Rajiformes

(vertebrate zoology) The equivalent name for Batoidea.

(Skates and rays)

Class: Chondrichthyes

Order: Rajiformes

Number of families: 20

Evolution and systematics

Despite the fact that skates and rays greatly outnumber their shark relatives within Chondrichthyes, they have received far less recognition. As of 2002 there were about 513 recognized species of skates and rays, compared with about 390 species of sharks. Sharks are topics of books, television documentaries, and news coverage, whereas skates and rays get little press.

Skates and rays, sharks, and chimaeroids are members of Chondrichthyes, the cartilaginous fishes. The cartilaginous fishes are distinguished from other jawed fishes (Osteichthyes, or bony fishes) by several characters. (1) The endoskeleton consists of calcified cartilage. (2) The tooth-bearing jaws are the palatoquadrate and Meckel's cartilages, rather than being comprised of dermal bones. (3) Lungs and swim bladders are absent. (4) A single boxlike skull supports the brain and sense organs. (5) Males possess copulatory organs that are extensions of the pelvic girdle and internally fertilize the females.(6) Fins are supported by elastic connective tissue rays, or ceratotrichia. (7) The body is covered with dermal denticles or placoid scales, toothlike structures with enameloid crowns and dentine bases.

Rajiformes include the electric rays, sawfishes, guitarfishes, skates, and stingrays; their fossil record dates back to the Lower Jurassic (150 million years ago [mya]) (guitarfishes). All of the major taxa are known by the Upper Cretaceous (100 mya) to the Paleocene (50 mya). A majority of the early fossils come from northern Africa and southern Europe, areas that in the late mid-Mesozoic and Lower Cenozoic were part of the Tethys Sea, a shallow tropical sea that separated the northern and southern continents over much of this period. Although the fossil record spans more than 150 million years, the record is very incomplete, owing to the paucity of hard body parts of skates and rays. Unlike the bony fishes, skates and rays (and all chondrichthyans for that matter) lack large bony external and internal structures that readily fossilize. In many cases, the fossil chondrichthyans are represented solely by teeth or enlarged scales. Teeth and scales serve to identify the fossils as skates and rays but provide little information on body structure or phylogenetic relationships.

Skates and rays have been classified variously within the cartilaginous fishes. Traditionally, they have been considered an equivalent group or sister group of the sharks. More recently, they have been grouped within a subsection of the sharks. Currently, based on morphological characters, they are considered to be a sister group of the angelsharks and sawsharks within the squalomorph sharks. The squalomorph sharks, in turn, are the sister group of the galeomorph sharks, a group that includes the horn sharks, carpet and nurse sharks, catsharks, mako sharks and white sharks, and requiem sharks. The squalomorph sharks include the sixgill and sevengill sharks and dogfish sharks in addition to the angelsharks, sawsharks, and skates and rays. These relationships make intuitive sense, because both squalomorph sharks, except for the most primitive members, and skates and rays lack an anal fin. Moreover, both groups are, for the most part, adapted for a benthic existence, and the sequential squalomorph sister taxa of skates and rays, angel- and sawsharks, share many anatomical characters with the skates and rays.

Recent molecular data, however, offer some support for the traditional relationship of skates and rays as a sister group of the sharks. If the molecular data are correct, suggesting that sharks and skates and rays shared a common ancestor, then squalomorph sharks and skates and rays independently acquired their adaptations for benthic habitats. As of 2002 the relationships of the skates and rays to the remainder of the cartilaginous fishes remain uncertain. Problems with classification of both the skates and rays and the sharks are due to their geological age and mediocre fossil record and, possibly, their parallel evolutionary trajectories.

As of 2002 the 513 species of skates and rays are classified within 63 genera and 20 families. The families are classified into eight suborders, although the higher-level classification is a work in progress. Most of the species are in the electric ray family Narcinidae (about 30 species), the guitarfish family Rhinobatidae (about 40 species), the skate family Rajidae (about 250 species), the round ray family Urolophidae (about 25 species), the freshwater stingray family Potamotrygonidae (about 25 species), and the stingray family Dasyatidae (about 63 species).

