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Overview

Brief Summary

Biology

All skates are oviparous (8) (egg-laying), with large female thornbacks laying up to 140 to 160 egg capsules per year (6). The species moves into shallower, inshore waters in spring when ready to spawn, laying eggs between March and September, which take a further four to six months to hatch (6). Young feed on small crustaceans and other bottom-dwelling creatures, predating more on fish and larger crustaceans such as crabs as they develop (5) (6). Maturity is reached at around eight years of age (5), and the maximum lifespan is at least 15 years (6).
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Description

The thornback skate is probably one of the commonest skates encountered by divers (3), being among the most abundant rajids in the north-eastern Atlantic and Mediterranean (1). As with all skates, the body is flattened and disc-shaped, with the pectoral fins broadly expanded and joined to the head and body (4). The tail is distinctly demarcated from the disc-like body, relatively narrow, and about as long as body length (3) (4). The upper surface of the disc and tail are covered with numerous thorns, which become thickened with button-like bases (known as bucklers) once the skate is sexually mature, hence the species' common name (3) (5). Only the snout and margins of the disc are prickly in young, and the underside is only prickly in large, mature females, which also possess more developed 'bucklers' on their back and tail (2) (3). The colour varies from light brown to grey on the upper surface, variegated with dark and light spots and blotches that camouflage the skate in the dappled light of the sea bed, while the underside is creamy-white (2) (3).
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Comprehensive Description

Biology

Inhabits shelf and upper slope waters. Depth range from 10-300 m (Ref. 4426, 82319, 88187), and from 300-577 m in the eastern Ionian Sea (Ref. 56504); most common in coastal waters between 10-60 m depth (Ref. 58137). Tolerates low salinities (Ref. 88171). Found on mud, sand and gravel bottoms, rarely on rougher bottoms (Ref. 6808, 58137). Nocturnal species. Feeds on all kinds of bottom animals, preferably crustaceans (Ref. 3167). Undertakes migrations with mean distances of 54-117 km per month; shows a clear annual migration cycle (Ref. 89017, 89018), moves from deeper offshore waters (10-30 m) in autumn and winter to shallower areas (
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Description

 A short-snouted ray with typical diamond shape and sharply angled pectoral fins approaching 90 degrees. The colour is variable, usually a mottled, blotchy brown to grey, with numerous small dark spots and yellowish patches. In young fish the yellow patches form distinct spots, each outlined with smaller dark spots. The underneath is an off-white with grey to black margins. Dorsally the body is covered with coarse prickles, which are present from hatching. In addition, large, backward pointing thorns (called bucklers) are scattered over the dorsal surface, each of which has a thick button-like base. These thorns are particularly numerous on the tail and in females present in a line along the back. Raja clavata can grow up to 1 m long however 85 cm is the usual maximum length. Females are larger than males.On account of its abundance, Raja clavata is an important fish commercially. Most of the skate found in fishmongers is likely to be thornback ray also known as the 'roker'. Thornback rays lay up to 150 egg cases a year. The empty egg cases are a common sight washed up on the shore and are known as mermaids purses. Thornback rays tend to lie covered in sand during the day and feed at night on a range of bottom-dwelling animals. They will eat fish such as sand eels, herrings, sprats and small flatfish however shore and swimming crabs and brown shrimps are its main food. Raja clavata resembles other rays found in area, but is distinguished by the large 'buckler' thorns scattered on dorsal surface.
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Description

The thornback ray is probably one of the commonest rays encountered by divers. Like all rays it has a flattened body with broad, wing-like pectoral fins. The body is kite-shaped with a long, thorny tail. The back is covered in numerous thorny spines. In sexually mature fish some of the spines are thickened with button-like bases (known as bucklers). These are particularly well developed on the tail and back of sexually mature females. The colour varies from light brown to grey with darker blotches and numerous small darker spots and yellow patches. Sometimes the yellow patches are surrounded by small dark spots. The underside is creamy-white with a greyish margin. Adult fish can grow to 1m in length although most are less than 85cm. Other rays have spiny thorns, especially on their tails however only the thornback has large spines with button-like bases (bucklers).
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Distribution

