Myliobatidae

Diversity

Rays of the family Myliobatidae are well known for their extreme grace and great size. With three subfamilies containing seven genera and about 42 species, the family includes the eagle rays, manta or devil rays, and cownose rays (see Systematic/Taxonomic History). These are free-swimming rays with broad, powerful pectoral fins that can measure over 6 m from tip to tip (see Physical Description). Many members of the family are able to leap completely out of the water into the air. Rays, like their shark relatives (both fall within the superorder Euselachii ), have a reproductive strategy in which a great deal of energy is invested in relatively few young over a lifetime, which may last several decades. Myliobatids generate, depending on the subfamily, only one to six embryos each year, and these young are born live after growing to considerable size inside the mother’s uterus (see Development, also Reproduction). Despite the imposing size attained by many members of the family, these rays are not dangerous to humans. Mantas have tiny teeth and strain planktonic organisms (and sometimes small schooling fishes) from the water. Eagle and cownose rays have pavement-like teeth suited for grinding mollusks (see Food Habits). Their tail spines, when present, are used for defense. The worst damage caused by these rays is economic, for they are capable of destroying entire beds of cultivated mollusks or oysters (see Economic Importance to Humans). As of 1994 only one species was listed as vulnerable to extinction, but due to their “slow” reproductive strategy rays may have difficulty replenishing their numbers if human activity threatens them.

Geographic Range

Eagle, cownose, and manta or devil rays occur in tropical and warm temperate seas worldwide. They are found in the Atlantic, Pacific, and Indian oceans, as well as smaller seas. In the Atlantic some migrate as far north as the British Isles and Cape Cod.

• 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

Habitat

Myliobatidae is a marine family, although some eagle and cownose venture into estuaries and mangrove areas. Cosmopolitan in tropical and warm temperate seas, this family can be found near reefs, in coastal lagoons, and, especially in the case of manta rays, far out to sea. Many members of the family make summertime migrations into temperate waters. Although cownose and eagle rays spend time feeding on mollusks and other invertebrates on or near the substrate, they, along with the plankton-feeding manta rays, are free-swimming.

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

• Aquatic Biomes
• pelagic pelagic
• reef reef
• 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 Myliobatidae (eagle rays), Rhinopteridae (cownose rays), and Mobulidae (manta or devil rays) each stand as a separate family. Here we follow Nelson (1994), who acknowledges Compagno’s work but places the three groups, as subfamilies, into the single family Myliobatidae. The three subfamilies are Myliobatinae (eagle rays), with four genera and about 24 species; Rhinopterinae (cownose rays) with one genus and five species; Mobulinae (manta rays and devil rays) with two genera and 13 species.

Physical Description

In general, eagle, cownose, and manta or devil rays are medium to large, heavy-bodied fishes with winglike pectoral fins forming a disc that is much wider than it is long. The tail is much longer than the disc, has a filamentous end, and in many species bears one or more serrated stinging spines near its base, close behind the pelvic fins. In these rays the head is elevated and protrudes in front of the disc, but an anterior portion of the pectoral fins continues forward to form a subrostral lobe under the snout. In the subfamily Myliobatinae (eagle rays) this lobe is rounded. The subfamily Rhinopterinae gets its common name, cownose ray, from the indentation in the middle of the snout and forehead that forms a double fleshy lobe. In manta or devil rays (subfamily Mobulinae) this lobe takes the form of two long, mobile, cephalic fins, making manta rays the only living vertebrates with three pairs of functional limbs. These can be rolled into a spiral while swimming or extended like a funnel to guide planktonic organisms into the ray’s mouth. Mantas take in water through their mouths and filter out organisms with a highly developed branchial sieve. Their mouths are large and terminal or subterminal, and they have very small, cuspidate or flat-crowned teeth. The other two subfamilies, cownose and eagle rays, have small, subterminal mouths with one to seven rows of large, flat teeth that form pavement-like plates for crushing mollusks. The middle rows of teeth are much wider than the outer rows.

Some common characteristics shared by all three subfamilies are the presence of a moderately large dorsal fin over or just behind the pelvic fins, a very small or absent caudal fin, eyes lateral on the head, and two spiracles (respiratory openings) on top of the head just behind the eyes. Like other batoids, the lower eyelid is intact while the upper eyelid is fused to the eyeball. There are five pairs of gill openings, large in Mobulinae but small in Myliobatinae and Rhinopterinae. These rays are generally countershaded, ventrally white or pale and dorsally black, olive, gray, or brown. Some (certain species of eagle ray) have dorsal spots or bands. The disc may be naked or may be covered with small denticles. All three subfamilies can be large fish, with adults measuring between one and five m long. Manta rays (subfamily Mobulinae) are the largest of all rays, with some attaining pectoral fin spans of over 6 m, and weighing more than 1360 kg. Members of this subfamily are reported to be the morphologically the most highly evolved of all living elasmobranch fishes.

The male sexual organs are called mixopterygia or claspers, and can be found at the rear bases of the pelvic fins. There are two claspers, and each has a groove that fills with sperm before the clasper is inserted into a female. Female eagle, cownose, and manta rays have a uterus that is specially adapted to nurturing embryos outside of the egg (see Development and Reproduction below).