Physical characteristics

Skates and rays share a large number of characters, and as of 2002 there is little doubt that they form a natural group of fishes. They are defined largely by their adaptations for a benthic existence. In fact, they may have the ideal structure among vertebrates for such an existence. All taxa are flattened, at least anteriorly, with the pectoral fins joined to the head and trunk to form a disc. Eyes and spiracles are located on the upper side of the head; the nostrils, mouth, and gill slits are found on the ventral side of the head. Only a few sharks, orectolobids (carpet sharks), and squatinids, but no bony fishes, approach skates and rays in their degree of dorsoventral flattening. Flatfishes (bony fishes) are greatly laterally flattened or compressed, with both eyes on the same side of the head and the mouth contorted slightly to greatly. In other words, to achieve the degree of flatness of skates and rays, flatfishes had to become asymmetrical. Apparently, sharks and rays have the evolutionary potential to become depressed, whereas bony fishes are morphologically constrained from assuming such a posture.

Many of the distinguishing characteristics of skates and rays are concerned with the structural demands of their depressed shape. The vertebrae between the cranium and the shoulder girdle are fused into a tube (synarchial), the upper cartilage of the shoulder girdle (suprascapula) is either joined or articulated with the vertebral column or synarchial, the anterior pterygium cartilage of the shoulder girdle indirectly or directly joins the side of the cranium, and the upper jaw lacks an articulation with the cranium. All but the last of these adaptations provide the structure to support the expanded disc. The pectoral fin is supported anteriorly by the cranium, medially by the vertebral column, and posteriorly by the trunk, and this support system has allowed massive pectoral fins to develop in the majority of skates and rays. The massive pectoral fins made it possible for these ray fishes to swim by means of undulating or oscillating their pectoral fins. Freeing the upper jaw from the cranium provided greater versatility in feeding both on and in the benthic habitat.

Skates and rays vary in their degrees of flatness and disc development. The more primitive taxa, sawfishes (pristids) and guitarfishes (rhinids, rhinobatids, platyrhinids, and zanobatids), have rather small discs and stout, sharklike tails. These fishes swim by laterally undulating the trunk muscles like the sharks. Electric rays, thought to be the most primitive of the skates and rays, likewise have stout tails but rather expansive discs. They retain a large number of primitive characteristics, however. The large discs house the branchial electric organs that distinguish the group, and large discs may have been independently derived in this group to house the electric organs.

The skates (rajids) and stingrays (plesiobatids, hexatrygonids, urolophids, potamotrygonids, urotrygonids, dasyatids, gymnurids, myliobatids, rhinopterids, and mobulids) have very large, laterally expanded discs and slender to very slender tails. The tails of skates are slender, whereas those of stingrays are slender to very slender and mostly whiplike, and they bear one or more serrated spines. Both skates and stingrays swim by vertically undulating their discs or, in the case of the more derived stingrays, by vertically oscillating the discs like birds in flight. Stingrays have a ball-and-socket connection between the shoulder girdle and the vertebral column and an extra synarchial behind the shoulder girdle. Some of the derived forms of stingrays can generate enough speed to leap clear of the water. Skates lack the ball-and-socket connection, but they have bilobed pelvic fins with a finger-like anterior lobe. The anterior lobe can be used to "walk" or "punt" along the bottom. Punting is a unique locomotive gait of skates.

The arrangement of the eyes on the upper surface and the mouth on the lower surface means that skates and rays are unable to see their prey except at a distance. In fact, vision may play only a secondary role in feeding. Like sharks and some bony fishes, skates and rays have electric organs, ampullae of Lorenzini, symmetrically arranged around their mouths. The ampullae are at the end of pores and tubes filled with a conductive substance and are capable of responding to small electric currents, such as those produced by the muscle contractions of prey organisms. Skates and rays also have closed lateral line systems on the ventral surface that are sensitive to small jets of water pressure, such as those produced by bivalve mollusks that are often the prey of these fishes. When a jet of water strikes the surface in the vicinity of the canal system, it causes the fluid in the canal system to flow. Sensory cells lining the canal system perceive the moving current.

