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  To support such wide pectoral fins, parts of the cartilage are fused to the vertebral column. Between the cranium and shoulder girdle the skate’s vertebrae are fused into a tube called the synarchial. The disc is further supported by the trunk. Another feature of the adaptive flattening of the skate is its upper jaw, which has no attachment to the cranium and is thus more mobile in feeding at substrate level. The skate’s mouth is surrounded by ampullae of Lorenzini, which enable the animal to detect prey beneath the sand or mud. A ventral lateral line is able to detect the small jets of water emitted by bivalves. The skate also has electrical facilities on its tail, but these are thought to be used only in communication, particularly prior to mating. Skates are oviparous and the eggs laid may take up to a year to hatch.

  The thorny skate (Amblyraja radiata) is well-named:

  As females mature sexually, they become increasingly thornier with increasing prickles between the thorns. Males, on the other hand, lose most of the thorns on the pectoral fins except along the anterior margins. Typically for adult males, the outer corners of the pectoral fins have two rows of hooked, erectile thorns.92

  This skate inhabits cool waters in the western and eastern Atlantic Ocean, as far north as Greenland and Scandinavia. The similarly named thornback skate (Raja lemprieri), common in the temperate waters of south-eastern Australia and Tasmania, is no less well-equipped:

  Orbital ridge with 6–10 thorns (forming a rosette in juveniles); median disc thorns commencing just in advance of nuchal area, usually extending along disc midline and onto tail; media thorns with 2–6 additional midlateral rows extending posteriorly . . . Dorsal surface of disc and tail with fine granulations (very dense in adults); alar thorns retractable; malar thorns well developed . . .93

  The boreal skate (Raja hyperborean) has probably the widest range of the skates, being found in deep cold waters off southern Australia, New Zealand, northern Japan, the east coast of the United States, the tip of southern Africa and north of the English Channel to Greenland. It is also known as the Arctic skate; catch studies reveal large individuals being taken at depths of about 700 metres in the Norwegian Sea.94 Its far south equivalent is the Antarctic skate (Bathyraja eatonii). There are far fewer species of shark and ray in stable temperature Antarctic waters than in seasonal Arctic waters.

  What, then, do skates—slow movers designed for the seafloor—eat? Their diet is surprisingly varied. The large barndoor skate (Dipturus laevis), confined to the east coasts of the United States and Canada, preys upon

  . . . polychaetes, gastropods, bivalve mollusks, squids, crustaceans and fishes. Small individuals subsist on benthic invertebrates such as polychaetes, copepods, amphipods, isopods, crangon shrimp, and euphausiids. Larger specimens are capable of capturing larger and more active prey, including razor clams, large gastropods, squids, cancer crabs, spider crabs, lobsters and fishes . . . the thorns on the snout of barndoor skates are worn smooth, as though the snout is used to dig in the mud or sand to obtain bivalve mollusks.95

  The Melbourne skate (Raja whitleyi) reaches up to two metres in length and is found in number from south-western Western Australia to the mid-north coast of New South Wales. It is Australia’s largest skate, weighing up to 50 kilograms. Never to be outdone in the naming stakes, however, is New South Wales. The Sydney skate (Raja australis), found along the New South Wales coast, grows to a comparatively small 50 centimetres. Its short, thickish tail sprouts between three and five rows of large, sharp thorns.

  The maugean skate (Zearaja maugeana), also known as the Port Davey skate, is a recently discovered Tasmanian endemic. It has just two known habitats, Bathurst Harbour and Macquarie Harbour, and is mostly confined to their shallow upper reaches. There are some 250 species of skate worldwide; Z. maugeana is one of very few known to inhabit fresh water.

