Shark Page 12
• Bottom dwellers in shallows down to 1300 metres or more
The angel shark—so named for its angel-like ‘wings’—is one of the planet’s oldest extant sharks. The lack of diversity in the order Squatiniformes is a pointer to its ancient lineage: this shark reached its evolutionary endpoint early in elasmobranch diversification. Angel shark fossils have been found dating to the Triassic Period, over 200 million years ago, not long after the demise of the xenacanths. The forepart of an angel shark is flattened and raylike, while the trunk and tail are sharklike. Angel sharks are well adapted to benthic dwelling but, despite their flatness, they are true sharks, not rays. Although their pectoral fins are greatly enlarged they are lobed; that is, they develop from the body as ‘limbs’, whereas the pectoral fins of rays and skates are fused as a single disc. And the angel shark’s barbeled mouth is terminal, not ventral like the rays’. A few species exceed two metres in length, with most averaging about 130 centimetres.
The Australian angel shark (Squatina australis) inhabits coastal waters from southern Western Australia east to New South Wales. It is a specialist ambush predator, burying itself in the sand and using the force of its lean, muscular body to lunge up at prey, which it grabs with long, thin, sharp teeth. American studies have shown that the angel shark actively travels from one location to another, where it fashions a pit in the sand and lies in wait for its prey, which includes small teleosts, cephalopods and bivalves.
Hexanchiformes
The frilled shark, placed here, is classified by some scientists in its own family, Chlamydoselachiformes. One reason is that, unlike the other hexanchids, it has a terminal mouth. (Chlamy means cloak or mantel, which is a description of their gills.)
Classification
• Three genera in two families
• Approximately six species
Biology
• Elongated eel-like body
• Wide terminal mouth
• Six (or seven) pairs of gill slits
• Small pectoral fins well back near the anal fin
• One small dorsal fin near the tail
• Large anal fin
• Large extended caudal fin
• Ovoviviparous reproduction
Habitat
• Cold deep waters
Frilled shark (Chlamydoselachus anguineus) (Plate 3)
The frilled shark grows to a length of just under two metres and is often referred to as ‘primitive’ because it has scarcely changed since the Jurassic shark radiation 190 million years ago. Some palaeoichthyologists believe it has an even older lineage, dating back to the Devonian well over 300 million years ago. The body of the frilled shark is elongated and muscular and it has a long, wavy tail. Emerging from a broad, reptilian head are distinctive frilly gill margins which resemble a collar or ruff. The first pair of gills are joined, that is, they form a single gill slit running under the throat.
The frilled shark is an elusive deep-water dweller, inhabiting continental shelves and slopes to depths of well over 1000 metres. This and its rarity make it difficult to study. Even so, specimens are occasionally found in shallower waters, often in the seas around Japan. The frilled shark was described and classified in 1884 by Samuel Walton Garman, the first official curator of fishes, amphibians and reptiles at the Harvard Museum of Comparative Zoology:
For several years, [Garman] had been working on an elasmobranch that he found quite remarkable . . . This was a rare opportunity to examine the soft tissue of what appeared to be a living fossil. While many disparaging comments have been made about Garman as a systematist, no one would find fault with his anatomical skills . . . He turned his knife on the frilled shark with great success. After his initial description of the species, he published seven other papers on the anatomy of this odd creature; eventually dissecting the type specimen nearly into oblivion.13
The frilled shark’s jaws have about 300 trident-shaped, ultra-sharp teeth set in 25 rows—ideal for impaling slippery prey such as deepsea squid. It is possible that the shark might use its elongated body to strike whiplike at prey. In January 2007, a live specimen was captured by staff of the Awashima Marine Park south of Tokyo. Although it died soon after, rare video footage was taken which clearly shows its unusual features.
Japanese studies suggest that the gestation period of the frilled shark could exceed three years. This would make it by far the longest period for an elasmobranch and, for that matter, any vertebrate.