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

Development

Members of the family Myliobatidae, 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. One species, Aetobatus narinari , is known to reach sexual maturity at four to six years of age, but others may not reach maturity until much older. Once sexually mature, rays have only one litter per year, and in manta rays and some cownose rays a litter consists of only one embryo. 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. 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, so much so that in Rhinoptera bonasus , for example, the embryo’s net weight increases to 3,000 times that of the egg. Development from egg to term (birth) usually takes about two to four months. At birth the ray is fully developed and the umbilicus completely absorbed.

Reproduction

Only a few species of elasmobranch (subclass including all sharks and rays) fishes have been observed during courtship and mating. However, rays have a system that involves internal fertilization, and 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 in general are reported to engage in prolonged mating that may last up to four hours. According to one observation of manta rays, mating occurred right at the surface of the water, with a graceful undulating motion of their bodies, and the male alternately inserting his claspers (paired male reproductive organs, also known as mixopterygia) into the female. The pair did not copulate continuously, but swam about for short periods.

Rays bear young on a yearly cycle, although pregnancy usually lasts only several months, generally spanning some period in the spring, summer, and fall. Only a few species, like Myliobatis californicus with its nearly year-long gestation period, are known to differ from this pattern. 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. Eagle rays bear up to six young at a time, but all manta rays and at least some cownose rays bear only one pup in each cycle. A milky fluid (histotroph) secreted by the uterus nourishes the embryos (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 Myliobatidae 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

There was little specific information information found regarding lifespans in Myliobatidae, but in general rays, like their relatives the sharks, grow and mature slowly and are long-lived. It is known that spotted eagle rays reach sexual maturity after four to six years, and bat rays live about 23 years. 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.

Behavior

Eagle, manta, and cownose rays are more active than their bottom-dwelling stingray relatives. They swim freely in the water column, near reefs or over a continental shelf, often near the surface. Eagle rays occur alone, in pairs, or in schools of a hundred or more. Cownose rays have been observed in groups of 4000 to 6000, apparently migrating. Manta or devil rays, although frequently observed singly, also occur in small shoals of up to five individuals. Members of this family are famous for their ability to leap high into the air, sometimes turning a somersault. Some hypothesize that this activity serves to rid the rays of parasites, while others view the behavior as playful. Like other rays, they use their broad pectoral fins rather than the tail for forward propulsion. This results in their graceful “flying” motion through the water. Manta rays have been observed gliding near the surface with the upturned tips of their pectoral fins sticking up out of the water. Remoras often accompany manta rays, inside their mouths or clinging to their pectoral fins. Eagle rays seem to travel around in search of mollusks, which they dig up, using their snouts like a spade. Cownose rays often feed in schools, beating their powerful pectoral fins to expose buried shellfish.

• Key Behaviors
• natatorial natatorial
• diurnal diurnal
• 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 Myliobatidae cannot. Some of them are able, however, to inflict a venomous sting with their tail spine in defense. Rays sometimes let out a sound like a bark when captured.

• Communication Channels
• visual visual
• acoustic acoustic

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

Food Habits

Members of the subfamily Mobulinae, the manta and devil rays, strain their food from the water. The large cephalic fins on either side of the mouth (see Physical Description) can be extended to form a funnel that scoops planktonic organisms, small fishes and crustaceans into the ray’s mouth. Manta rays have a highly developed filter system on their gill arches that traps small animals taken in with water during respiration. The other two subfamilies, Myliobatinae (eagle rays) and Rhinopterinae (cownose rays), have interlocking plates of flat, pavement-like teeth suited for grinding mollusks and other hard-shelled prey. Some, however, have also been observed eating fish, octopus, and worms. Spotted eagle rays are able to crush clams and oysters and spit out the shells, accomplishing this so deftly that whole soft oysters, without the shell, have been found in the rays’ stomachs. Cownose and eagle rays use their powerful pectoral fins to fan the substrate, creating a suction that digs out buried clams, and then use the lower parts of their snouts to pry up the mollusks. All members of the family Myliobatidae appear to move around in search of concentrations of prey.

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

• Foraging Behavior
• filter-feeding filter-feeding

Predation

Although rays can grow very large, they are still preyed upon by other large fishes, especially 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.

Ecosystem Roles

Eagle, manta, and cownose rays are nearly cosmopolitan in tropical and warm temperate seas, and therefore are a consistent predator on populations of mollusks, crustaceans, planktonic organisms, and small fishes. They, especially smaller specimens, provide food for sharks and other large fishes.

Economic Importance for Humans: Positive

While little information was found regarding human use of the family Myliobatidae in particular, 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. Manta rays, however, along with rays that have two spines, are considered inedible. Aborigines use ray spines for spear tips, although this likely impacts stingrays of other families more than members of Myliobatidae. 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. Finally, some large rays are a popular part of public aquarium exhibits.

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

Economic Importance for Humans: Negative

Eagle rays and cownose rays are considered a nuisance wherever humans cultivate oyster or clam beds. They are capable of clearing an entire area of mollusks, and destroy the beds. If these rays perceive the need to defend themselves, they can deliver a stab with their tail spine that is reported to be excruciating. Members of Myliobatidae, however, cannot whip their tails about as effectively as other stingrays because their sting is located near the base of the tail. The largest members of the family, the manta rays, are plankton feeders and are harmless to humans and their commercial endeavors.

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

Conservation Status

As of 1994 only one species, giant devil ray , was listed as vulnerable to extinction. 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

Myliobatidae 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 Myliobatidae 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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