  This cartilaginous fish was first discovered in 1988, when University of Tasmania ecologist Dr Graham Edgar netted a specimen in Bathurst Harbour. Twenty-three specimens have since been collected, from both harbours, and divers have reported rare sightings, usually in shallow, brackish water, of the animal partly concealed in the sand. The recently named skate shares some close morphological characteristics with western Tasmania’s deepwater continental slope skates, rather than with the region’s inshore marine skates. This may relate to its feeding habits. Buttongrass tannins stain and darken the waters of both harbours, resulting in a lack of light and silty floor, a similar environment to that at continental slope depth. This means that there are no algal communities such as normally grow in light-penetrated water; in their place are invertebrate communities, not dissimilar to those found in the dark depths of the continental slope. Like its deepwater relatives, the Port Davey skate may use its elongated, electroreceptive snout to search for food in the silty substrate. Frills at the edge of a prominent nasal curtain may also be sensitive to prey.

  The Port Davey skate is quite small, averaging about 70 centimetres in length. Unique as a shallow brackish water specialist, it has an ancient, conservative lineage that may date back to the Cretaceous Period that ended 65 million years ago, when modern geological Tasmania was forming. It is not hard to imagine that even mild habitat disturbance will threaten the survival of such a geographically restricted species, of which there are estimated to be about 1000 individuals. It is currently listed as Endangered on the IUCN Red List of Threatened Animals, an assessment unlikely to change for the better.96

  Electric rays

  Classification

  • Eleven genera in four families

  • At least 56 species

  Biology

  • Platelike pectoral disc, thick-bodied

  • Large paired electric organs behind small eyes

  • Thick tail

  • Ovoviviparous

  Habitat

  • Temperate and tropical waters worldwide

  The electric rays (order Torpediniformes) are grouped in four families: numbfishes, sleeper rays, torpedo rays and coffin rays. The Latin word torpere means ‘to stiffen or paralyse’, which effectively sums up these rays’ principal weapon, their electricity-generating organs. While they bear a superficial resemblance to skates, their discs (incorporating the snout) are thicker and rounder and their tails short and broad with large dorsal and caudal fins. Extending well back and laterally behind the small eyes are the two electric organs with which the ray stuns its prey. Situated beneath the skin, through which they are visible in some species, they give the disc its bulk. The teeth are quite small. Electric rays are exclusively marine, preferring the temperate and tropical waters of the three major oceans. Their habitats range from shallow reef waters to continental shelves of hundreds of metres.

  The coffin ray (Hypnos monopterygium), so-named because in death it bloats into a coffin shape, is endemic to a broad band of inshore Australian waters, from tropical Western Australia to the Spencer Gulf and along the New South Wales coast to southern Queensland. It is small, seldom exceeding 50 centimetres in length, with an almost circular disc. Its pelvic fins are themselves joined to form a second disc, resulting in a miniature tail. It is a muddy brown colour with loose skin lacking either dermal denticles or thorns, a characteristic of the electric rays. The eyes are tiny and set on retractable stalks relatively well back from the straight front of the disc. These stalks can assume a vertical position, to increase the animal’s field of vision.

  The coffin ray is relatively common in intertidal zones, although it also inhabits rocky and coral reefs. By day, it camouflages itself in sand or mud or grassy bottoms. As a bottom feeder its usual prey includes molluscs, crustaceans and bony fishes, but it seems capable of ingesting larger prey. Although a slow swimmer, it can move surprisingly quickly and its mouth can open widely. A dead coffin ray found on Umina Beach on the central New South Wales coast in 2004 had the remains of a rat sticking out of its mouth, which led to correspondence with the Australian Museum: how could such a small ray, only 45 centimetres long, take on a whole rat? Setti
ng aside the separate puzzle of how the animal had come across a land mammal (unless it was the water rat, Hydro-mys chrysogaster), aquarium official Craig Henderson came up with empirical evidence of its ability to take large prey:

  As Unit Supervisor of Taronga Zoo Aquarium until its closure in 1992 I undertook a captive management study of Hypnos monopterygium as the Aquarium had never previously been successful in keeping them in captivity. We found that the rays would only feed on fish (only whole fish initially) that were presented to them on a plastic stick which was ‘swum’ past their nose. The rays required a soft substrate such as sand or fine shellgrit to bury into. Their attack of the prey was incredibly fast, without any warning, and was always accompanied by an electrical discharge. The prey item (including very large fish) would be swallowed head first, whole, very quickly before the animal would re-bury itself.97