Members of two suborders, electric rays and skates, produce electric currents by means of modified muscle cells. Some of the gill arch or branchial muscles of electric rays are modified into electric cells that can produce up to 200 volts. These cells occupy most of the lateral area of the disc and are used to stun prey and defend against predation. Some electric rays have an auxiliary electric organ behind the main one that produces weak electric currents that may be used for communication among members of a population. Discharges of one individual can be perceived by the ampullae of Lorenzini of another individual. Skates have weak electric organs along the sides of the tail that apparently are used in communication among members of a population.

Skates and rays vary considerably in body size. Some electric rays (Narcinidae) mature at about 4 in (10 cm) in total length. Some skates (Rajidae) mature at about 6 in (15 cm) in total length and are probably the lightest of the chondrichthyans because of their very slender tails and thin discs. Sawfishes, on the other hand, can reach up to 23 ft (7 m) in length and have tooth-bearing rostra almost 6.6 ft (2 m) long. Manta rays (Manta) can reach 22 ft (6.7 m) in width and have been reported to be up to 29.8 ft (9.1 m) in width.

The coloration of skates and rays appears to be related largely to camouflage. Species that occur in shallow water tend to be dark yellow-brown, various shades of gray, or brown to black dorsally and cream to white ventrally. Those in mucky waters tend to be unpatterned dorsally, whereas those in clear water often are patterned with wavy lines, stripes, bars, or ocelli. The patterning apparently functions in obscuring their outline and thus aids in making them unrecognizable to potential predators. There is little if any sexual dichromism. Species in deep water are typically dark colored dorsally and ventrally.

Distribution

Skates and rays are found worldwide in the marine environment from the shoreline to about 9,842 ft (3,000 m) and in many tropical freshwaters. The greatest diversity of species to subordinal taxa is in the tropical Indo-West Pacific, although not all of the higher taxa are represented in this region. The Indo-West Pacific region includes the tropical waters from the east coast of Africa to the east coast of Australia and Japan. With the exception of the stingrays, rajiforms are almost entirely absent from the coral islands of the central and western Pacific.

With few exceptions electric rays are limited to tropical and warm temperate seas over continental shelves. Torpeninid electric rays range into temperate latitudes, and some narcinid electric rays (Benthobatis) occur to depths of about 3,281 ft (1,000 m). Their eyes are minute and covered with skin, suggesting that they are either blind or respond only to light. Narkid and hypnid electric rays are limited to the tropical waters of the Indo-West Pacific.

Sawfishes are tropical and apparently limited to coastal, brackish, and freshwaters. Guitarfishes are tropical to warm temperate in coastal and brackish waters. One of the families is limited to the tropical Indo-West Pacific, and the other three families are most diverse in this region.

Skates (rajids) and stingrays (myliobatoids) largely have complementary distributions. In tropical waters skates are absent on inner continental shelves but are abundant in deeper water and in both the north and the south of tropicalsubtropical regions. Stingrays, on the other hand, are limited primarily to shallow tropical seas. The stingray families Plesiobatidae (with one species) and Hexatrygonidae (with one to five species) are exceptions and occur to depths of 1,640–3,281 ft (500–1,000 m) under tropical seas. Skates are the only rajiforms that occur at polar latitudes and that are common at great depths, to about 9,842 ft (3,000 m). Few species are found in estuaries, and only one species occurs in freshwater (Dipturus sp. from Bathurst Harbour, near Port Davey, Tasmania). Stingrays, on the other hand, are abundant in low-salinity regions, and a number are found strictly in tropical freshwaters. The stingray family Potamotrygonidae is limited to the freshwaters of South America.

