Raja clavata

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. (CHEVOLOT, et al., 2006a; CHEVOLOT, et al., 2006b; Ellis, 2005)

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. ("ARKive Images of Life on Earth", 2006; "ICES-Fish Map", December 2005; Ellis, 2005)

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. ("ARKive Images of Life on Earth", 2006; "Raja clavata Linnaeus, 1758 Thornback ray", October 6, 2010; Ellis, 2005)

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. (Ellis, 2005; Nottage and Perkins, 2006)

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. (Smith, 2004)

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. ("ARKive Images of Life on Earth", 2006; "ICES-Fish Map", December 2005; "Raja clavata Linnaeus, 1758 Thornback ray", October 6, 2010; Ellis, 2005; Holden and Vince, 1973)

There is no information available regarding parental care in Raja maderensis; however, studies on closely related elasmobranches indicate a high level of parental investment. (Ellis, 2005; Payne, et al., 2008)

In the wild, Raja maderensis lives between 12 and 15 years. There is no information available regarding the average lifespan of captive individuals. ("ARKive Images of Life on Earth", 2006; Holden and Vince, 1973)

Raja maderensis can often be found lying motionless on the bottom of the sea floor, drawing water into its gill slits through a spiracle located behind its eye. It travels in groups of similar sized fish. While traveling, R. clavata use a rajiform swimming method that is common among skates and rays. To achieve directional locomotion, R. clavata simultaneously undulates both pectoral fins in a wave-like manner. ("ICES-Fish Map", December 2005; Ellis, 2005; Hunter, et al., 2005)

There is no information available regarding the average home range size of Raja maderensis.

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. (KALMIJN, 1971; Lowenstein and Roberts, 1950; Lowenstein, et al., 1964; Maruska, 2001; Parker, 1905)

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. (KALMIJN, 1971; Lowenstein and Roberts, 1950; Lowenstein, et al., 1964; Maruska, 2001; Parker, 1905)

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. (KALMIJN, 1971; Lowenstein and Roberts, 1950; Lowenstein, et al., 1964; Maruska, 2001; Parker, 1905)

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. (Ellis, 2005; "ICES-Fish Map", December 2005; Ellis, 2005)

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. ("ICES-Fish Map", December 2005)

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. (Ellis, 2005)

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. ("Thornback", 1890; Stevens, et al., 2000)

There are no known adverse effects of Raja clavata on humans. (Ellis, 2005)

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. ("ARKive Images of Life on Earth", 2006; "ICES-Fish Map", December 2005; Ellis, 2005; Stevens, et al., 2000)