Rajiformes

(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

List of Species

Roughtail stingray

Dasyatis centroura

family

Dasyatidae

taxonomy

Raja centroura Mitchill, 1815, Long Island coast, New York.

other common names

French: Pastenague épineuse; Italian: Trigone spinoso.

physical characteristics

Size 83 in (210 cm) in disc width as an adult and 13–15 in (34–37 cm) in disc width at birth. The head, pectoral fins, and trunk are flattened and joined to form a broad, rectilinear disc. The disc is about as broad as it is long, and the outer corners are subangular. Tail 2.4 to 26 times the disc length, slender, and whiplike. It lacks a fleshy dorsal keel and dorsal fins, and the caudal fin but has one or more serrated spines and a low ventral fold. Snout moderately long and very obtuse. Mouth moderately wide and moderately arched. Specimens larger than 20 in (50 cm) in disc width have large tubercles or bucklers along the midline and central area of the disc and along the upper surface of the tail. Coloring brown to olive dorsally and white to whitish ventrally.

distribution

Tropical to warm temperate regions in the Atlantic. Ranges from Massachusetts to the northern Gulf of Mexico and Uruguay in the western Atlantic and from the Bay of Biscay to Angola, including the Mediterranean and Madeira.

habitat

Benthic habitats on soft bottoms from near shore to about 899 ft (274 m) in the western Atlantic and to about 197 ft (60 m) in the eastern Atlantic.

behavior

Moves northward and shoreward in the spring and southward and offshore in the autumn.

feeding ecology and diet

Prey include polychaetes, cephalopods, crustaceans, and bony fishes.

reproductive biology

Viviparous, with litters ranging from two to six neonates.

conservation status

Not listed by the IUCN.

significance to humans

Capable of inflicting painful wounds in waders and swimmers that come into contact with them in inshore areas.

Spiny butterfly ray

Gymnura altavela

family

Gymnuridae

taxonomy

Raja altavela Linnaeus, 1758, Mediterranean Sea.

other common names

French: Raie-papillon épineuse; Spanish: Raya mariposa.

physical characteristics

Size 80 in (202 cm) in disc width. The head, pectoral fin, and trunk are flattened and joined to form a very broad rectilinear disc. Disc 1.5 times broader than it is long, and the outer corners are abruptly rounded. Tail very short and slender, about one-fourth of the disc width. It lacks dorsal and caudal fins but has one or more serrated spines and dorsal and ventral ridges. Snout short and very obtuse. Mouth moderately arched. There is a tentacle-like structure on the inner posterior margin of the spiracle. The body is naked, except for small denticles over central areas of the disc. Coloring dark brown and white ventrally. Dorsal surface patterned with small dark and light spots.

distribution

Tropical to warm temperate waters of the Atlantic. Ranges from southern Massachusetts to Rio de la Plata, Argentina, in the western Atlantic and from Portugal to Angola, including Madeira, the Canary Islands, and the Mediterranean Sea, in the eastern Atlantic.

habitat

Benthic habitats on soft bottoms from near shore to about 197 ft (60 m).

behavior

Nothing is known concerning the behavior of this species.

feeding ecology and diet

Prey include mollusks, crustaceans, and bony fishes.

reproductive biology

Viviparous, with litters ranging from four to seven neonates. The gestation period is about six months.

conservation status

Not listed by the IUCN.

significance to humans

Capable of inflicting painful wounds in waders and swimmers that come into contact with them in inshore areas.

Atlantic manta

Manta birostris

family

Mobulidae

taxonomy

Raja birostris Walbaum, 1792, type locality not specified.

other common names

English: Giant manta; French: Mante géante; Spanish: Manta voladora.

physical characteristics

Size 22 ft (6.7 m) in disc width as an adult and 47 in (120 cm) in disc width at birth. The head, pectoral fin, and trunk is flattened and joined to form a broad rectilinear disc. The disc is much broader than it is long, and the outer corners are slightly falcate. Tail very short and whiplike. It lacks folds, keels, and a caudal fin but has a dorsal fin at the base and either has or lacks a small serrated spine. The anterior section of the pectoral fin forms a narrow, vertically oriented cephalic fin that is attached to the head and is free of the remainder of the fin. Head slightly elevated and very broad. Mouth is terminal. Teeth are small and located only on the lower jaw. Brown to olive in color dorsally and white to whitish ventrally.