Habitat

The majority skates and rays are benthic in marine habitats. The more sharklike forms, such as the sawfishes and guitarfishes, rest on the bottom and swim immediately over the bottom. The more depressed forms, such as the electric rays, skates, and most of the stingrays, rest and swim close to the bottom and often partially bury themselves in the bottom. They undulate their greatly expanded discs while lying over soft substrates to cover themselves partially. When partially covered with sediment, skates and rays ventilate by bringing water in through their spiracles and expelling the water through their gill slits. One species of Dasyatidae (the pelagic stingray, or Pteroplatytrygon violacea), the Myliobatidae (eagle rays), Rhinopteridae (cownose rays), and Mobulidae (manta rays) are largely pelagic or at least capable of sustained swimming. Pteroplatytrygon violacea and the mobulids spend most of their time and feed in the water column and are at least partially oceanic. The mobulids, however, appear to feed near continents, where upwelling of currents leads to high concentration of zooplankton. Myliobatidae are capable of leaping from the water like the mobulids, but they feed on the bottom.

As a group, chondrichthyans are uncommon in freshwater. Their absence in freshwater is related at least in part to the retention of urea in their tissues. Urea acts as a salt and makes chondrichthyans about as salty as seawater. This reduces the costs of osmoregulation in marine waters but increases its costs in freshwaters. Chondrichthyans that enter freshwater apparently have the ability to reduce the urea content in their tissues. The ability to inhabit freshwater is more widespread among skates and rays than sharks. All species of sawfishes and many species of stingrays either move back and forth between saltwater and freshwater or reside permanently in freshwater. Some sawfishes become more freshwater tolerant with age. Numerous dasyatid stingrays move in and out of freshwater. Some dasyatid stingrays reside in freshwater, and the stingray family Potamotrygonidae is limited to the freshwaters of South America. In fact, the potamotrygonids have lost the ability to conserve urea.

Behavior

The majority of skates and rays are rather docile, both on a daily basis and over long periods of time. Some of the electric rays and stingrays that live in shallow water may limit daily excursions to moving in and out with the tide. During high tide they move shoreward and burrow into sandy bottoms; at low tide they abandon these depressions and construct similar abodes in deeper water. Limited data from tagging studies suggest that skates are rather provincial. Templeman found that most specimens of a particular skate (Amblyraja radiata) tagged off Newfoundland were recaptured within 60 mi (97 km) up to 20 years from the time that they were tagged. In temperate regions benthic species of skates and rays move northward and southward with vernal warming and cooling. More active species, such as eagle rays, cownose rays, and mantas, may be wide ranging, although there are reports that individuals of Manta birostris return to the same feeding areas on a yearly basis.

The social behavior and communication of rajiforms are poorly known, but some observations suggest that skates use electrical discharges of their tail electric organs for communication. The organs discharge posteriorly, and males follow directly behind females during mating. All skates have weak electric organs, and the histologic characteristics of the electric cells vary among species. It is thus possible that different species have distinctive electric discharges and that these differences may be used as mate-recognition systems to aid in seeking mates of the same species. Such systems are well known for elephantnose fishes.

Little is known concerning the social relationships between skates and rays and other organisms. There are numerous observations, however, that Manta birostris enter shallow water reef areas to be cleaned of ectoparasites by cleaning bony fishes. One species of remora often hitches a ride on Manta birostris and even enters the ray's cloaca for extended periods of time. The remora probably feeds on ectoparasites and possibly on the ray's feces.

