Shark Page 11
• Ovoviviparous reproduction
Habitat
• Estuarine, coastal, oceanic
• No freshwater species
• Widespread globally from Arctic to Antarctic waters
• Mostly bottom dwellers
• Nightly vertical migration common in many species
Dwarf lanternshark (Etmopterus perryi)
Found in deep Atlantic and Caribbean waters off South America, the adult of this tiny shark reaches a maximum of nineteen centimetres in length. What does such a truly diminutive predator eat? Probably larger krill and shrimps, and it is thought that dwarf lanternsharks feed cooperatively, attacking squid larger than themselves. Discovered in 1985, in the Caribbean Sea, the dwarf lantern shark is currently listed as the world’s smallest known shark.
The much larger southern lanternshark (Etmopterus baxteri) grows to about 60 centimetres and inhabits cool southern Pacific and Atlantic waters. Lanternsharks congregate over seamounts in waters near Tasmania, and as a result became an orange roughy bycatch, but the soft flesh is not considered commercially viable.
Spined pygmy shark (Squaliolus laticaudus)
A widely distributed deepwater tropical pelagic species, the adult of which grows to just 22 centimetres. This shark spends its days at the bottom of the ocean and at night follows its vertically migrating prey—deepsea squid, bristlemouth fish and lanternfish. Bioluminescent organs on the shark’s belly attract prey in the lightless depths.
Little is known of its biology.
Prickly dogfish (Oxynotus bruniensis) (Plate 1)
Also known as the roughshark, the prickly dogfish inhabits deep southern Australian and New Zealand waters. A small shark growing to at least seventy centimetres, it has dermal denticles that are large and prickly and firm ridges running the length of its belly. Colouration is generally brown to grey. Contrary to the popular image of a shark as sleek and torpedo-shaped, the prickly dogfish is best described as humpbacked or cambered in shape. It also has large floppy dorsal fins, with barely visible spines firmly embedded at their bases.
Not much is known about the biology or behaviour of this species, although there is some speculation about its close relative, Oxynotus centrina: ‘Angular roughsharks feed upon polychaetes, perhaps utilising their very large livers to hover at neutral buoyancy above the seafloor with limited forward motion whilst seeking prey’.2 The teeth of O. bruniensis are lanceolate in the upper jaw and blade-like in the lower jaw, to maximise its ability to clutch and cut its prey. Interestingly, this tooth structure is similar to that of the cookie-cutter shark.
Cookie-cutter shark (Isistius brasiliensis)
Out of all the ways in which one creature can eat another, the cookie-cutter shark has evolved a method that is truly audacious. This tiny shark reaches about 50 centimetres in length when fully grown, but preys upon marine creatures hundreds of times larger than itself. The cookie-cutter shark is a deepwater dweller and its skin is sprinkled with photophores, which make it glow greenly in the dark to attract prey such as squid, but the cookie-cutter’s primary weapon is its basihyal. In conjunction with powerful, suction-plated lips, the basihyal vacuum seals the rounded mouth onto the chosen pelagic prey—whale, tuna, marlin, seal, dolphin. The little mouth is full of teeth, of varying shapes, which bite into the victim as the cookie-cutter twists its body around, extracting a round plug of flesh and leaving a wound up to seven centimetres deep. Predator or parasite? Both. Officially, the cookie-cutter shark is a facultative ectoparasite: it exists both as an external parasite and independently of parasitism.
Portuguese shark (Centroscymnus coelolepis) (Plate 2)
The body plan and teeth structure of this shark are very similar to those of the Greenland shark, but there the similarity ends, for despite the close taxonomic relationship, the Portuguese shark seldom grows to more than a metre in length, while the Greenland shark may exceed seven metres. The Portuguese shark is a benthic dweller, reflected in its colouring: the pup’s skin is almost black, becoming chocolate-brown as it matures. This species was heavily fished off Portuguese waters, hence its common name. It is also found in waters off Newfoundland, Brazil, southern Australia, western Namibia and the western Pacific Ocean. A Portuguese shark has been caught at 3576 metres—the record known depth for a shark. The Portuguese shark’s large denticles overlap closely. It seems to have a fairly broad diet.
