SKATES Family Rajidae P.R

19 SKATES Family Rajidae P.R. Last, B. Séret, M.F.W. Stehmann & S. Weigmann Members of the family Rajidae, known as skates or hardnose skates, are small to very large rays (adults 33 cm to more than 2 m TL) with a depressed body, almost circular to rhombic disc, and pectoral-fin apices broadly rounded to angular. A firm, slender tail is well demarcated from the disc. The snout is more or less elongated and pointed, supported by a stiff rostral cartilage, and often has a short lobe at its tip. The anterior nasal flaps are expanded to form an incomplete nasal curtain. These flaps usually reach the mouth but their posterior margins are not joined like in some other ray groups, such as the stingrays. Pelvic fins are notched with distinct anterior and usually larger posterior lobes. Two small dorsal fins are located near the end of the tail and their bases are often joined. The caudal fin is greatly reduced in size with the lower lobe (when present) smaller than the upper lobe. The skin is sometimes naked, but the disc and tail are usually partly covered with dermal denticle patches, particularly along the anterior disc margins. Thorns, usually present on the upper disc of juveniles (and most adults), are variably located on the orbital rims (around upper half of eye), nuchal (nape) and scapular (shoulder) regions, and along the median disc and tail (additional lateral thorn rows on the tail of several species). Adult males have a well-defined, longitudinally arranged patch of retractable alar thorns on the mid-outer regions of each pectoral fin, and some species have additional anteriorly positioned thorn patches, known as malar thorns. The family includes at least 154 valid described species in 17 genera: Amblyraja, Beringraja, Breviraja, Dactylobatus, Dentiraja, Dipturus, Hongeo, Leucoraja, Malacoraja, Neoraja, Okamejei, Orbiraja, Raja, Rajella, Rostroraja, Spiniraja and Zearaja. However, several unnamed genera and species are known to exist, and some species are likely to be re-assigned to other genera. Specialised skeletal features, particularly those of the clasper cartilages, are important in distinguishing genera and species. The rostral cartilage varies from being moderately slender to stiff and stout from its base to the snout tip (except in Breviraja), and its base extends anteriorly to, or noticeably forward of, the leading edges of the nasal capsules. The anteriormost part of pectoral-fin skeleton is well separated from the tip of snout (by a semi- translucent area, except in Breviraja), and the ventral terminal cartilage of the clasper has a characteristic sharp, lateral edge forming an external component, the shield (see Glossary). Most skates are demersal in cold water on continental slopes and abyssal plains to depths of more than 4000 m, but some temperate and polar species occur inshore on continental shelves, and 1 lives primarily in brackish/freshwater habitats. The group is cosmopolitan with representatives in all oceans. Some larger, more mobile benthopelagic species are wide-ranging, but most are primarily benthic with restricted distributions, often within small geographical confines. Skates are mainly carnivorous, feeding on benthic invertebrates (i.e. crustaceans, polychaetes, bivalve molluscs, and cephalopods) and fishes living on or in sediments of the seafloor. All species are oviparous, with females laying their eggs in cases (also known as mermaid’s purses). Egg cases are rectangular and flattened in shape with a tough protective outer coating of keratin, and often a horny structure at each corner. Embryos develop slowly, taking up to 2 years to hatch. Skates are caught worldwide, mainly as bycatch of trawl fisheries. The flesh is edible and some species are retained as food for humans. K EY TO RaJID GENERA 205 KEY TO RAJID GENERA The hardnose skates are very difficult to characterise based solely on external features because skeletal structures, particularly of the claspers, are more important in distinguishing groups. The following key is incomplete; clearly separating some genera but failing to distinguish others. For example, genera in couplet 14 could not be distinguished so readers need to use distributional ranges (which largely do not overlap) to help identify species. Where a specimen fails to key out correctly, we suggest proceeding again using the alterative option in couplets. The key below has incorpo- rated some technical characters in order to distinguish between genera (see the Glossary for further explanation). fig. 1 1. Anterior margin of snout very obtuse (fig. 1); anterolat- eral margin of ventral disc with band of claw-like denticles (fig. 2); Western Atlantic ................................... ............... Dactylobatus (2 species; fig. 1, pp. 229–230) Anterior margin of snout somewhat obtuse (fig. 7) to acute (fig. 10); no enlarged claw-like denticles on ante- rolateral margin of ventral disc .................................... 