Range Description

The Thornback Skate, or roker, is one of the most abundant rajids in north European coastal waters and can be the dominant rajid in commercial landings and research vessel catches (Rousset 1990a, Ellis unpubl.). It is widely distributed from Iceland and Norway (south of the Arctic Circle), to the North Sea (where it is now less abundant in south-eastern areas (Walker 1998), the Mediterranean, the western Black Sea, Madeira, the Atlantic coasts of Africa, and as far south as South Africa and the south-western Indian Ocean (Stehmann 1995). The status of this species in West and South African waters, and its relationship with Raja (Raja) cf. clavata, which is reported from the waters off Namibia and southern Africa (Macpherson 1986, Ebert et al. 1991, Smale and Cowley 1992) needs further research.
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Eastern Atlantic: Iceland, Norway, North Sea and the western Baltic southward to Morocco and Namibia, including the Mediterranean and the Black Sea. Although reported from southern Africa, its status in the area is uncertain (Ref. 5578).
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Geographic Range

Raja maderensis is wide spread in coastal waters from Iceland to Norway. Its geographic ranges extends into the North Sea, the Mediterranean Sea, the western Black Sea, Madeira Island, the Atlantic coasts of Africa, as well the waters off of South Africa and the south-western Indian Ocean. Adult R. clavata remains inshore during summer and move off shore into deeper waters during winter.

Biogeographic Regions: palearctic (Native ); ethiopian (Native ); mediterranean sea (Native )

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Baltic Sea, North Sea, Mediterranean Sea, Black Sea, eastern Atlantic: Iceland to Namibia; possibly South Africa and Mauritius (Mascarenes).
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Range

Found in the eastern Atlantic from Iceland, Norway, North Sea and the western Baltic southward to Morocco and Namibia, including the Mediterranean and the Black Sea (2). Also reported from South Africa eastward into the south-western Indian Ocean (4).
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This species is common all around the coasts of Britain and Ireland.
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Physical Description

Morphology

Dorsal spines (total): 0; Analspines: 0; Analsoft rays: 0
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Physical Description

Raja maderensis shares its general body shape with all rays in that it is a flattened, disc shape with broad pectoral fins connected to the head and body with triangular pelvic fins at the base of the body. Raja maderensis has a long, narrow tail that is roughly equivalent to the length of the body. As the common name denotes (thornback ray), the upper portion of the body and tail are covered with thorn-like projections. When R. clavata reaches sexual maturity the bases of the thorns thicken to resemble small button like projections called buckler. The snout and small portions of the body are covered in thorns in sub adults and the underside may be thorny in large females. Adults typically have between 21 and 25 large thorns running from the nape to its first pectoral fin.  It is sexually dimorphic, as females have a maximum length of 118 cm and males have a maximum length of 98 cm. The dorsal surface of R. clavata exhibits a wide range of coloration from light brown to grey and is sprinkled with light and dark spots of varying size. Its underside is solid white except for the snout which is grey. The largest specimen ever recorded weighed 18 kg.

Range mass: 18 (high) kg.

Range length: 98 to 120 cm.

Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry

Sexual Dimorphism: female larger

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Size

Maximum size: 1010 mm TL
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Max. size

105 cm TL (male/unsexed; (Ref. 53748)); 139 cm TL (female); max. published weight: 18.0 kg (Ref. 35388); max. reported age: 15 years (Ref. 41305)
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Diagnostic Description

Sub-rhomboid in shape with pointed wing-tips; disc-width 1,25 to 1,36 times in its length, its length 1,70 to 1,83 times in total length. Short rostrum, rounded at his extremity; pectoral fins with clear angles on lateral side; triangular pelvic fins (Ref. 39246). Dorsally prickly; large females also prickly throughout their ventral surface; juveniles and large males prickly along the margins of their discs and the underside of their snout. 30-50 thorns form a median row from the nape to the first dorsal fin; additional large 'buckler' thorns with swollen bases scattered on upper surface of disc in adults (Ref. 3167). Coloration is highly variable, with the dorsal surface having all shades of brown with dark and light spots and blotches; ventral surface white (Ref. 3167, 58137).
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Ecology

Habitat

Known from seamounts and knolls
  • Stocks, K. 2009. Seamounts Online: an online information system for seamount biology. Version 2009-1. World Wide Web electronic publication.
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Habitat and Ecology