Whitespotted spurdog (Squalus acanthias)
This shark has a variety of names including spiny dogfish and piked dogfish and it also apparently attains a variety of maximum sizes according to habitat, from about one metre in Australian waters to twice that in the Black Sea. Furthermore, it has the longest known gestation period of any shark—up to two years—with a lifespan of about 70 years. It is widespread globally in both inshore and offshore waters, forms large schools, and is highly migratory. Unfortunately for the whitespotted spurdog, it could also be known as the ‘fish-and-chips’ shark. It has been so overfished in the north-eastern Atlantic that it is listed as endangered on the IUCN Red List and strict limits have been set on catches. In Australia it is abundant and does not have much culinary appeal, being considered ‘to be rather coarse’.3 In fact Australians came in for a bit of a scolding in the early 1960s for their unadventurous taste buds:
On occasions many tons of the Dogfishes have been brought up in a short period. At first practically all had to be thrown back as there was no market for the fish; but gradually the public were educated to buying ‘Flake’, the name under which the flaked Dogfish flesh is well (and so favourably) known in Great Britain, and some soon learned to appreciate it. But most of the Australian people are extremely touchy about trying anything in the fish line that they cannot determine on its appearance; so it is always an uphill fight to introduce them to, and to get them used to buying and using for the table, anything that seems to them to be ‘new’.4
Kitefin shark (Dalatias licha)
This shark is somewhat similar in appearance to the Portuguese shark, but is larger, reaching an average of about 1.4 metres in length. It has a patchy worldwide distribution, with concentrations in the eastern Atlantic and in southern Australian and New Zealand waters. The kitefin shark is found at depths of around 600 metres but has been recorded at depths of nearly 2000 metres. Its colouration is variously described as dark chocolate brown, cinnamon, and violet brown. This is a hovering shark, its large liver enabling it to hang almost motionless over the outer shelves and slopes of its deepwater habitats. Prominent features of this shark are its large eyes and thick lips in which are powerful jaws containing large teeth: the kit of a formidable predator. In turn, it is targeted by larger sharks occupying the same niche. The second dorsal fin is slightly larger than the first. It has one of the largest elasmobranch livers—about one fifth of its body weight.
Greenland shark (Somniosus microcephalus)
This shark is popularly described as a sluggish carrion eater (and its scientific name translates as ‘sleepy tiny-brain’) but the truth is that it is one of the planet’s cold saltwater apex predators (another being the orca). The Greenland shark commonly exceeds four metres in length and specimens measuring more than seven metres have been recorded. The Greenland shark weighs up to 1000 kilograms and its primary habitat is the seas and fjord systems of Greenland and the Canadian Arctic, although it is found throughout Arctic waters. Southern populations inhabit Canadian waters including the Gulf of St Lawrence. To the east, they are common in Scandinavian waters. It is the only shark known to live beneath Arctic ice sheets, where it has been recorded in water temperatures of minus 1.94 degrees Celsius. An unmanned submarine recorded a six-metre Greenland shark off the east coast of the United States at a depth of 2200 metres. The Greenland shark has high concentrations of TMAO (trimethylamine oxide), which acts as an anti-freeze and a protein stabiliser at great depths, and as in other shark species is an important component of the osmoregulatory system. This compound is present at neurotoxic concentration
s and renders the flesh toxic unless correctly processed.
Greenland shark (Somniosus microcephalus). (Jeffrey Gallant/GEERG.ca)
The Greenland shark ranges in colour from dark brown to purplish. It has a heavy build with small dorsal and pectoral fins, big eyes behind a short, round snout and large nostrils, and a broad, thin-lipped mouth. Both upper and lower teeth are diminutive when compared with those of other large carnivorous sharks. The upper arcade consists of layers of teeth which overlap and form a dental band that is very sharp and thin, while the pointed lower teeth are separate but densely placed, with wider cusps, and probably act in a holding/gripping role during predation. The tail is moderately large. None of this suggests a dashing, top-order predator and yet the Greenland shark, as well as feeding on teleosts, squid, sharks, rays and eels, eats mammals such as seals, which suggests a turn of speed not immediately apparent in the body plan. It is very slow-growing, due to its cold water habitats.
