Young Raja clavata 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.
Raja clavata 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
Raja clavata 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 clavata 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
In the wild, Raja clavata 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.
Raja clavata 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 clavata 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
Raja clavata 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 )
Raja clavata 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)
Raja clavata 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.
Raja clavata 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
There are no known adverse effects of Raja clavata on humans.
Raja clavata 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
Like most other elasmobranchs, Raja clavata 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 clavata 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 clavata 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
Little is known of the mating behavior of many skates, including Raja clavata. 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 clavata 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 clavata; however, studies on closely related elasmobranches indicate a high level of parental investment.
The thornback ray (Raja clavata), or thornback skate, is a species of ray fish in the family Rajidae.[2]
The Thornback ray is found in the Atlantic coastal waters of Europe and western Africa. It is also present from south Africa to the southwestern Indian Ocean [2] and in the Mediterranean and Black Seas.[3] It is native possibly as far south as Namibia and South Africa.[2]
Its natural habitats are open large seas and shallow seas. It is sometimes seen trapped in large estuarine pools at low tide.[2]
The thornback ray is probably one of the most common rays encountered by divers.
The thornback ray is usually found on sedimentary 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.
Like all rays, the thornback ray 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.[2][4]
Adult fish can grow to 1 metre (3.3 ft) in length, although most are less than 85 centimetres (33 in). This ray can weigh from 4.5 to 8.75 lb (2 to 4 kg).[5]
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.[6][7]
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.[7]
Raja clavata, the thornback ray (or thornback skate, roker), was named by Carl Linnaeus in the 10th edition of Systema Naturae in 1758, in the genus Raja of the Order Rajiformes in the Family Rajidae.[2]
It is one of about 13 species of skate (family Rajidae) that are known from the North Sea and adjacent Atlantic waters.
Common names include:[8]
Dorsal spines : 0; Anal spines: 0; Anal soft rays: 0. ; disc-width 1,25 to 1,36 times in its length, its length 1,70 to 1,83 times in total length; pectoral fins with clear angles on lateral side; triangular pelvic fins . Dorsally prickly; large females also prickly throughout their ventral surface; young and large males prickly along the borders 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 . Max length : 105 cm male/unsexed; 139.0 cm (female); common length : 85.0 cm ; max. weight: 18.0 kg.
Oviparous. Polyandrous species. Paired eggs are laid and deposited on shallow sand, mud, pebble or gravel bottoms . Up to 170 egg cases can be laid by a single female in a year, average fecundity around 48-74 eggs. In northwestern Europe, egg cases are laid during spring, and in the Mediterranean during winter and spring. Egg cases are oblong capsules with stiff pointed horns at the corners, each containing one embryo. Capsules are 5.0-9.0 cm long without the horns and 3.4-6.8 cm wide. Egg cases are anchored with an adhesive film.
Embryos feed solely on yolk. Egg cases hatch after about 4–5 months and pups are about 11–13 cm.
Mating season is from February to September, peaking in June. Adults observed to form same-sex aggregations during the mating season with females moving to shallower inshore waters approximately a month before the males. Mating does not occur in the Baltic Sea.[2] This is one of the fish used by the marine leech Pontobdella muricata as a host.[9]
A search about the growth and maturation of Raja clavata in the Solway Firth (part of the border between Cumbria, England and Dumfries and Galloway, Scotland) shows that the males and females appear to mature at 42 and 45 cm in disc width respectively. The Solway population is heavily exploited by an unrestricted commercial fishery and a considerable proportion (48.6%) of the retained catch is immature. It is suggested that fishing pressure has brought about a reduction in the size at which female fish mature.[10]
The thornback ray (Raja clavata), or thornback skate, is a species of ray fish in the family Rajidae.