Habitat and Ecology
The Thornback Skate is a demersal coastal species which inhabits a variety of substrates, including mud, sand, shingle, gravel and rocky areas, in water down to 300 m, although it is most abundant in 10? 60 m of water off coastal areas (Wheeler 1969, Stehmann and Buerkel 1984). Rousset (1990a) studied the elasmobranch assemblage off the coast of Brittany and found that R. clavata was the most abundant rajid due to the fact that it was able to inhabit this range of benthic habitats. Thornback Skate is the second most important species, after the Smallspotted Catshark (Scyliorhinus canicula), in the demersal elasmobranch assemblage in the northern Bristol Channel and constitutes between 7.4?8.8% of the elasmobranch biomass (Ellis unpubl.). The age and growth of this species have been studied by examining the vertebrae of fish caught in the Bristol Channel (Ryland and Ajayi 1984), although Brander and Palmer (1985), using length-frequency analysis, have indicated inconsistencies for the smaller sized fish used in this work. Ryland and Ajayi (1984) gave maximum age and length of 12 years and 1,047 mm (k = 0.090). Vertebral growth rings have been shown to be annual (Holden and Vince 1973). Growth has also been estimated from tagging studies (Holden 1972). Validated age studies of specimens from the southern North Sea have shown thornback skates to have a maximum length of 118 cm for females and 98cm for males (Walker 1998). The size at maturity for females and males have been estimated at 45?50 cm disc width (DW) and 38?44 cm DW (Fitzmaurice 1974); 85 cm total length (TL) (54 cm DW) and 75 cm TL (48 cm DW) (Capapé 1976); 45 cm DW and 42 cm DW (Nottage and Perkins 1983) and 59 cm TL and 60 cm TL (Ryland and Ajayi 1984). Walker (1998) estimated length at 50% maturity to be 77 cm TL for males and 68 cm TL for females. The corresponding ages at 50% maturity were eight and seven years. It has been reported that R. clavata first spawn in their fifth year (Ryland and Ajayi 1984). Eggs are laid during a protracted breeding season from February to September (Holden 1975), with a peak in May and June. However, this is for the population as a whole and the egg-laying period for individual fish may be shorter. Ellis and Shackley (1995) maintained one female in captivity and reported that egg laying lasted six weeks, with a mean egg-laying rate of 1.07 eggs per day, a pair of eggs being laid on alternate days. These data concur with the observations of Holden (1971). Development lasts 16?20.5 weeks (Ellis and Shackley 1995), although this period may vary with temperature. The young hatch at a length of 10?13 cm. The nursery areas used are coastal, estuarine and tidal flat areas (e.g., the Wash and Thames estuary in the UK). The fecundity of R. clavata in British waters has been estimated at 150 eggs per year (Holden 1971), 140 eggs per year (Holden 1975) and 100 eggs per year (minimum of 62? 74) (Ryland and Ajayi 1984). Capapé (1976, 1977a) estimated a fecundity of 70?167 eggs per year in Tunisian waters, although it may be as low as 48 (Ellis and Shackley 1995). The feeding habits have been well documented from many areas over its geographical range, including British waters (Holden and Tucker 1974, Ajayi 1982, Ellis et al. 1996), Ireland (Fitzmaurice 1974), France (Du Buit 1968, 1978?79, Quiniou and Andriamirado 1979), Portugal (Marques and Re 1978, Cunha et al. 1986), the Mediterranean (Capapé 1975, 1977; Abdel-Aziz 1986), the Southeast Atlantic waters off Namibia (Macpherson 1986) and southern Africa (Ebert et al. 1991). Young and juvenile R. clavata predominantly eat small crustaceans, such as shrimps, mysids, amphipods and small crabs. Larger specimens prey on larger crustaceans, including prawns and crabs and will also consume fish. The migratory habits have been studied by Steven (1936) who found that very little movement occurred, especially in young fish, with 71% of tagged fish moving less than five miles. Fish tagged in the southern North Sea also showed a sedentary pattern, with 80% being recaptured within 40 nautical miles of their release position (Walker et al. 1997). The recapture percentage was nearly 30%. Fitzmaurice (1974) studied the populations within two bays in Ireland and reported a sex ratio of 1:1 and, of 71 tagged, eight (11.3%) recaptures. Rousset (1990b) observed that mature females were more common in exposed areas and juveniles and mature males were more common in more sheltered areas.

Systems
  • Marine
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Environment

demersal; marine; depth range 10 - 577 m (Ref. 56504)
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Raja maderensis lives at the bottom of the continental shelf and upper slope of the coastal sea floor. It inhabits a wide range of sea floor habitats including mud, sand, shingle, gravel and rocky areas. Raja maderensis has been found in coastal waters as deep as 300 m but is most abundant in waters between 10 and 60 meters. Juveniles are often found further inshore in more shallow waters than their adult counterparts.

