Gymnuridae

Diversity

The family Gymnuridae contains two genera and about 12 species. Their pectoral fins form a disc that is much broader than it is long, forming “wings” that give the family its common name, butterfly rays. These rays are marine and occasionally estuarine, found worldwide in tropical and subtropical seas. They live on the bottom in open sandy areas, where they feed on a variety of invertebrates, fishes, and other small organisms. Some bear a venomous spine on their short tails, but butterfly rays are considered far less dangerous than the long-tailed stingrays. Like other rays, butterfly rays are viviparous, giving birth to live young after nourishing them inside the uterus (see Development and Reproduction).

Geographic Range

Members of the family Gymnuridae can be found worldwide in tropical and warm temperate seas. Their range in the Atlantic includes the Black Sea, and they are widely distributed in the Pacific and Indian oceans.

• 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
• atlantic ocean atlantic ocean
  • native native
• pacific ocean pacific ocean
  • native native
• mediterranean sea
  • native native

Habitat

Gymnuridae is a marine family, but are occasionally found in estuaries and river mouths as well. Butterfly rays are benthic (bottom-dwellers), most commonly found on the upper continental shelf. They prefer stretches of open sand and frequent shallow beaches and bays.

• 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

As their name indicates, butterfly rays have extremely wide pectoral fins that resemble butterfly wings. These fins create a disc that is more than 1.5 times as broad as it is long and extends forward to include the head, ending in a blunt, angular, short snout. The snout is rounder and blunter on females than on males, at least in the case of Gymnura micrura . The disc is brown, grayish, purplish or greenish in color, sometimes with pale and dark spots, and it may be naked or covered with small denticles. The head is even with the disc rather than elevated, giving the fish a very flat appearance from the side view. The eyes are located on the sides of the top of the head, with the spiracles (respiratory openings) close behind the eyes. At least one species, Gymnura australis , has a long tentacle near the back edge of the spiracles. Like all rays, butterfly rays have ventral gill openings. These form five small pairs and the internal gill arches do not have filter plates. Their mouths are relatively large and located under the end of the snout. Since their mouths are directed downward and often placed against the sand, bottom-living rays 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. Butterfly rays have small, cuspidate teeth that do not form the crushing plates found in many other rays. Their dorsal fins, if present, are small. They lack anal fins, and rather than a caudal fin they may have low dorsal and ventral ridges on the tail. The slender tail is much shorter than the disc. Some butterfly rays have a stinging spine behind the pelvic fins. These fish range in size from less than 0.5 m long to over 2 m long.

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

• Sexual Dimorphism
• sexes alike
• sexes shaped differently

Development

Members of the family Gymnuridae, 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. 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 Gymnura micrura , for example, the young ray’s net weight increases by 4900% from egg to birth, and are between 15-22 cm when they are born. Development in the uterus usually takes about two to four months. One species of butterfly ray, Gymnura natalensis , is born with an almost triangular shape, but develops much wider pectoral fins as it grows, and in adults the disc is more than twice as broad as it is long.

Reproduction

Only a few species of elasmobranch (subclass including all sharks and rays) fishes have been observed during courtship and mating. However, butterfly rays 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. 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. Butterfly rays usually bear between two and six live 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 Gymnuridae 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 passage 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 Gymnuridae 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 Gymnuridae 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 Gymnuridae does not include the largest rays and may not reach such extremes of longevity.

Behavior

At least one species of butterfly ray ( Gymnura micrura ) is migratory, moving into temperate waters during the warm season. Butterfly rays forage for food over the sandy bottoms on which they live. Some butterfly rays are equipped with a venomous tail spine that they use in defense. Due to their short tails and smaller spines, however, their stings are generally less fierce than those of their stingray relatives.

• Key Behaviors
• natatorial natatorial
• motile motile
• migratory migratory

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 Gymnuridae cannot. Some are able, however, to inflict a 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

Butterfly rays feed upon crabs, shrimps, various invertebrates, fishes, small crustaceans, and mollusks. They generally seek prey near the bottom of warm waters over sandy stretches.

• 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, many rays have drab coloring that matches the sand or mud bottom. Several species of butterfly ray can quickly change color to blend in with their background.

• 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

Although butterfly rays do not have any particular commercial importance, 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. In Australia, cuts from one species of butterfly ray are sometimes sold as skate flesh. 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.

• Positive Impacts
• food food

Economic Importance for Humans: Negative

Butterfly rays have little negative impact on humans. They are not known to destroy shellfish beds, and their spines, when present, are much less dangerous than those of their long-tailed relatives ( Dasyatidae ).

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

Conservation Status

Currently, there is no conservation threat to any member of this family.

Additional Links

Encyclopedia of Life

References

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