Feeding ecology and diet

The majority of skates and rays can be considered generalist benthic predators that feed on the more abundant benthic invertebrates and small to moderately sized bony fishes. Some groups, such as the electric rays and sawfishes, have specialized devices for capturing food. The branchial electric organs of electric rays are used to stun fishes and invertebrates, which then are quickly swallowed. Sawfishes use their tooth-bearing rostral blades to disable schooling fishes and to dislodge invertebrates from the substrate. Myliobatid and rhinopterid stingrays have jaw teeth that are fused into crushing plates that enable these fishes to crush bivalve clams, oysters, and mussels. Mobulids have specialized lateral extensions of their rostra (cephalic fins); large, oval-shaped mouths; and filter plates running between their gill arches that allow them to strain zooplankton from the water column. The cephalic fins direct the zooplankton into the mouth, and the filter plates separate the zooplankton from the water that flows over the gill slits. The ampullae of Lorenzini and the closed lateral line systems of skates and stingrays permit these fishes to sense infaunal organisms that can be sucked out of the sub-stratum by means of their protrusible jaws and the sucking action of the mouth and gill cavities.

Skates and rays are preyed upon by sharks, and small skates and rays, including egg capsules of skates, are occasionally consumed by large skates and rays.

Reproductive biology

Mating has not been observed frequently in rajiforms, but all species practice internal fertilization. Males possess extensions of their pelvic fin cartilages (claspers) that are inserted into the cloaca of females and serve to transmit sperm into the oviduct. Only one of the pair of claspers is inserted at a time. Generally, copulation occurs between a single male and a single female. In many cases the males bite the anterior margin of the female's disc, to enable them to insert the clasper. Males of many taxa of skates and stingrays have sharp, pointed teeth, unlike the flat teeth of females; these teeth enable the males to remain in contact during copulation. Skates also have sharp, pointed and often barbed, clawlike thorns near the outer corners of their discs, which are used as additional points of contact during copulation.

Fertilization takes place in the anterior section of the oviduct of the female, and the fertilized egg then is encapsulated in the oviductal gland. The encapsulated egg descends into the uterus; in most rajiforms the egg is retained in the uterus, and development is termed "viviparity without a placenta," also termed ovoviviparity. The egg capsules are generally thin, and the embryos may be encapsulated only during the early stages of development. In addition to the yolk supplied with the encapsulated eggs, nutrients are available to the embryos from the uterine wall of the female. Internal development extends from several months to nearly a year among the various taxa of rajiforms. The young or neonates are immature copies of the adults at birth, and the mother provides no parental care.

In skates the encapsulated egg is shed to the environment, and development is termed "oviparity." As with the other rajiforms, the egg is fertilized and encapsulated in the oviductal gland, but the egg capsule is thick, collagenous, and rectangular shaped, with a horn at each corner. The young remain in the capsule from several months to more than a year. Females do not offer any protection for the egg capsules, but it is possible that they seek special areas in which to release them.

Conservation status

Skates and rays have long been exploited by artisanal fisheries and small-scale fisheries in developing countries, but they have not been targeted by large-scale fisheries. Despite the lack of directed fisheries, humans have had a negative impact on populations of many species over the past half-century. Slow growth rates and low reproductive potentials make chondrichthyans, including skates and rays, vulnerable even to modest rates of exploitation. Chondrichthyans have much lower growth rates and fecundity than bony fishes. Thus a fishery directed at bony fishes may inadvertently negatively affect skates and rays before the fishery overexploits the targeted bony fishes. Inshore tropical habitats occupied by many skates and rays have been degraded by human activities, and commercial shrimp fishing has accidentally captured inshore species, such as guitarfishes and sawfishes. Bottom trawling (for shrimps, for example) unintentionally captures large quantities of skates and rays.

As of 2002 the IUCN listed 26 skates and rays as Vulnerable, Endangered, or Critically Endangered. These are species of sawfishes, guitarfishes, skates, or stingrays, and they occupy tropical freshwaters, inshore tropical waters, or continental shelf habitats in temperate regions that are under heavy fishing pressure. All seven species of sawfishes are listed as Endangered (5 species) or Critically Endangered (2 species). Two guitarfishes are listed as either Vulnerable or Critically Endangered. A total of six skates are listed as Near Threatened/Lower Risk (3 species), Vulnerable (1 species), or Endangered (2 species). Eleven stingrays are listed as Near Threatened/Lower Risk (1 species), Vulnerable (4 species), Endangered (5 species), or Critically Endangered (1 species). The Thailand population of Himantura chaophraya is Critically Endangered.