Range depth: 10 to 300 m.

Average depth: 60 m.

Habitat Regions: saltwater or marine

Aquatic Biomes: coastal

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Depth range based on 12415 specimens in 1 taxon.
Water temperature and chemistry ranges based on 5046 samples.

Environmental ranges
  Depth range (m): -9 - 820
  Temperature range (°C): 3.997 - 17.167
  Nitrate (umol/L): 0.887 - 34.321
  Salinity (PPS): 31.839 - 38.670
  Oxygen (ml/l): 3.290 - 6.588
  Phosphate (umol/l): 0.098 - 2.461
  Silicate (umol/l): 0.987 - 37.958

Graphical representation

Depth range (m): -9 - 820

Temperature range (°C): 3.997 - 17.167

Nitrate (umol/L): 0.887 - 34.321

Salinity (PPS): 31.839 - 38.670

Oxygen (ml/l): 3.290 - 6.588

Phosphate (umol/l): 0.098 - 2.461

Silicate (umol/l): 0.987 - 37.958
 
Note: this information has not been validated. Check this *note*. Your feedback is most welcome.

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 Raja clavata frequents a wide variety of grounds from mud, sand, shingle and gravel. Found to a depth of 300 m but most common between 10 60 m. Although mainly a non-migratory species, the fish often moves close inshore during the winter and spring.
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Depth: 0 - 300m.
Recorded at 300 meters.

Habitat: demersal. Found in shelf and upper slope waters from the coastal line to about 300 m. Feeds on all kinds of bottom animals, preferably crustaceans. Oviparous, about 150 egg-cases are laid in a year, in spring (northwestern Europe) or winter and spring (Mediterranean). Utilized fresh and frozen (Ref. 9987).
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A bottom-dwelling species occupying continental shelf and upper slope waters to depths of 300 m, but mainly from 10 to 60 m (6). However, off South Africa this species occurs to 1000 m (4). Usually found on sediment type sea beds such as mud, sand or gravel, occasionally adjacent to rocky reefs (3) (7).
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The thornback ray is usually found on sediment type seabeds such as mud, sand or gravel at depths between 10-60m. Juvenile fish feed on small crustaceans, particularly amphipods and bottom-living shrimps, Adults feed on crabs, shrimps and small fish.
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Trophic Strategy

Nocturnal predator. Feeds on all kinds of bottom animals (Ref. 3167). Polychaetes and crustaceans are important prey items of smaller individuals, bony fishes become more important prey items of larger individuals (Ref. 28070).
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Food Habits

Raja maderensis is carnivorous. Juveniles feed predominantly on bottom dwelling aquatic crustaceans such as amphipods, mysids and crangonid shrimps. Larger adults feed on larger crustaceans such as swimming crabs and fish, such as sandeels, small gadoids and dragonets.

Animal Foods: fish; aquatic crustaceans

Primary Diet: carnivore (Piscivore , Eats non-insect arthropods)

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Associations

Ecosystem Roles

Raja maderensis feeds on a variety of benthic animals. Juveniles feed on small crustaceans such as amphipods, mysids and crangonid shrimps, and adults feed on fish and larger crustaceans such as swimming crabs and fish such as sandeels, small gadoids and dragonets. This species is also known to feed on conspecifics as well. The eggs of R. clavata act as prey for some fish and mollusks such as necklace shells. Juveniles are preyed upon by common skate and adult R. clavata. Due to its high placement in the costal sea floor food web, R. clavata has a significant effect on trophic levels of the benthos. Parasites of this species have not been documented.

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Predation

Raja maderensis eggs are preyed upon by some fish and mollusks, such as necklace shells. Juveniles are commonly preyed upon by larger fish, specifically common skates and adult R. clavata. The coloration of R. clavata likely helps reduce risk of predation.