Strange feeding habits are attributed to this shark. One is that the parasites called copepods (Ommatokoita elongata) which hang off the eyes of most adults—apparently causing near-blindness—act as bioluminescent prey attractors. This is unlikely to be true. Also unsubstantiated is the belief that Greenland sharks ambush caribou drinking at the mouths of Arctic rivers or attack the animals while they are crossing rivers. West Greenland longline halibut fishermen report that Greenland sharks eat their way up the longlines, so that when they themselves are finally caught their stomachs are full of hooks. While the ‘gut full of hooks’ claim may be an exaggeration, the shark is undoubtedly a pest to under-ice fisheries in the Arctic, as it is to the Greenland halibut fishery in Lancaster Sound in the Canadian Arctic.
A recently formed Canadian-based entity, the Greenland Shark and Elasmobranch Education and Research Group, has made many new discoveries about this intriguing predator. According to a 2006 statement by one of the group’s founders, University of British Columbia marine biologist and veterinarian Chris Harvey-Clark, ‘all the papers published on the species, including magazine articles, can barely fill two shoeboxes’.5 Harvey-Clark and his colleague Jeffrey Gallant became the first divers to study and report on the behaviour of free-swimming Greenland sharks. They did so in 2003 following the discovery of a resident population in Canada’s St Lawrence Estuary near the city of Baie-Comeau. (Sharks had been seen there in the past by fishers and workers at a pier construction site in the 1930s.) Their findings were numerous and revelatory. Males and females undertook vertical movements at night, regularly swimming up to the surface and back down throughout the course of the night; they exhibited classical defensive/threat postures to divers, with pectoral fins and head pointed down, the back arched, the mouth slightly open; they also exhibited curiosity about the divers. Certainly, they were not in any way lacking vision (almost none had copepods):
Most sharks in the St. Lawrence have beautiful, crystal clear eyes and are quite visual. As you swim by, their eyes swivel and follow you, which sets them apart from the population in the arctic . . . They are likely to see relatively well [and] using a variety of other sensory modalities, they are very effective, stealthy predators and could take out an agile seal in zero visibility without alerting it . . . My take on the Greenland sharks is that they’re probably like hyenas, capable of both predating and scavenging. They have lower teeth like an old-fashioned straight razor that takes a five kilogram chunk out of a whale like an ice cream scoop. But they can also suck up a large flounder like a vacuum cleaner. It gives you pause when you are diving with them.6
Such a description is far removed from the notion of a shark so slow and myopic that Inuit fishermen catch it by the tail. The St Lawrence researchers concluded that Greenland sharks were ‘capable of rapid acceleration . . . were highly maneuverable and were capable of changing depth and direction rapidly’.7 Perhaps the Greenland shark is entitled to an updating of its current scientific name. It is not to be confused with the similar-looking, equally large southern sleeper shark (Somniosus antarcticus) scattered in the Southern Ocean.
Bramble shark (Echinorhinus brucus)
This shark has a patchy worldwide distribution and is most commonly found in the eastern Atlantic Ocean, the Mediterannean Sea, southern Africa, Sri Lanka, southern Australia and New Zealand. It inhabits deep waters, to about 900 metres, towards the bottom of the continental shelf and the upper slope. The bramble shark grows to about three metres. It is a heavy-bodied shark with a pointy snout, eyes set in front of the mouth, small pectoral fins, small rearward dorsal fins, and a large sweeping tail. The very similar prickly shark (Echinorhinus cookei) grows to at least 2.6 metres.
The bramble shark’s Latin name translates as ‘hedgehog shark’, with good reason. Its skin, which has a purplish tinge, is profusely covered with large, hornlike denticles. About the width of a thumbnail at the base, the denticles are tipped with cone-shaped spikes. It is thought that these spikes either generate a metallic reflection or are luminescent, for attracting prey in deep, dark waters. Bramble sharks also have a strong-smelling, thick coat of mucus over their thin skin. Their small fins, rough, denticled skin and heavy, cylindrical shape indicate sluggish swimmers. Bramble sharks have been observed hovering almost motionless just above the ocean floor. Speculation is that it is a slow-growing and late-maturing shark. Furthermore, ‘almost nothing is known of the species’ biology’.8 Although the bramble shark has the bladelike teeth of a predator, it may be that its large pharynx also has a prey-inhaling role in feeding. The small dorsal fins are set well back.
