Rajiformes

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Rajiformes
Temporal range: Pliensbachian–Recent [1]
Little skate (Leucoraja erinacea)
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Elasmobranchii
Infraclass:
Euselachii
Clade:
Neoselachii
Superorder: Batoidea
Order: Rajiformes
L. S. Berg, 1940

Rajiformes

mermaid's purse").[citation needed
]

Characteristics

Rajoids typically have a dorsoventrally flattened body. The snout is slender and pointed and the wide mouth, often covered with a fleshy nasal flap, is on the underside of the head. The eyes and well-developed spiracles are located on the top of the head. In most species, the spiracles are large and are the main means of drawing water in for respiration. There is no

dermal denticles on their skin, often with a row of large denticles along the spine.[3]

The pectoral fins are large, but not clearly demarcated from the body, and together with the body are known as the disc. They start from the side of the head in front of the gill openings and end at the

caudal peduncle. There are up to two dorsal fins but no anal fin. A slender tail is clearly demarcated from the disc. The caudal fin varies in size between species and the rays have a whip-like tail with no caudal fin.[3]

Distribution

Species of the order Rajiformes are found throughout the world's oceans, from Arctic to Antarctic waters, and from shallow coastal shelves to open seas and abyssal regions. A few are found in rivers and some in estuaries, but most are marine, living near the sea bed at depths down to 3,000 m (9,800 ft) or more.[3]

Diversity

Families

Four extant and five extinct families of rajoid have been described:[4][5][6]

Formerly, some Rhinopristiforms were also included.

The smooth skates, the

Anacanthobatidae, contain a single genus, Anacanthobatis, of about 10 species. They are small fish living on the continental slopes of tropical and subtropical waters, and are native to Natal, South Africa, tropical West Africa, and Taiwan. Smooth skates have a filament extending from a rounded protuberance on the snout. Both the dorsal and ventral surfaces are smooth and have no dermal denticles. The tail is slender and a little shorter than the body. No dorsal fins are present and the caudal fin is small and membranous.[7]

Raja texana
, the roundel skate

The skate family

Rajidae contains 14 genera and around 200 species. They are found worldwide, but are relatively uncommon near coral reefs and in shallow tropical seas. A few species occur in brackish water. The disc shape is rhomboidal, and the tail long. Two dorsal fins are found and the caudal fin is much reduced. The pelvic fins have two lobes. Most species have rough skins with dermal denticles which are especially obvious along the spine. The eggs are laid in a protective hard case with string-like elongations at the four corners.[8]

The extinct families

Sclerorhynchiformes containing Sclerorhynchidae and Ischyrhizidae and forming a sister group to sawfish and all other rays, with the clade containing sawsharks and rays being coined Pristiorajea.[9][10] However, further studies have affirmed that sawsharks are true sharks and a member of Selachimorpha. Later studies have instead found Sclerorhynchiformes to form a sister group to Rajidae, and thus downgraded it to being the suborder Sclerorhynchoidei of the order Rajiformes.[6][11][12][13]

Biology

electric field to locate their prey.[14]

In most rajoids, water for breathing is taken in through the spiracles rather than through the mouth and exits through the gill slits. Most species swim by undulating their enlarged pectoral fins, but the guitarfish propel themselves through the water with sideways movements of their tail and caudal fin. Most species are

pelagic, and continually cruise the ocean.[3]

References

Wikimedia Commons has media related to Rajiformes.
  1. doi:10.1007/s12542-019-00451-4
    .
  2. S2CID 14029833
    .
  3. ^ a b c d Martin, R. Aidan. "Batoids: Order Rajiformes". ReefQuest Centre for Shark Research. Retrieved 2013-09-01.
  4. ^ Vanden Berghe, Edward (2013). "Rajiformes". WoRMS. World Register of Marine Species. Retrieved 2013-08-16.
  5. ISBN 9780643109148
    .
  6. ^
    S2CID 239067365
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  7. ^ "Family Anacanthobatidae - Smooth skates". FishBase. Froese, Rainer; Pauly, Daniel (eds.). 2011. Retrieved 2013-08-15.
  8. ^ "Family Rajidae - Skates". FishBase. Froese, Rainer; Pauly, Daniel (eds.). 2011. Retrieved 2013-08-15.
  9. ^ Kriwet, Jürgen (2004). "The systematic position of the Cretaceous sclerorhynchid sawfishes (Elasmobranchii, Pristiorajea)". Mesozoic Fishes 3: Systematics, Paleoenvironment and Biodiversity.
  10. ^ Watson, Joshua (2019-10-13). "Infraclass Euselachii sharks and rays and related fossils - Fishes". gibell Aquarium Society. Retrieved 2020-10-17.
  11. S2CID 210302939
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  12. S2CID 145940997
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  13. ISSN 0024-4082
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  14. S2CID 15603518
    .
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