Known Predators:

Anti-predator Adaptations: cryptic

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Life History and Behavior

Behavior

Communication and Perception

Like most other elasmobranchs, Raja maderensis uses a diverse array of senses that allow it to perceive orientation as well as locate potential predators and prey. In order locate predators and prey, it depends on its acute sense of smell and a lateral line system that allows it to detect changes in pressure in the local environment. The lateral line system is made up of individual mechano-receptors called neuromasts which run the length of the body. These neuromasts are hair filled structures surrounded with a jelly like substance. These are connected to lateral line canals which interact with the environment. Pressure changes in the water displace the hair and fires impulses to the organism’s brain alerting it to the direction and strength of the disturbance. Raja maderensis also depends on electrosensory perception organs known as Ampullae of Lorenzini. The ampullae, which consists of a series of gelatin filled pits that create an electrically sensitive network along the ventral surface of the animal, help sense minor electrical pulses created by the muscle contractions of organisms in the local environment. The ampullary organs interact with local electric fields through an opening in the skin on the snout of the animal that leads to a jelly filled canal. The electrical fields produced by animals dissipate rather quickly so R. clavata can only perceive the weak current at short range. Within that short range the thornback ray can accurately determine the location of the prey, even if buried in the sand. Some studies have indicated that R. clavata as well as other sharks and rays can use their Ampullae of Lorenzini to detect the strength and direction of the Earth’s magnetic field as a sort of rudimentary positioning system. Studies have shown that the thornback ray does not rely sight for locating prey.

Raja maderensis acquires information from its three otilith organs (the utriculus, the sacculus, and the lagena) to maintain spatial equilibrium in its marine habitat. These organs are located in the inner ear vestibular system where the semicircular canals converge next to the cochlea. The three organs respond to tilts in both the longitudinal and transverse axes. The utriculus and sacculus produce similar responses and thus create mutually reinforcing signals. The macula lagena is made up of gravity receptors which provide a marker for R. clavata upright position. The macula utriculus produces an increase in discharge to signal side-up and nose-up displacements while the macula sacculus signals side-up and nose-down displacements. All three maculae act as an "out-of-position receptor" when the individual is subjected to a constant speed spatial deviation.

Thornback rays have a very acute sense of smell that they use for locating prey. They have two openings called nares that are located ventrally on the organism’s snout. Water enters the nares and passes through a structure known as the olfactory sac. The olfactory sac is lined with a series of folded tissue called the olfactory lamellae which provide increased surface area for molecule-receptor interactions. Molecules dissolved in the water bind to neuroreceptors in the olfactory lamellae to provide chemosensory information to the brain.

Perception Channels: visual ; acoustic ; vibrations ; chemical ; electric ; magnetic

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Life Cycle

Oviparous. Distinct pairing with embrace. Polyandrous species (Ref. 89024). Paired eggs are laid and deposited on shallow sand, mud, pebble or gravel bottoms (Ref. 205, 89025). Up to 170 egg cases can be laid by a single female in a year (Ref. 3167), but average fecundity is much lower (around 48-74 eggs) (Ref. 3603, 31302). In northwestern Europe, egg cases are laid during spring (Ref. 3167) and in the Mediterranean during winter and spring (Ref. 3167). Egg cases are oblong capsules with stiff pointed horns at the corners, each containing one embryo (Ref. 205). Capsules are 5.0-9.0 cm long without the horns (Ref. 41250, 88187) and 3.4-6.8 cm wide (Ref. 41250). Egg cases are anchored with an adhesive film (Ref. 82399). Embryos feed solely on yolk (Ref. 50449). Egg cases hatch after about 4-5 months and pups are about 11-13 cm TL (Ref. 88864). Mating season from February to September, peaking in June (Ref. 74501). Adults observed to form same-sex aggregations during the mating season with females moving to shallower inshore waters approximately a month before the males (Ref 3603, 58137, 74501). Mating does not occur in the Baltic Sea (Ref. 82311).
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Development

Young Raja maderensis hatch at 11 to 13 cm in length and 8 to 9 g in weight with a male:female sex ratio of 1:1. Juveniles remain in shallow water. As they increase in size, juveniles grow large button like structures called bucklers. When R. clavata reaches sexual maturity, usually around 8.8 years, it begins seasonally migrating. During summer, adults move in shore during the summer and migrate into deeper waters during the winter.

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Life Expectancy

Lifespan/Longevity

In the wild, Raja maderensis lives between 12 and 15 years. There is no information available regarding the average lifespan of captive individuals.

Typical lifespan

Status: wild:
12 to 15 years.