distribution

Tropical to warm temperate regions worldwide.

habitat

Pelagic species in near shore to oceanic waters but is most common in coastal waters.

behavior

Performs somersaults during feeding. Occasionally leaps partially or completely out of the water. Enters shallow reef areas to be cleaned of ectoparasites by small bony fishes.

feeding ecology and diet

Prey include zooplankton and nektonic crustaceans.

reproductive biology

Viviparous. Litters size unknown.

conservation status

Listed by the IUCN as Data Deficient.

significance to humans

Harpooned or gill-netted for human consumption in some parts of the world. It is the focus of underwater scuba-based ecotourism.

Spotted eagle ray

Aetobatus narinari

family

Myliobatidae

taxonomy

Raja narinari Euphrasen, 1790, Brazil.

other common names

French: Aigle de mer léopard; Spanish: Chucho pintado.

physical characteristics

Size 130 in (330 cm) in disc width as an adult and 7–14 in (18–36 cm) in disc width at birth. The head, pectoral fins, and trunk are flattened and joined to form a broad rectilinear disc. The head is elevated from the disc, and the anterior section of the pectoral fins forms a subrostral lobe above the mouth. Disc 2.1 times broader than long, and outer corners are slightly falcate. Tail very long, slender, and whiplike. It lacks folds, keels, and a caudal fin, but it has a dorsal fin at the base and one or more serrated spines. Snout moderately short; mouth is straight. Teeth flattened and pavement-like and aligned in series. The body is naked. Color ranges from olivaceous to dark brown dorsally and white ventrally, except for a dusky subrostral line and dusky pelvic fins. The dorsal surface is patterned with small white, greenish, or yellow spots.

distribution

Tropical to warm temperate regions worldwide. Species may represent a series of cryptic species, each limited to a particular location. It ranges from North Carolina to southern Brazil, including the Gulf of Mexico in the western Atlantic, Cape Verde to Angola in the eastern Atlantic, the entire tropical and subtropical region of the eastern Pacific, and throughout the tropical and subtropical Indo-West Pacific.

habitat

Benthic. Occurs from near shore to about 197 ft (60 m).

behavior

Spends much of its time actively swimming in the water column by the oscillatory action of its pectoral fins. It is capable of leaping clear of the water.

feeding ecology and diet

Prey include shellfish, such as clams, oyster, whelks, and other mollusks.

reproductive biology

Viviparous, with litters ranging up to four neonates.

conservation status

Listed by IUCN as Data Deficient. There is a possibility that future research may reveal that threatened classification is appropriate, but currently information is not sufficient to list the species.

significance to humans

Capable of inflicting painful wounds in waders and swimmers that come into contact with it in inshore areas. Occasionally, it is captured by gill net fisheries for human consumption.

Lesser electric ray

Narcine bancrofti

family

Narcinidae

taxonomy

Torpedo bancrofti Griffith, 1834, Jamaica.

other common names

Spanish: Raya eléctrica torpedo.

physical characteristics

Adults up to 23 in (58 cm) in total length but only 3.5–3.9 in (9–10 cm) in total length at birth. The head, pectoral fins, and trunk are flattened and joined to form a fleshy disc. Tail is stout, and the caudal fin well developed. Snout is very blunt, with a narrow, greatly protrusible mouth that forms a short tube. Kidney-shaped electric organs are located on either side of head, giving the skin surface a honeycomb appearance. Coloring yellowish brown to grayish brown or dark brown dorsally and white to creamy white ventrally. The dorsal surface is patterned with dark blotches, spots, and crossbars.

distribution

Tropical to warm temperate waters of the western Atlantic. Ranges from North Carolina to Venezuela, including the northern and western Gulf of Mexico, the Greater and Lesser Antilles, Yucatan, Belize and northern South America.

habitat

Benthic habitats on soft bottoms in shallow water.

behavior

Nothing is known concerning the behavior of this species.

feeding ecology and diet

Electric organs are used to stun prey, which consist of polychaetes, other invertebrates, and small fishes.

reproductive biology

Viviparous without a placenta. Litter size can be as many as 18 embryos.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Freshwater stingray