Significance to humans

For the most part, skates and rays are not considered high-quality food items. They do enter artisanal fisheries, however, and are landed by numerous commercial fisheries in the Far East and in Europe. Skates and rays are consumed fresh, dried, or salted.

The skin of skates and rays is very tough and can be used as leather. Handles of samurai swords may be covered with guitarfish skin. Various ethnic groups of the Indo-West Pacific once used the teeth of sawfishes and the serrated spines of stingrays as war clubs. Native Americans of the Amazon and Orinoco River drainages capture freshwater stingrays for food and use their serrated spines for arrowheads or as implements for self-mutilation. Today, dried, mutilated skates and rays are sold in seashore curiosity shops.

With the exception of stingrays and large electric rays, rajiforms are not harmful to humans. Bethnic stingrays, , often lie partially buried in the sand along beaches frequented by human bathers. Bathers who are unfortunate enough to step on a partially buried ray may receive a nasty wound and poison from a gland associated with the spine. This gland at the base of the spine releases neurotoxins and proteolytic toxins.

Some stingrays have contributed to the ecotourism industry. Tourists visit Stingray City in the Cayman Islands to feed large stingrays (Dasyatis americana). Scuba expeditions are conducted in Hawaii, the northern Gulf of Mexico, and various other areas, to observe manta rays at their feeding sites.

Species accounts

Resources

Cappetta, H. Chondrichthyes. II. Mesozoic and Cenozoic Elasmobranchii. New York: Gustav Fischer Verlag, 1987.

Carroll, Robert L. Vertebrate Paleontology and Evolution. New York: W. H. Freeman and Company, 1988.

Hamlett, William C., ed. Sharks, Skates, and Rays: The Biology of Elasmobranch Fishes. Baltimore: Johns Hopkins University Press, 1999

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

McEachran, John D., and Janice D. Fechhelm. Fishes of the Gulf of Mexico. Vol. 1, Myxiniformes to Gasterosteiformes. Austin, TX: University of Texas Press, 1998.

McEachran, John D., K. A. Dunn, and T. Miyake. "Interrelationships of the Batoid Fishes (Chondrichthyes: Batoidea)." In Interrelationships of Fishes, edited by M. L. J. Stiassny, L. R. Parenti, and G. D. Johnson. New York: Academic Press, 1996.

Paxton, J. R., and W. N. Eschmeyer. Encyclopedia of Fishes. New York: Academic Press, 1994.

Taylor, L. R., editor. Sharks and Rays. Alexandria, VA: Nature Company Guides, Time-Life Books, 1997.

Lovejoy, Nathan R., E. Bermingham, and A. P. Martin. "Marine Incursion into South America." Nature 396 (December 1998): 421–422.

McEachran, John D., and K. A. Dunn. "Phylogenetic Analysis of Skates, a Morphologically Conservative Clade of Elasmobranches (Chondrichthyes: Rajidae)." Copeia 1998, no. 2 (1998): 271–290.

Rosenberger, Lisa J. "Pectoral Fin Locomotion in Batoid Fishes: Undulation Versus Oscillation." Journal of Experimental Biology 204, no. 2 (2001): 379–394. ——. "Phylogenetic Relationships Within the Stingray Genus Dasyatis (Chondrichthyes: Dasyatidae)." Copeia 2001, no. 3 (2001): 615–627.

Rosenberger, Lisa J., and M. W. Westneat. "Functional Morphology of Undulatory Pectoral Fin Locomotion in the Stingray Taeniura lymma (Chondrichthyes: Dasyatidae)." Journal of Experimental Biology 202, no. 24 (1999): 3523–3539.