Bluntnose sixgill shark (Hexanchus griseus)
Like the frilled shark, this large predator also traces its lineage back some 200 million years to the Jurassic Period. The bluntnose sixgill shark is recorded as reaching a maximum length of about five metres and weighs about 600 kilograms, although some researchers think that as a deep benthic dweller it could be considerably larger and heavier.
The shark’s upper body is shaded through grey to dark brown and its underside is pale. It has a broad, muscular and heavy body, with relatively small fins, except for the elongated upper caudal lobe which propels it through the water. The single dorsal fin is as far back as most other sharks’ second dorsal fin. The snout is rounded, as is the large mouth, which has six sets of large sawing teeth in the lower jaw and smaller curved single-cusped teeth in the upper jaw.
The small, green eyes have no iris, so that they can take in as much light as possible in deep, dark water. Gestation is thought to last up to two years and the resulting litters can number 100 or more. Some researchers think that such large litters may be to compensate for high mortality rates. These sharks can live for about 80 human years, but their slow, cold-water growth makes them particularly vulnerable to habitat disruption.
Despite its traditional reputation as an ancient and sluggish leviathan of the deep, the bluntnose sixgill has shown itself to be a significant apex predator with a global (but not pelagic) range perhaps exceeded only by that of the blue shark. It feeds upon all the usual prey, but also seals, dolphins and whales (blubber found in sixgill stomachs has been assumed to have been scavenged). It is reported to have an impressive burst of speed which enables it to ambush prey. Teleosts are swallowed whole, often face-first:
I observed a 13-foot (4-metre) male Bluntnose Sixgill capture and eat a Lingcod. The shark came up along a canyon wall, swam up and over the Ling, pushing it down against the bottom with its snout. Thus pinning the Ling to the rocky substrate, the Sixgill with its tail nearly vertical spun around until it could swallow its prey head-first. This maneuver is similar to one reported in a Great Hammerhead (Sphyrna mokarran) feeding upon a Southern Stingray (Dasyatis americana) off Bimini (Strong et al., 1990) and likewise seems very economical and efficient, its speed of execution belying the normally languid cruising pace of Sixgill Sharks.14
Bluntnose sixgill sharks are solitary, migrating vertically at night to feed. They confound perceptions again, however, by undertaking significant annual inshore migrations in order to mate and produce their live young in safe conditions. The same individuals return to the same inshore grounds year after year. Intensive studies of migratory inshore populations in Puget Sound in the northwest of the United States and Flora Islets in the Strait of Georgia, British Columbia, have provided observers with opportunities and insights into the species that would be impossible on the continental shelf and upper slope where the animals live for most of each year. Researchers at Puget Sound were able to identify the same individuals on their yearly return and noted that they appeared ‘to have established movement corridors and territories that remain relatively fixed over time’.15
The bluntnose sixgill shark seems to be emerging from its deeps. A YouTube video shows one experimentally biting a submarine cable, before slowly swimming on. Some suggest that it may even be actively competing with the great white for prey, in waters off the Cape of Good Hope in South Africa. Unfortunately, the bluntnose sixgill shark has become targeted as the major sport fish to be had off Ascension Island in the southern Atlantic Ocean, an area tha
t is—or was—one of its major pupping grounds.
Broadnose sevengill shark (Notorynchus cepedianus)
There are just two genera of sevengill sharks, commonly known as the sharpnose shark (Heptranchias perlo) and the broadnose shark. Together they are called cow sharks and are among the most ancient extant elasmobranchs, their body plans having undergone very little change over millions of years.
The broadnose sevengill shark has a worldwide distribution in specific, generally inshore, locations, including the western coastline of the US north to British Columbia, western and eastern South American populations, southern Africa, and from Japan south to Australia and New Zealand. It is often referred to as the Tasmanian tiger shark, because the specimen first described by the French naturalist Francois Peron in 1807 was taken at Adventure Bay, Bruny Island, which is separated from mainland Tasmania by the narrow D’Entrecasteaux Channel.