2 fig. 2 2. Anteriormost extension of pectoral-fin skeleton abutting or nearly abutting tip of snout, not separated from pectoral fin ray snout by largely translucent area (fig. 3); tip of snout pliable ................................................................................ 3 Anteriormost extension of pectoral-fin skeleton distinctly separated from tip of snout by semi-translucent area (fig. 4); tip of snout rather firm and not pliable ............................................................................................. 4 3. Triangular patch of thorns present on nuchal and shoulder region (fig. 7); length of anterior pelvic-fin fig. 3 fig. 4 lobe 65–80% of posterior lobe (fig. 5); tail width at base slightly greater than orbit diameter; Western Atlantic skeleton of head ....................... Breviraja (5 species; fig. 7, pp. 224–228) No triangular patch of thorns on nuchal and shoulder region (nuchal thorns and 1 or 2 scapular thorns are sometimes present, but these do not form a distinct triangular patch) (fig. 8); length of anterior pelvic-fin lobe 80–100% of posterior lobe (fig. 6); tail width at base less than orbit diameter; Eastern and North-West Atlantic ........... Neoraja (5 species; fig. 8, pp. 297–301) fig. 5 fig. 6 triangular thorn patch fig. 7 fig. 8 206 RAYS OF THE WORLD 4. Upper disc with 2 prominent pairs of dark pectoral markings with undulate edges (fig. 9); North-West Pacific ........................ Hongeo (1 species; fig. 9, p. 279) Upper disc without pectoral markings, or if present only 1 pair prominent ..................................................... 6 5. Distal quarter to third of tail without distinct thorns, instead uniformly covered with small denticles (fig. 13); Eastern and North-West Atlantic �� ................. Malacoraja (4 species; fig. 10, pp. 293–296) Distal third to quarter of tail with distinct thorns, similar to those on anterior tail (fig. 14) ....................... 7 6. Sensory pores and often canals on ventral surface of fig. 9 disc darkly pigmented (fig. 12), most obvious on head (except in Australian Okamejei) ..................................... 5 Sensory pores and canals on ventral surface of disc not darkly pigmented (pores not strikingly obvious) .... 10 7. Typically large skates with an elongate to very elongate snout (fig. 11); internarial width usually less than 60% of distance from tip of snout to anterior margin of nostril in adults; end of clasper narrowly rounded or pointed; cosmopolitan ....................................................... .................... Dipturus (41 species; fig. 11, pp. 241–278) Typically small to medium-sized skates with a short (fig. 16) to elongate (fig. 15) snout; internarial width usually exceeding 60% of distance from tip of snout to anterior margin of nostril in adults; end of clasper fig. 10 narrowly rounded, pointed, or flattened and spatula- like ..................................................................................... 8 fig. 11 fig. 13 fig. 12 fig. 14 ventral head rear of tail K EY TO RaJID GENERA 207 8. Snout long and narrowly pointed (fig. 15); end of greatly enlarged clasper flattened and spatula-like (fig. 18); temperate South Pacific and South-West Atlantic ........................ Zearaja (4 species; fig. 15, pp. 360–363) Snout short to moderately elongate; end of moderately large clasper narrowly rounded (fig. 19) or pointed (fig. 20) ....................................................................................... 9 9. Long interdorsal and postdorsal tail sections (fig. 16); end of clasper in all species, except O. ornata, with small soft spiny structure near its tip (component funnel) (fig. 22); Indo–West Pacific .................................. .... Okamejei (in part, 12 species; fig. 16, pp. 302–313) fig. 15 Short interdorsal and postdorsal tail sections (fig. 17); clasper without component funnel; Australia ............... ...... Dentiraja (in part, 8 species; fig. 17, pp. 231–240) 10. Median thorns on disc very large and with stellate bases (fig. 23); end of massive, broad clasper club- shaped and widened (fig. 21); cosmopolitan .................. .................. Amblyraja (8 species; fig. 25, pp. 210–217) Median thorns on disc (if present) small to moderate in size and without stellate bases (fig. 24); end of clasper narrowly rounded (fig. 19) or pointed (fig. 20) ........... 11 fig. 16 spatula-like narrowly rounded fig. 18 fig. 19 pointed club-shaped fig. 20 fig. 21 shapes of right clasper fig. 17 clasper glans funnel fig. 22 side view of fig. 23 median thorns

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