  The torpedo rays are the largest of the four ray families, in some cases reaching two metres. The large disc is uniformly oval, although the tail can be either long or short. Torpedo rays are found in all three major oceans. The short-tail torpedo ray (Torpedo macneilli) is endemic to Australian waters and common along the island’s coasts excepting the Top End. Its bulging electric organs can emit a charge of up to 200 volts:

  There are several reports of fishermen being thrown several feet after unwittingly touching large specimens of this ray. The electric organs consist of a battery of hexagonal cells (sometimes visible through the skin in smaller rays) that form a honeycomb structure on each side of the body. Each cell is filled with a jelly-like fluid and is connected to an elaborate network. When fully charged, the organs operate much like a battery, with the upper surface having a positive charge and the lower surface being negative.98

  Descriptions of the two remaining families, the numbfishes and sleeper rays, is difficult, as this 2007 IUCN report demonstrates:

  Aspects of the general biology of the numbfishes are poorly-known. The family, like other electric rays is yolk-sac viviparous, but details on the reproductive cycle, fecundity and maturity, as well as age and growth information is generally lacking . . . The Taiwanese blind numbfish B. yangi is known only from a limited number of specimens detailed in its original description while the dark blindar B. moresbyi is known only from five specimens . . . There is minimal information on the biology of the sleeper rays. The biology of the rare Natal sleeper ray Heteronarce garmani is ‘virtually unknown’ . . . The two endemic New Zealand sleeper rays of the genus Typhlonarke are also rare and poorly-known, although they reportedly have up to 11 pups per litter . . .99

  Stingrays

  Classification

  • At least 25 genera in nine families

  • At least 189 species

  Biology

  • Pectoral disc triangular, oval, rhomboid, often winged

  • Usually slender tail with one or more serrated stinging spines

  • Six gills in some species

  • Ovoviviparous

  Habitat

  • Temperate and tropical waters worldwide

  • Marine, estuarine and freshwater

  The order Myliobatiformes consists of three genera, stingray, stingaree and ray, grouped in nine families totalling nearly 100 species: stingarees; giant stingarees; sixgill stingrays; river stingrays; whiptail stingrays; butterfly rays; eagle rays; cownose rays; and devil rays. They range in size from small to very large, and are found worldwide in warm and temperate waters, inshore and offshore as well as in freshwater environments. This diversity of species types and habitat is a good indicator of a successful, adaptable life form. Disc shapes vary considerably. Stingray tails are generally long and very thin (with the exception of stingarees); but almost all have toxic, replaceable stinging spines embedded in the upper section of the tail. They are mostly bottom dwellers.

  The stingaree’s tail is moderately thick and short and it is the only stingray to have a caudal fin. That being said, there are at least 35 species of stingaree; a profusion which perplexes rather than assists researchers. Stingarees have a ‘peculiar and disjunct’ habitat distribution: ‘very diverse’ in the eastern Indian Ocean but not known in the western or northern parts of that ocean; very diverse in the western Pacific off Australia and the eastern Pacific to Chile and California, but not known in the Central Pacific; and scattered species found in waters off Japan and Korea.100

  Stingarees are found inshore but also at depths of up to 700 metres on continental slopes. They hunt in a manner known as hydraulic mining, in which they emit a powerful jet of water into the seabed to uncover prey. They also fan their pectoral fins to blow away sand. Stingarees not only bury themselves during the day, they also have a considerable array of camouflage, as evidenced by some of their common names: striped, sandyback, banded, patchwork, spotted, brown, yellowback and greenback. This is as much to avoid being preyed upon as to ambush prey. This shy batoid will use its stinging spine to defend itself only as a last resort. A detailed study of stingaree tail spines (from 423 individuals representing 46 Pacific Ocean species) concluded that the spine lengths and their number of serrations related to specific habitat—open water, midwater or benthic.101 Perhaps, then, the spine plays a more complex role in the stingaree’s world than is commonly assumed.

  There are many species of stingray, the largest of which is the short-tail stingray (Dasyatis brevicaudata) which is common in waters at depths of 100 metres, especially off southern Africa, Australia below the tropics and New Zealand. Its thick, massive disc can exceed two metres in width. The body length is at least four metres, and it can weigh in excess of 350 kilograms.