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Lifespan, longevity, and ageing

Maximum longevity: 12 years (wild) Observations: Unverified estimates suggest these animals may live up to 23 years (http://www.fishbase.org/).
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Reproduction

Little is known of the mating behavior of many skates, including Raja maderensis. Some speculate that this is because the animals mate in the cover of night. Studies of sting rays may provide insight into the mating behaviors of elasmobranchs. During mating, females swim in shore and spend the morning either buried in the sand or lay atop each other in large groups. Those females buried are attempting to avoid males that already mated or are not yet ready for reproduction whilst those in the aggregate groups are prepared for mating. Meanwhile, males swim up and down the beach searching for potential mates. In the afternoon, they switch roles and the males bury themselves in the sand while the females become mobile to search for food in the grassy beds. This cycle can continue for multiple weeks.

Thornback rays are oviparous with females migrating further in shore to lay their eggs. Egg-laying season for Raja maderensis occurs between March and September. Mature females deposit one egg at a time in sandy or muddy substrate close to shore. Eggs range in length from 5 to 9 cm long and 3.8 to 6.4 cm wide. Mature females can lay between 140 and 160 eggs in a single year. Eggs are rectangular and oblong in shape and are marked by rigid horns on each corner. During development, embryos feed solely on their yolk. Embryonic development typically takes between 4 and 6 months and is largely dependent on ambient water temperature.

Breeding season: Thornback rays breed from March to September.

Range number of offspring: 140 to 160.

Range gestation period: 4 to 6 months.

Average age at sexual or reproductive maturity (female): 8.8 years.

Average age at sexual or reproductive maturity (male): 7.1 years.

Key Reproductive Features: iteroparous ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (Internal ); oviparous

There is no information available regarding parental care in Raja maderensis; however, studies on closely related elasmobranches indicate a high level of parental investment.

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Molecular Biology and Genetics

Molecular Biology

Barcode data: Raja clavata

The following is a representative barcode sequence, the centroid of all available sequences for this species.


No available public DNA sequences.

Download FASTA File
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Statistics of barcoding coverage: Raja clavata

Barcode of Life Data Systems (BOLDS) Stats
Public Records: 11
Specimens with Barcodes: 100
Species With Barcodes: 1
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Conservation

Conservation Status

IUCN Red List Assessment


Red List Category
NT
Near Threatened

Red List Criteria

Version
3.1

Year Assessed
2005

Assessor/s
Ellis, J.

Reviewer/s
Musick, J.A. & Fowler, S.L. (Shark Red List Authority)

Contributor/s

Justification
This assessment is based on the information published in the 2005 shark status survey (Fowler et al. 2005).

This demersal batoid is one of the most abundant elasmobranchs in the Northeastern Atlantic. The Thornback Skate (Raja clavata) is widespread, although the taxonomy of specimens from South Africa requires additional study. There is some limited evidence of a decline in landings in the northern part of the East Atlantic range of this species and management of the fishery is required. However, declines have not been as serious as reported for other large rajids.
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Raja maderensis is classified as near threatened on to the ICUN's Red List of Threatened Species. Declines in R. clavata landings are thought to be due to over exploitation and poor species identification. It is caught primarily as by-catch in both otter and beam trawl fisheries, and is targeted with gillnets, long-lines, and recreational anglers. Landings of rays and skates are often lumped together under one generic category (i.e., Raja spp.), but market samplings suggest that R. clavata is the most dominant species landed in the North Sea and Skagerrak. Over exploitation in this species is especially relevant as it has slow growth, long life span, and long period before individuals become reproductively mature make. Research suggests that R. clavata may be extinct from the North Sea due to over fishing. In an effort to help R. clavata recover (as well as other batids), total allowable catch (TAC) for all skates and rays was reduced by 47% from 1999 to 2005. Some fisheries in the United Kingdom have also implemented a minimum landing size. Unfortunately, localized management efforts are not expected to have an impact on regional population conservation.

