the subfamily Dasyatinae spend much of the time partially buried under sand or mud in warm shallow areas. They can, however, swim powerfully; some migrate seasonally as far north as the British Isles, and at least one species, the pelagic stingray lives swimming freely in the water column. They can be seen in large groups when migrating. At least one species, Taeniura lymma , moves in large numbers into shallow sandy areas along the Australian coast during high tide, and at low tide retreats to caves and ledges for shelter. Fishes in the freshwater subfamily Potamotrygoninae, or river stingrays, also lie half buried on the bottom. Members of both subfamilies, if stepped on, can whip their tails upward to deliver a penetrating sting with their spines.

• Key Behaviors
• natatorial natatorial
• motile motile
• migratory migratory
• 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, stingrays 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 Dasyatidae cannot. They are able, however, to inflict a venomous sting with their tail spine in defense.

• Communication Channels
• visual visual
• chemical chemical

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

Food Habits

Stingrays feed on mollusks, worms, crustaceans, fishes, clams, crabs, and shrimps. They uncover buried organisms by scooping the sand or mud with their pectoral fins. For some, turbulent coastal surf provides a constant flow of invertebrates. The pelagic stingray eats squid and jellyfish along with crustaceans and fish.

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

Predation

Although rays can grow very large, they are still preyed upon by other large fishes, especially sharks. Stingray 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, most stingrays have drab coloring that blends in with the sand or mud bottom. The color of Dasyatis americana , for example, varies depending on the color of the surface on which it lies.

• Anti-predator Adaptations
• cryptic cryptic

Ecosystem Roles

Stingrays are nearly cosmopolitan in tropical and warm temperate seas, and therefore are a consistent predator on populations of mollusks, crustaceans, worms, and fishes. They, in turn, provide food for sharks and other large fishes. Remoras sometimes accompany adult rays.

Economic Importance for Humans: Positive

Cartilaginous fishes in general are important to humans in a number of ways. Australian Aborigines have eaten rays for centuries. They determine whether a seasonal catch is ready to eat by checking a ray’s liver; if it is oily and pinkish white, the ray is suitable for eating. Rays that have two spines, however, are considered inedible. Australian Aborigines, Malayans, tribes in South and Central America, and West Africa, and peoples of the Indo-Pacific have used ray spines for spear tips, daggers, or whips. Rays are considered food fish in Australia, Europe, and parts of Asia, and in some places are among the most highly priced fishes. Like shark fins, fins of some rays are harvested in Asia for soup and as an aphrodisiac. Cartilaginous fishes are used for medical purposes as well. Chondroiten, used as skin replacement for burn victims, is derived from the fishes’ cartilage. Other extracts from cartilage help suppress tumors and may assist cancer treatment. Some large rays are a popular part of public aquarium exhibits. Reportedly there has been an increase in the aquarium trade of fishes in the subfamily Potamotrygoninae as well.

• Positive Impacts
• pet trade pet trade
• food food
• source of medicine or drug source of medicine or drug

Economic Importance for Humans: Negative

Some stingrays, like Dasyatis fluviorum , wreak havoc with oyster farms and cultivated clam beds. They can crush large quantities of this favored prey, which can be costly for the owners of the beds. The most well known negative impact of stingrays is the excruciatingly painful sting they can inflict. Stingrays and river rays frequently bury themselves in the sand or mud in shallow water, which makes it probable that waders will step on them. If stepped on, a ray will thrust its tail forcefully upward into the victim. The serrated, barbed spine not only delivers venom, but also creates a deep wound often worsened by the thrashing of the ray. Fishermen in rivers and coastlines of many parts of the world fear the often large, abundant stingrays. Victims of stings generally recover, but fatalities have occurred. Reportedly, soaking the injured part of the body in hot water (about 50 degrees Celsius) for 30 to 90 minutes can alleviate pain from the venom.

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

Conservation Status

As of 1994 eight species within Dasyatidae were listed as endangered or vulnerable to extinction, with one other species nearing vulnerable status. Five species of river ray were listed as potentially endangered, but there was insufficient data to make a definite determination. However, sharks and rays in general are vulnerable to overfishing. They grow and mature slowly, and the size of the adult population closely determines the number of young produced, due to their “slow” reproductive strategy of investing a great deal of energy in relatively few young over a lifetime.

Other Comments

Dasyatidae is a family of cartilaginous fishes (class Chondrichthyes ), the oldest surviving group of jawed vertebrates. This group was the first to bear live young, nourish developing embryos by means of a placenta, and to regulate reproduction and embryonic growth hormonally. Batoids (skates and rays) split off from the sharks in the early Jurassic period. Fossil records of Dasyatidae date back to the upper Cretaceous period.

Additional Links

Encyclopedia of Life

Contributors

Monica Weinheimer (author), Animal Diversity Web.

R. Jamil Jonna (author), Animal Diversity Web.

References

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