Batoidea
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 Spotted eagle ray, Aetobatus narinari
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Rajiformes - common rays and skates |
Batoidea is a superorder of cartilaginous fishes containing more than 500 described species in thirteen familes. They are commonly known as rays, but that term is also used specifically for batoids in the order Rajiformes, the "true rays". Batoids include stingrays, skates, electric rays, guitarfishes and sawfishes.
Batoids are most closely related to sharks and young batoids look very much like young sharks. Indeed according to recent DNA analyses the catshark is more closely related to the batoids than to other sharks.[citation needed]
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[edit] Anatomy
Batoids are flat-bodied, and, like sharks, are a species of cartilaginous marine fish, meaning they have a boneless skeleton made of a tough, elastic substance. Batoids also are like sharks in having slot-like body openings called gill slits that lead from the gills. 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 eyes and spiracles are located on top of the head.
[edit] Reproduction
Batoid eggs, unlike those of most other fishes, are fertilized inside the female's body. The eggs of all batoids except for the skates (family Rajidae) hatch inside the female and are born alive (ovoviviparous). Female skates lay internally fertilized flat, rectangular, leathery-shelled eggs, with tendrils at the corners for anchorage. Hatched eggs of this type can be found on beaches and are known as mermaids’ purses.
[edit] Habitat
Most species live on the sea floor, in a variety of geographical regions - many in coastal waters, few live in deep waters, 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.
[edit] Feeding
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.
[edit] Classification
The classification of batoids is currently undergoing revision. This article follows FishBase in dividing batoids into three orders. Some taxonomists argue in favour of placing all batoids in a single order, Rajiformes; others propose a division into five or six orders. The additional orders in these systems are 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 sharkfin guitarfishes).
[edit] Order Rajiformes (true rays)
- Family Anacanthobatidae (smooth skates)
- Family 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.
- Family Gymnuridae (butterfly rays)
- Family Hexatrygonidae (sixgill stingrays)
- Family Myliobatidae (eagle rays). The largest of rays, including the giant manta rays. Most eagle rays have one poison-carrying spine.
- Family Plesiobatidae (deepwater stingrays)
- Family Potamotrygonidae (river stingrays)
- Family Rajidae (skates)
- Family Rhinobatidae (guitarfishes). They have a body structure similar that of the sawfishes, but are not thought to be closely related.
- Family Urolophidae (round rays)
[edit] Order Pristiformes (sawfishes)
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 6 ft (1.8 m) long, and 1 ft (30 cm) wide, and are used for slashing and impaling small fishes and to probe in the mud for imbedded animals. Sawfishes can enter freshwater rivers and lakes. Some species reach a total length of 20 ft (6 m).
- Family Pristidae
[edit] Order Torpediniformes (electric rays)
Electric rays have organs in their wings that generate electric current. They are used to immobilize prey and for defense. The current is strong enough to stun humans, and it is said that the ancient Greeks used these fish for shock therapy.
- Family Narcinidae
- Family Torpedinidae
[edit] References
- 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.
