Temporal range: Early Triassic-Recent
|Devil fish, Mobula mobular|
Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, approximately 560 described species in thirteen families. They are in the fish subclass Elasmobranchii, along with sharks, to which they are closely related. Rays are distinguished by their flattened bodies, enlarged pectoral fins that are fused to the head, and gill slits that are placed on their ventral surfaces.
Batoids are flat-bodied, and, like sharks, are cartilaginous marine fish, meaning they have a boneless skeleton made of a tough, elastic substance. Most batoids have five ventral slot-like body openings called gill slits that lead from the gills, but the Hexatrygonidae have six. Batoid gill slits lie under the pectoral fins on the underside, whereas a shark's are on the sides of the head. Most batoids have a flat, disk-like body, with the exception of the guitarfishes and sawfishes, while most sharks have a streamlined body. Many species of batoid have developed their pectoral fins into broad flat wing-like appendages. The anal fin is absent. The eyes and spiracles are located on top of the head. Batoids have a ventrally located mouth and can considerably protrude their upper jaw (palatoquadrate cartilage) away from the cranium to capture prey. The jaws have euhyostylic type suspension, which relies completely on the hyomandibular cartilages for support.
Batoids reproduce in a number of ways. As is characteristic of elasmobranchs, batoids undergo internal fertilisation. Internal fertilisation is advantageous to batoids as it conserves sperm, does not expose eggs to consumption by predators, and ensures that all the energy involved in reproduction is retained and not lost to the environment. All skates and some rays are oviparous (egg laying) while other rays are "aplacental viviparous", meaning that they give birth to young which develop in a womb but without involvement of a placenta.
The eggs of oviparous skates are laid in leathery egg cases that are commonly known as mermaid's purses and which often wash up empty on beaches in areas where skates are common.
Most species live on the sea floor, in a variety of geographical regions — many in coastal waters, few live in deep waters to at least 3,000 metres (9,800 ft), most batoids have a somewhat cosmopolitan distribution, in tropical and subtropical marine environments, temperate or cold-water species. Only a few species, like manta rays, live in the open sea, and only a few live in freshwater. Some batoids can live in brackish bays and estuaries. Bottom-dwelling batoids breathe by taking water in through the spiracles, rather than through the mouth as most fishes do, and passing it outward through the gills.
Most batoids have developed heavy, rounded teeth for crushing the shells of bottom-dwelling species such as snails, clams, oysters, crustaceans, and some fish, depending on the species. Manta rays feed on plankton.
|Phylogenetic tree of Batoidea[page needed]|
The classification of batoids is currently undergoing revision; however, molecular evidence refutes the hypothesis that skates and rays are derived sharks. Nelson's 2006 Fishes of the World recognizes four orders. The Mesozoic Sclerorhynchoidea are basal or incertae sedis; they show features of the Rajiformes but have snouts resembling those of sawfishes.
|Myliobatiformes||Stingrays and related species||10||29||223||1||16||33||Myliobatiformes include stingrays, butterfly rays, eagle rays, and manta rays. They were formerly included in the order Rajiformes, but more-recent phylogenetic studies have shown that they are a monophyletic group, and that its more-derived members evolved their highly flattened shapes independently of the skates.|
|Rajiformes||Skates and related species||5||36||270||4||12||26||Rajiformes include skates, guitarfishes, and wedgefishes. They 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. Most species give birth to live young, although some lay eggs inside a protective capsule or mermaid's purse.|
|Torpediniformes||Electric rays||4||12||69||2||9||The electric rays have electric organs in their pectoral fin discs that generate electric current. They are used to immobilize prey and for defense. The current is strong enough to stun humans, and the ancient Greeks and Romans used these fish to treat ailments such as headaches.|
The sawfishes are shark-like in form, having tails used for swimming and smaller pectoral fins than most batoids. The pectoral fins are attached above the gills as in all batoids, giving the fishes a broad-headed appearance. They have long, flat snouts with a row of tooth-like projections on either side. The snouts are up to 1.8 metres (6 ft) long, and 30 centimetres (1 ft) wide, and are used for slashing and impaling small fishes and to probe in the mud for embedded animals. Sawfishes can enter freshwater rivers and lakes. Some species reach a total length of 6 metres (20 ft).
- Family Pristidae (sawfishes)
- Family Anacanthobatidae (smooth skates)
- Family Rajidae (skates)
- Family Rhinidae (bowmouth guitarfishes)
- Family Rhinobatidae (guitarfishes)
- Family Rhynchobatidae (wedgefishes)
- Family Dasyatidae (whiptail stingrays)
- Family Gymnuridae (butterfly rays)
- Family Hexatrygonidae (sixgill stingrays)
- Family Myliobatidae (eagle rays)
- Family Platyrhinidae (thornbacks)
- Family Plesiobatidae (deepwater stingrays)
- Family Potamotrygonidae (river stingrays)
- Family Urolophidae (round stingrays)
- Family Urotrygonidae (American round stingrays)
- Family Zanobatidae (panrays)
Difference between sharks and rays
Sharks and rays are both cartilaginous fishes which can be contrasted with bony fishes. Rays are basically flattened sharks, adapted for feeding on the bottom. Guitarfish are somewhat between sharks and rays show characteristics of both ( though they are classified as rays).
|Comparison of sharks, guitar fishes and rays|
|Shape||laterally compressed spindle||dorsoventrally compressed (flattened) disc||dorsoventrally compressed (flattened) disc|
|Spiracles||not always present||always present|
|Habitat||usually pelagic surface feeders, though carpet sharks are demersal bottom feeders||demersal/pelagic mix||usually demersal bottom feeders|
|Eyes||usually at the side of the head||usually on top of the head||usually on top of the head|
|Gill openings||on the sides||ventral (underneath)|
|Pectoral fins||distinct||not distinct||not distinct|
|Tail||large caudal fin used for propulsion||caudal fin that can be used for propulsion||varies from thick tail as extension of body to a whip that can sting to almost no tail.|
|Locomotion||swim by moving their caudal (tail) fin from side to side||Guitar fish and sawfish have a caudal fin like the shark.||swim by flapping their pectoral fins like wings.|
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- Shark references. Database of bibliography of living/fossil sharks and rays (Chondrichtyes: Selachii) with more than 15.000 listed papers and many download links.
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- McEachran, JD; Aschliman, N (2004), "Phylogeny of batoidea", in Carrier, JC; Musick, JA; Heithaus, MR, Biology of sharks and their relatives, Boca Raton: CRC Press, pp. 79–114.
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