Paratrygon aiereba

family

Potamotrygonidae

taxonomy

Trygon aiereba Müller and Henle, 1841, Brazil.

other common names

English: Discus ray.

physical characteristics

Size 43 in (110 cm) in disc width. The head, pectoral fin, and trunk are flattened and joined to form an elliptical disc. The disc is longer than it is broad, indented anteriorly, and slightly broader across the anterior third than across the posterior third. Pelvic fins are covered entirely by the disc. Tail relatively short, broad at the base, and filamentous distally. The preoral snout length is 30–38% of the disc width. The posterior outer margin of the spiracle bears a knoblike process. Eyes small and located just in front of the spiracles. Mouth small; teeth small and few in number. The dorsal surface is covered with small dermal denticles. Light brown dorsally and white ventrally. The dorsal surface is patterned with dark reticular or vermicular blotches.

distribution

Freshwaters of South America from northern Bolivia and eastern Peru to northern Brazil and Venezuela in the Amazon and Orinoco drainages.

habitat

Benthic habitats on soft bottoms in rivers.

behavior

Nothing is known concerning the behavior of this species.

feeding ecology and diet

Prey include benthic organisms.

reproductive biology

Viviparous.

conservation status

Not listed by the IUCN.

significance to humans

Capable of inflicting very painful wounds in waders and swimmers that come into contact with it.

Smalltooth sawfish

Pristis pectinata

family

Pristidae

taxonomy

Prisitis pectinata Latham, 1794, type locality not specified.

other common names

French: Poisson scie tident; Spanish: Pejepeine.

physical characteristics

Reaches 217 in (550 cm) in total length as an adult but only 24 in (60 cm) in total length at birth. Body elongated and moderately depressed, and the snout is prolonged into a long, narrow, flattened blade bearing 24 to 32 pairs of teeth. The blade is about 25% of the total length. The caudal fin is without a distinct ventral lobe. Color dark brownish gray dorsally and grayish white ventrally.

distribution

Tropical to warm temperate western Atlantic from New York to central Brazil, including the Gulf of Mexico, the Caribbean Sea, and Bermuda. It has been recorded from the eastern Atlantic, Mediterranean Sea, the eastern Pacific, Indian Ocean, and the Indo-Pacific region but these records need to be verified.

habitat

Lives near shore in bays, estuaries, and freshwater.

behavior

Nothing is known concerning the behavior of this species.

feeding ecology and diet

The blade is used to dislodge invertebrates and to disable fishes.

reproductive biology

Viviparous without a placenta. Litters range from 15 to 20 young.

conservation status

Listed as Endangered by the IUCN. It is protected in Florida and Louisiana state waters.

significance to humans

None known.

Atlantic torpedo

Torpedo nobiliana

family

Torpedinidae

taxonomy

Torpedo nobiliana Bonaparte, 1835, Italy, iconograph.

other common names

French: Torpille noire; Spanish: Tremolina nigra.

physical characteristics

Reaches 71 in (180 cm) in total length in adulthood, and 7.9–9.8 in (20–25 cm) in total length at birth. The head, pectoral fin, and trunk are flattened and joined to form a fleshy disc. The tail is stout, and the caudal fin is well developed. The species has a very blunt snout, with a wide and slightly protrusible mouth. Kidney-shaped electric organs are located on either side of the head, giving the skin surface a honeycomb appearance. Coloring varies from brown to purplish brown dorsally and is white with a dark margin ventrally.

distribution

Tropical to temperate waters of the North Atlantic. Ranges from southern Nova Scotia to Venezuela, including the northern Gulf of Mexico, Cuba, and Trinidad in the western Atlantic and the British Isles and Mediterranean Sea to South Africa in the eastern Atlantic.

habitat

Benthic habitats on soft bottoms from the shoreline to the upper continental slope at 1,739 ft (530 m).

behavior

Nothing is known concerning the behavior of this species.

feeding ecology and diet

Electric organs are used to stun prey, which consist of fishes.

reproductive biology

Viviparous without a placenta. The litter size is unknown.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Clearnose skate

Raja eglanteria

family

Rajidae

taxonomy

Raja eglanteria Bosc, 1800, Charleston Bay, South Carolina.

other common names

French: Raie blanc nez; Spanish: Raya hialina.