  The pelagic stingray (Pteroplatytrygon violacea) is unusual in being an open ocean dweller, as well as frequenting inshore waters. It is virtually the only ray that doesn’t bury itself in sand but cruises the ocean’s upper layer preying on shoalfish such as herring and mackerel, squid, jellyfish and also small crustaceans. Not surprisingly it is a regular bycatch of pelagic fisheries.

  The white-blotched river stingray (Potamotrygon leopoldi) is one of at least 22 species of freshwater stingray. It has a very rounded disc, covered in large, distinctive white blotches. These exclusively freshwater elasmobranchs are found only in tropical rivers of South America, particularly the Amazon River, where there are frequent reports of humans being stung, occasionally causing death. Biologically, their adaptation to freshwater environments has rendered them incapable of tolerating saline water; they don’t have urea in their bloodstream, nor a specialised rectal gland to expel salt. It’s not known how they came to evolve in this way, but one theory is that they derive from a saltwater Pacific Ocean species trapped by the creation of the Panama isthmus. Unfortunately, their brilliantly varied colouring means that P. leopoldi and its close relatives are heavily targeted for the worldwide aquarium trade.

  The browny-pink, plump sixgill stingray (Hexatrygon bickelli) may be unique. Common in the waters off South Africa, it is also found in the western Pacific. Its gills have large gill filaments and its spiracles have external rather than internal valve flaps. Another unusual feature is its elongated, translucent snout, measuring about one third of the total body length, which may have a function in rooting around for prey. The young develop inside the mother, nourished by uterine milk and their birth, too, may be unique: ‘According to one investigator, a young ray is rolled up like a cigar during birth, which . . . facilitates the birth of such proportionally large young. The young ray then unrolls and swims away’.102

  The cownose ray (Rhinoptera bonasus) is a fairly large species which inhabits the open ocean as well as inshore waters. It is particularly known for its long group migrations which are probably temperature-related and may be guided by a form of solar navigation. Its common name arises from the large indent at the front of its skull. The rear end of its tan-coloured disc is also snub-shaped, while the disc tapers to points, giving it a rather boxy shape. When hundreds of these cownose rays are seen migrating in clear waters, they look like underwater ki
tes. They are found off northwest Africa, northeast South America and from the Gulf of Mexico along the east coast of the United States. Their feeding method is intriguing:

  Cownose rays use a very specific mechanism to obtain deep-burrowing prey. They locate food on the bottom substrate (benthos) through mechano- or electroreceptive detection. Once they suspect prey is there, they employ a combination of stirring motions of the pectorals while sucking/venting both water and sediment out through the gills and away from the area to create a central steep-sided cavity depression. The continued movement of the pectoral fins aids in dispersing the sediments released from the gills and increases the depth of the depression. Eventually, the food is seized and drawn into the mouth. Common prey items include nekton, zoobenthos, finfish, benthos crustaceans, mollusks, bony fish, crabs, lobsters, bivalves, and gastropods.103

  The huge and majestic manta ray (Manta birostris) occurs globally in warm waters. As befits this underwater albatross, the manta’s great wings, measured as disc width, attain nine metres. Their power to propel it through the water aids its filter feeding. An adult can weigh over 1300 kilograms. Size apart, the manta’s most distinguishing features are its cephalic lobes, which extend forward of the head. These are malleable and act to herd plankton into the anterior mouth. Much of manta behaviour remains to be properly understood:

  Three types of jumps have been observed, forward jumps landing head first, forward jumps landing tail first, and somersaulting. Groups of these animals have been seen participating in this behavior, breaching one after the other. While it is not understood why this behavior is exhibited, some speculate it may play a role in attracting mates or is a form of play.104

  Chimaeriformes

  One of the two subclasses of the cartilaginous fishes, the chimaeras can be traced directly back to the experimental cartilaginous fishes of the Paleozoic Era and are, with some justification, described as ‘hybrids’ of sharks and teleosts, a kind of non-missing link.