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Skates are
cartilaginous fish Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue. ...
belonging to the
family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...
Rajidae in the superorder Batoidea of rays. More than 150 species have been described, in 17 genera.LAST, P.R. & SÉRET, B. & STEHMANN, M.F.W. & WEIGMANN, S. (2016) Skates, Family Rajidae. In: Last, P.R., White, W.T., Carvalho, M.R. de, Séret, B., Stehmann, M.F.W & Naylor, G.J.P (Eds.) Rays of the World. CSIRO Publishing, Melbourne: 204–363 Softnose skates and pygmy skates were previously treated as subfamilies of Rajidae (Arhynchobatinae and Gurgesiellinae), but are now considered as distinct families. Alternatively, the name "skate" is used to refer to the entire order of Rajiformes (families
Anacanthobatidae The Anacanthobatidae, the smooth skates or leg skates, are a family of skates found at depths below in the Indian, Pacific and Atlantic Oceans. They lack the dorsal denticles (sharp, tooth-like scales) of other rays, hence their name, from G ...
,
Arhynchobatidae Arhynchobatidae is a family of skates and is commonly known as the softnose skates. It belongs to the order Rajiformes in the superorder Batoidea Batoidea is a superorder of cartilaginous fishes, commonly known as rays. They and their close r ...
,
Gurgesiellidae Pygmy skates are cartilaginous fish belonging to the family Gurgesiellidae in the superorder Batoidea Batoidea is a superorder of cartilaginous fishes, commonly known as rays. They and their close relatives, the sharks, comprise the subclass ...
and Rajidae). Members of Rajidae are distinguished by a stiff snout and a rostrum that is not reduced.


Taxonomy and systematics


Evolution

Skates belong to the ancient lineage of cartilaginous fishes. Fossil denticles (tooth-like scales in the skin) resembling those of today's chondrichthyans date at least as far back as the
Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. T ...
, with the oldest unambiguous fossils of
cartilaginous fish Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue. ...
dating from the middle
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wh ...
. A
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...
within this diverse family, the Neoselachii, emerged by the
Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest per ...
, with the best-understood neoselachian fossils dating from the
Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
. This
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...
is represented today by
shark Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachi ...
s, sawfish,
ray Ray may refer to: Fish * Ray (fish), any cartilaginous fish of the superorder Batoidea * Ray (fish fin anatomy), a bony or horny spine on a fin Science and mathematics * Ray (geometry), half of a line proceeding from an initial point * Ray (gr ...
s and skates.


Classification

The skate belongs to the class
Chondrichthyes Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue. ...
. This class consists of all the cartilaginous fishes, including sharks and stingrays. Chondrichthyes is divided into two subclasses; of which Elasmobranchii includes skates, rays, and sharks. Skates are the most diverse elasmobranch group, comprising over 20% of the known species. The number of species is likely to increase as taxonomic issues are resolved and new species are identified. There are 17 genera recognized in the family Rajidae: * '' Amblyraja'' * '' Beringraja'' * '' Breviraja'' * '' Dactylobatus'' * '' Dentiraja'' * '' Dipturus'' * '' Hongeo'' * ''
Leucoraja ''Leucoraja'' is a genus of hardnose skates in the family Rajidae, commonly known as the rough skates. They occur mostly on continental shelves and slopes in the north-western and eastern Atlantic Ocean, the Mediterranean Sea, the south-wester ...
'' * '' Malacoraja'' * '' Neoraja'' * '' Okamejei'' * '' Orbiraja'' * ''
Raja ''Raja'' (; from , IAST ') is a royal title used for South Asian monarchs. The title is equivalent to king or princely ruler in South Asia and Southeast Asia. The title has a long history in South Asia and Southeast Asia, being attested ...
'' * '' Rajella'' * ''
Rostroraja ''Rostroraja'' is a genus of skate belonging to the family Rajidae, native to the Atlantic Ocean and East Pacific, including the Mediterranean, Caribbean, Gulf of Mexico and Gulf of California. The genus was considered monotypic in the past ( ...
'' * ''
Spiniraja The Melbourne skate (''Spiniraja whitleyi'') is a species of fish in the belonging to the skate family Rajidae. It is endemic to southern Australia. Its natural habitat is open sea The sea, connected as the world ocean or simply the ocea ...
'' * ''
Zearaja ''Zearaja'' is a small genus of skates in the family Rajidae. It currently consists of four described species found in oceans off New Zealand, Tasmania and southern South America. Species There are four species in the genus: * '' Zearaja a ...
'' Skates have more valid species (574) than any other group of cartilaginous fishes. Since 1950, 126 new species of skates have been discovered. Five scientists take credit for the rapid increase of findings. The Rajidae are considered monophyletic because of their similarity in appearance. There are 13 genera and about 245 valid species. However, there is little information about the diets of about 24% of these species. There are at least 45 dubious species of skates worldwide.