physical characteristics

Reaches 31 in (78.5 cm) in total length; at birth it measures 5–5.7 in (12.5–14.4 cm) in total length. The head, pectoral fins, and trunk are flattened and joined to form broad, spadeshaped disc. Tail moderately slender and makes up about half of the total length. The caudal fin is poorly developed. Snout moderately long and slightly obtuse, and mouth moderately wide and slightly arched. There are medium-size thorns on the head and in a row from the shoulder region to the first dorsal fin and irregular lateral rows located on either side of the tail. The dorsal surface is covered sparsely with dermal denticles. Coloring is brown to gray dorsally and whitish to yellowish ventrally. The dorsal surface is patterned with dark and light spots and transverse and diagonal dark bars.

distribution

Tropical to warm temperate regions of the western North Atlantic, ranging from Massachusetts to the northern Gulf of Mexico.

habitat

Benthic. Found on soft bottoms from near shore to about 390 ft (119 m).

behavior

In the northern part of its range this species migrates north and inshore in the spring and south and offshore in the autumn.

feeding ecology and diet

Prey include polychaetes, amphipods, shrimps, crabs, and bony fishes.

reproductive biology

Oviparous; neonates hatch from egg capsules after several months.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Atlantic guitarfish

Rhinobatos lentiginosus

family

Rhinobatidae

taxonomy

Rhinobatus lentiginosus Garman, 1880, Florida.

other common names

None known.

physical characteristics

Reaches 30 in (76 cm) in total length. The head, pectoral fins, and trunk are moderately expanded and joined to form a wedge-shaped disc. Nostril length equals or is slightly longer than the distance between the nostrils. The rostral cartilage of the snout is moderately long and expanded and bears conical tubercles near the tip. Tail stout, and dorsal fins and caudal fin well developed. Caudal fin lacks a distinct ventral lobe. Color gray to olive or dark brown dorsally and white to pale yellow ventrally. The dorsal surface generally is freckled with many small white spots.

distribution

Tropical to warm temperate waters of the western Atlantic and ranges from North Carolina to the southern Gulf of Mexico.

habitat

Benthic habitats on soft bottoms from the shoreline to 59 ft (18 m).

behavior

Nothing is known concerning the behavior of this species.

feeding ecology and diet

Feeds on benthic organisms.

reproductive biology

Viviparous without a placenta. The litter size is unknown.

conservation status

Not listed by the IUCN.

significance to humans

Found in local fish markets in the southern part of its range.

Yellow stingray

Urobatis jamaicensis

family

Urotrygonidae

taxonomy

Raia jamaicensis Cuvier, 1816, Jamaica.

other common names

None known.

physical characteristics

Reaches 28 in (70 cm) in total length. The head, pectoral fin, and trunk are flattened and joined to form an oval-shaped disc. The disc is longer than it is broad, and the outer corners are broadly rounded. Tail stout and relatively short, less than half its total length. It lacks dorsal and ventral folds and keels and dorsal fins but has one or more serrated spines and a small caudal fin. Snout moderately short and rounded; mouth small and straight. The body is naked, except for small denticles along the midbelt of the disc and tail. Green to grayish brown dorsally and yellowish, greenish, and brownish ventrally. The dorsal surface is patterned with fine reticulations or light-colored spots.

distribution

Tropical to warm temperate waters of the western Atlantic, ranging from Cape Lookout, North Carolina, to southern Florida, including the Gulf of Mexico, Bahamas, and Greater and Lesser Antilles.

habitat

Benthic. Found on soft bottoms from near shore, including bays and estuaries.

behavior

Docile. Spends much of its time partially buried in soft substrates.

feeding ecology and diet

Prey include benthic invertebrates and bony fishes.

reproductive biology

Viviparous, with litters ranging from two to four neonates.

conservation status

Not listed by the IUCN.

significance to humans

Capable of inflicting painful wounds in waders and swimmers that come into contact with it in inshore areas.

Resources

Books

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.

Periodicals

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.

Organizations

American Elasmobranch Society, Florida Museum of Natural History. Gainesville, FL 32611 USA. Web site: <http://www.flmnh.ufl.edu/fish/Organizations/aes/aes.htm>

John D. McEachran, PhD