American Elasmobranch Society, Florida Museum of Natural History. Gainesville, FL 32611 USA. Web site:

[Article by: John D. McEachran, PhD]

The noun has one meaning:

Meaning #1: fish with dorsoventrally flattened bodies; includes: rays; skates; guitarfishes; sawfishes
  Synonyms: order Rajiformes, Batoidei, order Batoidei

Rajiformes Fossil range: middle Eocene–Recent PreЄ Є O S D C P T J K Pg N [1]
Spotted eagle ray, Aetobatus narinari
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Subclass:	Elasmobranchii
Superorder:	Batoidea
Order:	Rajiformes
Families
Anacanthobatidae Arhynchobatidae Dasyatidae Gymnuridae Hexatrygonidae Myliobatidae Plesiobatidae Potamotrygonidae Rajidae Rhinobatidae Urolophidae Urotrygonidae

Rajiformes is the order of true rays and skates, flat-bodied cartilaginous fishes related to sharks.

Rajiforms are distinguished by the presence of greatly enlarged pectoral fins, which reach as far forward as the sides of the head, with a generally flattened body. The undulatory pectoral fin motion diagnostic to this taxon is known as rajiform locomotion. The eyes and spiracles are located on the upper surface of the body, and the gill slits on the underside. They have flattened, crushing teeth, and are generally carnivorous, although manta rays are filter feeders. Most species give birth to live young, although some lay eggs with a horny capsule ("mermaid's purse").^[1]

Classification

Onchopristis fossils

There are several proposed classifications of fish in the superorder Batoidea. In some classifications the order Rajiformes is split into two or three orders, the additional orders being Myliobatiformes, containing the eagle rays and their relatives; Rhinobatiformes, containing the guitarfishes (which may be further split into Rhynchobatiformes, containing the shovelnosed guitarfishes, and Rhiniformes, the guitarfishes).

• Anacanthobatidae (smooth skates)
• Arhynchobatidae (softnose skates)
• Dasyatidae (stingrays). Named for the venomous spines along the tail; these contain a poison that causes pain and may cause symptoms such as nausea, vomiting, fever, chills, muscle cramps, tremors, paralysis, fainting, seizures, elevated heart rate, and decreased blood pressure (depending on the species). In addition, some species' toxins can be fatal to humans.
• Gymnuridae (butterfly rays)
• Hexatrygonidae (sixgill stingrays)
• Myliobatidae (eagle rays). The largest of rays, including the giant manta rays. Most eagle rays have one poison-carrying spine.
• Plesiobatidae (deepwater stingrays)
• Potamotrygonidae (river stingrays)
• Rajidae (hardnose skates)
• Rhinobatidae (guitarfishes). They have a body structure similar that of the sawfishes, but are not thought to be closely related.
• Urolophidae (Indo-Pacific round rays)
• Urotrygonidae (American round rays)

Fossil genera are:

• Rhombodontidae from the Maastrichtian stage of the Cretaceous period of Europe, Asia, Africa and the Americas^[2].

References

1. ^ ^{a b} "Rajiformes". FishBase. Ed. Ranier Froese and Daniel Pauly. January 2009 version. N.p.: FishBase, 2009.
2. ^ http://www.elasmo.com/frameMe.html?file=genera/cretaceous/rhombodus.html&menu=bin/menu_genera-alt.html Rhombodus DAMES 1881 Extinct myliobatoid ray — Upper Cretaceous by Elasmo.com

• "Rajiformes". FishBase. Ed. Ranier Froese and Daniel Pauly. January 2006 version. N.p.: FishBase, 2006.
• Batoids: Sawfishes, Guitarfishes, Electric Rays, Skates, and Sting Rays
• J. D. McEachran, K. A. Dunn, T. Miyake (1996). "Interrelationships of the batoid fishes (Chondrichthyes: Batoidea)". In Interrelationships of Fishes, Academic Press.
• "Skate or Ray?". ReefQuest Centre for Shark Research. Accessed 20 March 2008.

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