US Federal List: no special status

CITES: no special status

State of Michigan List: no special status

IUCN Red List of Threatened Species: near threatened

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Status

Classified as Lower Risk/near threatened (LR/nt) on the IUCN Red List 2004 (1).
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Population

Population Trend
Decreasing
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Threats

Major Threats
Thornback Skate is a very important component of demersal fisheries in most European waters and is taken by trawl and gillnet, particularly as bycatch. There is or has been limited directed longlining and netting for the species. Landings of this species are not known, as landings of all rajids are combined in the records (ICES 1958?1987). Holden (1963) looked at the species composition of rajids landed by commercial trawlers at Milford Haven and Fleetwood, UK, during 1961 and 1962 and R. clavata accounted for 34.9% and 12.72% respectively. There is no evidence of severe population depletion, as has been documented for the Common Skate (Dipturus batis), although landings are considered to be in decline and a management strategy is required. Thornback Skates are also regularly caught by recreational anglers, although mortality from this source of fishing pressure will be of little impact for the population as a whole, particularly in areas where catch and release is practised.
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Near Threatened (NT)
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The thornback skate is an important component of mixed trawl fisheries, has been targeted by recreational anglers (1), and is also caught incidentally as bycatch in beam trawl fisheries (9). The thorniness of this species adds to their chance of being entangled in nets (9). There is some evidence of a decline in catch rates in north-west European waters, which could imply a decline in population numbers, but more data is required to confirm this (1). The impact of the fishing industry on rays and skates is the subject of increasing concern due to their slow growth rate, late maturity and low fecundity, making them particularly vulnerable to population collapse due to fishing activity (10).
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Management

Conservation Actions

Conservation Actions
Several of the UK?s local Sea Fisheries Committees have by-laws for a minimum landing size (e.g., 40 cm DW in the Southern and the Kent and Essex Sea Fisheries Districts). Such localised management initiatives will not, however, be of significant effect in conserving regional populations. Due to European rajid fisheries being a component of multispecies fisheries, which also target several species of flatfish and gadoid, gear restriction using mesh size is not a viable management measure. Minimum landing sizes have been implemented in some areas of the UK by Sea Fisheries Committees.
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Conservation

Fishing of the thornback skate is controlled through the authorities limiting the Total Allowable Catch (TAC), although the quota is set for all rajids combined, covering EU waters in the North Sea and Norwegian Sea. This measure helps ensure continuity of economic activity in the fisheries concerned, while giving depleted stocks a reasonable chance of recovery. The TAC for skates and rays has been reduced by approximately 47 % between 1999 and 2005, providing greater protection to the species by reducing the quota allowed to be caught. However, records show that the actual numbers caught each year have in fact been below what is permissible through the TAC (6). Thus, these skate are not currently considered to be particularly over-fished and are not seen as a high conservation priority.
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Relevance to Humans and Ecosystems

Benefits

Importance

fisheries: commercial; gamefish: yes
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Economic Importance for Humans: Negative

There are no known adverse effects of Raja clavata on humans.

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Economic Importance for Humans: Positive

Raja maderensis is more highly valued for its meat than other skates and rays. Thus, R. clavata plays an important economic role in fisheries, particularly those off the coast of Portugal.

Positive Impacts: food

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Wikipedia

Thornback ray

The thornback ray (Raja clavata) or thornback skate is a species of fish in the Rajidae family. It is found in coastal waters of Europe and the Atlantic coast of Africa, possibly as far south as Namibia and even South Africa. Its natural habitats are open seas and shallow seas. It is sometimes seen trapped in large estuarine pools at low tide.

Description[edit]

The thornback ray is probably one of the most common rays encountered by divers. Like all rays, it has a flattened body with broad, wing-like pectoral fins. The body is kite-shaped with a long, thorny tail. The back is covered in numerous thorny spines, as is the underside in older females.[1] Adult fish can grow to 1 m (3.3 ft) in length, although most are less than 85 cm (33.67 in). This ray can weigh from 4.5 to 8.75 lb (2 to 4 kg).[2]

In sexually mature fish, some of the spines are thickened with button-like bases (known as bucklers). These are particularly well developed on the tails and backs of sexually mature females. Their colours vary from light brown to grey with darker blotches and numerous small darker spots and yellow patches. Sometimes the yellow patches are surrounded by small dark spots. The underside is creamy-white with a greyish margin.When threatened they can appear black.

Habitat[edit]

The thornback ray is usually found on sedimentary seabeds such as mud, sand or gravel at depths between 10 and 60 m. Juvenile fish feed on small crustaceans, particularly amphipods and bottom-living shrimps; adults feed on crabs, shrimps and small fish.

References[edit]

  1. ^ "Morphology" in
  2. ^ Kindersley, Dorling (2001,2005). Animal. New York City: DK Publishing. ISBN 0-7894-7764-5. 

http://www.habitas.org.uk/marinelife/species.asp?item=ZF1360

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