Description


General Batoidea characteristics

Skates are cartilaginous fishes like other
Chondrichthyes Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue. ...
, however, skates, like rays and other Rajiformes, have a flat body shape with flat pectoral fins that extend the length of their body. A large portion of the skate's dorsal body is covered by rough skin made of
placoid scales A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as ...
. Placoid scales have a pointed tip that is oriented caudally and are homologous to teeth. Their mouths are located on the underside of the body, with a jaw suspension common to Batoids known as euhyostyly. Skate's
gill slit Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of cartilaginous fish such as sharks and rays, as well as deep-branching vertebrates such as lampreys. In co ...
s are located ventrally as well, but dorsal spiracles allow the skate to be partially buried in floor sediment and still complete respiratory exchange. Also located on the dorsal side of the skate are their two eyes which allow for predator awareness. In addition to their pectoral fins, skates have a first and second
dorsal fin A dorsal fin is a fin located on the back of most marine and freshwater vertebrates within various taxa of the animal kingdom. Many species of animals possessing dorsal fins are not particularly closely related to each other, though through c ...
,
caudal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...
and paired
pelvic fin Pelvic fins or ventral fins are paired fins located on the ventral surface of fish. The paired pelvic fins are homologous to the hindlimbs of tetrapods. Structure and function Structure In actinopterygians, the pelvic fin consists of two ...
s. Distinct from their rhomboidal shape is a long fleshy slender tail. While skate anatomy is similar to other Batoidea, features such as their
electric organ An electric organ, also known as electronic organ, is an electronic keyboard instrument which was derived from the harmonium, pipe organ and theatre organ. Originally designed to imitate their sound, or orchestral sounds, it has since develope ...
and
mermaid's purse An egg case or egg capsule, often colloquially called a mermaid's purse, is the casing that surrounds the eggs of oviparous sharks, skates and chimaeras. Egg cases typically contain one embryo, except for big skate and mottled skate egg case ...
create clear distinctions.


Skate specific characteristics


Mermaid's purse

Skates produce their young in an egg case called a mermaid's purse. These egg cases have distinct characteristics that are individualized to each species. This makes a great tool for identifying different species of skates. One of these identifiable structures is the keel. The keel is a flexible ridge that runs along the outside of the structure. Another characteristic is the number of embryos in the egg case. Some species contain only one embryo while others can have up to seven. The size of the fibrous shell around the case is another characteristic. Some species have thick layers on the exterior.


Electric organ

The
electric organ An electric organ, also known as electronic organ, is an electronic keyboard instrument which was derived from the harmonium, pipe organ and theatre organ. Originally designed to imitate their sound, or orchestral sounds, it has since develope ...
(EO) is a characteristic exclusive to aquatic species. Among the Chondrichthyes, the only species to possess electric organs are the electric ray and the skate. Unlike many other electrogenic fishes, the skate is unique in that it has two paired electric organs, which run longitudinal through the tail in the lateral musculature of the notochord.Macesic, Laura J.; Kajiura
"Electric organ morphology and function in the lesser electric ray, Narcine brasiliensis"
(PDF). ''Zoology''. 112: 442–450 – via ScienceDirect.
The impulses put out by the electric organs of the skate are considered to be weak, asynchronous, long-lasting signals. Although the anatomy of the skate EO is now commonly studied and well understood, there is not enough strong evidence to suggest what the actual function of the EO is. Some research suggests the electric impulses are too weak to be a mechanism used for defense or hunting. It is also too irregular to be useful for electrolocation purposes. The most reasonable explanation in the literature suggests that the electric organ discharges may be used as a form of communication used for reproduction purposes.