The relative rarity of the species was recognised by the founding director of Australia’s National Museum of Victoria, Frederick McCoy. In 1886 he took possession of a specimen that had been line-caught off Portland. He had it stuffed and mounted, labelling it the Spinous Shark. In 2004 Museum Victoria ran an ‘Out of the Vaults’ exhibit, in which 24 items were displayed. Visitors were invited to vote for the most popular item. McCoy’s bramble shark came fifteenth (just above an FX Holden; the winner was a collection of Aboriginal stone tools called Kimberley points).
Pristiophoriformes
The sawshark, a true shark, looks like a diminutive sawfish, a batoid.
Classification
• Two genera in one family
• Approximately nine species
Biology
• Elongated snout with paired barbels
• Large spiracles behind the eyes
• Five (or six) gill slits on each side of the head
• Two spineless dorsal fins
• No anal fin
• Ovoviviparous reproduction
Habitat
• Estuarine, coastal, deep water
• No freshwater species
• Temperate and tropical waters
• Bottom dwellers over sandy substrate
Common sawshark (Pristiophorus cirratus)
This medium-sized shark is described as belonging to a ‘minor group of harmless bottom sharks’.9 It is endemic to the southern Australian continental shelf and upper slope. It grows to a length of about 1.5 metres, at least a third of which is the extended snout, the cartilaginous rostral saw which gives the shark its common name. There are rows of twenty or more large rostral teeth on each side of the saw, the teeth alternating between long and short. A pair of long ventral barbels, at about the midway point of the snout, once gave it the nickname the ‘Fu Manchu’ shark. The saw is also studded with ampullae of Lorenzini and has a lateral line. It efficiently detects buried prey, then sifts through the sand to get to the hidden meal. It also acts as a ‘slice-and-dice’ implement that chops up prey into bite-sized pieces. The rostral teeth are continually replaced. The sawshark has a small mouth and tiny teeth.
The rest of the shark’s body is reasonably typical of a shark, being ‘subcylindrical to slightly depressed’.10 The spiracles behind the eyes are large. The gestation period is about twelve months, with about twenty live young born in a litter, the newborn measuring 3
0 or more centimetres in length. The pups’ rostral teeth are folded back during birth to prevent injury to the mother, but then quickly become upright.
The common sawshark is fished commercially as bycatch in southern Australian waters. Together with elephant fish, sawsharks are taken by gummy shark trawlers. According to the IUCN/SSC Shark Specialist Group’s 2004 Report,
There are no useful biological data available for this species, and no assessment of the impact of commercial fishing. Although they are caught only as bycatch, the fisheries are large and have the potential to impact on the populations. Further research is needed to fully determine the status of this species . . .11
Sixgill sawshark (Pliotrema warreni)
This is one of the very few sharks to have more than five gills. Its range is restricted to the warm waters off the southern African and Madagascar coasts. A study of species interaction in the region’s underwater canyons suggested that sixgill sawsharks ‘are too large at birth to be easily swallowed by Latimeria [coelacanth], and the sharp rostral teeth of juvenile sawsharks would also discourage a hungry coelacanth’.12
Squatiniformes
Angel sharks superficially resemble rays because they have flattened bodies adapted to life on the ocean floor.
Classification
• One genus in one family
• Approximately eighteen species
Biology
• Dorsoventrally flattened body
• Terminal mouth
• Nasal barbels
• Eyes on top of the head
• Five pairs of ventral gill slits on the side of the head
• Winglike pectoral and pelvic fins
• Two spineless dorsal fins
• No anal fin
• Caudal fin has two large lobes
• Ovoviviparous reproduction
Habitat
• Estuarine, coastal, oceanic
• No freshwater species
• Relatively widespread in temperate and tropical waters