Distribution and habitats

Skates are primarily found from the intertidal down to depths greater than . They are most commonly found along outer continental shelves and upper slopes. They are typically more diverse at higher latitudes and in deep-water. In fact, skates are the only cartilaginous fish taxon to exhibit more diversity of species at higher latitudes. A cool, temperate to polar water in the deep sea can be a favorable environment for skates. As the water becomes more shallow and warmer, skates are seen to be replaced by stingrays. Skates are absent from brackish and freshwater environments. However, there is a single estuarine species that has been found in Tasmania, Australia. Also, the Connecticut Department of Environmental Protection has caught and studied skates within the Long Island Sound estuary. Some skate fauna have been found inhabiting areas of rock cobble and high rocky relief.


Behavior and ecology


Reproduction

Skates mate at the same nursery ground each year. In order to fertilize the egg, males use claspers, a structure attached to the pelvic fins. The claspers allow them to direct the flow of semen into the female's
cloaca In animal anatomy, a cloaca ( ), plural cloacae ( or ), is the posterior orifice that serves as the only opening for the digestive, reproductive, and urinary tracts (if present) of many vertebrate animals. All amphibians, reptiles and birds, ...
. Skates are
oviparous Oviparous animals are animals that lay their eggs, with little or no other embryonic development within the mother. This is the reproductive method of most fish, amphibians, most reptiles, and all pterosaurs, dinosaurs (including birds), and m ...
, meaning they lay eggs with very little development in the mother. This is one major difference from rays, which are
viviparous Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the ...
, meaning they give birth to live young. When a female skate is fertilized, a protected case forms around the embryo called an egg case, or more commonly mermaid's purse. This egg case is then deposited out of the mother's body onto the ocean floor where the skates develop for up to 15 months before they enter the external environment.


Diet and feeding

The majority of skates feed on bottom dwelling animals, such as shrimp, crab, oyster, clams, and other invertebrates. To feed on these animals they have grinding plates in their mouths. Filter feeding rays that eat plankton have gill rakers. Skates are an influential part of the food webs of demersal marine communities. They utilize similar resources to those of other upper trophic-level marine predators, such as seabirds, marine mammals, and sharks. The flattened body shape, ventral eyes and well developed spiracles of the skate allows them to live benthically, buried in the sediment or using a longitudinal undulation of the pectoral fins known as Rajiform locomotion to glide along the water floor. Current research suggests that some species of skates, in addition to their Rajiform locomotion, use their pelvic fins to perform ambulatory locomotion. This form of locomotion performed by the skate is being explored as a possible origin for our own development of walking by looking for similar neural pathways used for movement between skates and animals walking on land.


Skates versus stingrays

Skates are like stingrays in that they have five pairs of gill slits that are located ventrally, which means on the underside of their body (unlike sharks that have their gills located on their sides). Skates and rays both have pectoral fins that are flat and expanded, which are typically fused to the head. Both skates and stingrays typically have their eyes on top of their head. Skates also share similar feeding habits with rays. Skates are different from rays in that they lack a whip-like tail and stinging spines. However, some skates have electric organs located in their tail. The main difference between skates and rays is that skates lay eggs, whereas rays give birth to live young. Moreover, skates can be more abundant than rays, and are fished for food in some parts of the world.


Conservation

Skates have slow growth rates and, since they mature late, low reproductive rates. As a result, skates are vulnerable to
overfishing Overfishing is the removal of a species of fish (i.e. fishing) from a body of water at a rate greater than that the species can replenish its population naturally (i.e. the overexploitation of the fishery's existing fish stock), resulting in t ...
and appear to have been overfished and are suffering reduced population levels in many parts of the world. In 2010, Greenpeace International added the barndoor skate, bottlenose skate, and maltese skate to its seafood red list. "The Greenpeace International seafood red list is a list of fish that are commonly sold in supermarkets around the world, and which have a very high risk of being sourced from unsustainable fisheries."


See also

* Jenny Haniver, a fake sea monster created from a skate carcass. * Hongeohoe, a Korean dish made from fermented skate. * Mokpo, a South Korean city famous for its skate cuisine.


References


External links

* ARKive �
images and movies of the common skate ''(Dipturus batis)''
* Kliman, Todd.

, ''Washingtonian'', May 1, 2006. {{Authority control . Ray families Cretaceous fish Extant Late Cretaceous first appearances Taxa named by Charles Lucien Bonaparte