This shark grows to about three metres and can weigh in excess of 100 kilograms. It is silvery or browny grey and is distinctively spotted. It has a liking for deep inshore channels and bays and seems to regulate its movements according to tides. In the arcuate mouth (curved like a bow) are sharp, jagged upper teeth for grasping and comb-shaped lower teeth for cutting and sawing. Its size, weight and dentition make for a shallow-water shark that is an active and powerful predator.
It is noted for its indiscriminate feeding habits, possibly equalled only by the ‘other’ tiger shark, Galeocerdo cuvier. Teleosts, sharks and rays, mammals and carrion are all eaten. Furthermore, this is one of the few shark species known to hunt cooperatively in pursuit of seals and dolphins. It is described as aggressive in its feeding behaviour. Despite this, predation on humans is virtually unknown.
THE GALEOMORPHS: MACKEREL SHARKS, GROUND SHARKS, BULLHEAD SHARKS, CARPET SHARKS
Carcharhiniformes
There is uncertainty over the precise number of genera and families in this large order, the systematics of which is described as ‘the most complex and contentious of all shark groups’.16 The Greek Carcharhinus means ‘sharp nose’ and this feature is characteristic of the requiem sharks, although many of the other families in the order are described as having short, blunt snouts.
Classification
• 48 genera in eight families
• At least 279 species
Biology
• Nictitating membrane
• Five gill slits
• Two dorsal fins
• Anal fin
• All three forms of reproduction
Habitat
• Most marine waters
• Fresh water
Pygmy ribbontail catshark (Eridacnis radcliffei)
This shark grows to just 24 centimetres and lives in varying depths from about 70 to 750 metres in the Indo-West Pacific, from east Africa across to Vietnam and the Philippines. It has a much greater range than other members of its genus. Dark brown in colour, it has blackish markings on the dorsal fins and prominent dark bands on the tail. It hunts along the ocean floor, preying on teleosts and crustaceans. The pygmy ribbontail catshark is ovoviviparous and gives birth to just one or two pups in a litter. This means that population numbers are difficult to sustain and the shark is exceptionally vulnerable to any kind of threat.
Swell sharks
There are at least seven known species of swell shark. They are members of the most numerous shark family, the catsharks. Rarely exceeding a metre in length, swell sharks are slow-moving bottom dwellers with large mouths containing hundreds of tiny pointed teeth, with which they prey on crustaceans and small fish.
All catsharks are small. There are several dwarf species and the average length across the family’s 160 or so species is about 80 centimetres. It’s such a large family that the species’ distinguishing characteristics are usually identified by their common names: flatnose catshark; brown catshark; stout catshark; longfin catshark; smallbelly catshark; Iceland catshark; Borneo catshark; longhead catshark; broadmouth catshark; fat catshark; chain catshark (which has an exquisite chain pattern on the skin); and the imaginatively named spatulasnout catshark. Most catsharks prefer deepwater slopes, although they can be found in shallower coastal ranges. The draughtboard swell shark (Cephaloscyllium isabellum), so named because of its skin patterning, is one of many shark species on the IUCN Red List; once abundant in waters off New Zealand, it was heavily exploited for its liver oil and, although no longer targeted, continues to suffer considerable casualties as a commercial fishery bycatch. The swell shark’s common name is derived from the defence mechanism which enables it to inhale air or water when threatened, greatly inflating its stomach.
River sharks
Rare and mysterious, these are among the least known of all elasmobranchs. There are thought to be at least six species of river shark (although this remains in doubt). They are moderately stout fusiform sharks and they range in adult size from about two to three metres, with broad rounded snouts and small eyes. The speartooth shark (Glyphis glyphis) has been identified in a few Queensland, Northern Territory and New Guinea rivers. It is also described, however, as inhabiting inshore estuarine, brackish and low- to reduced-salinity waters. In 2003 a speartooth shark was found for the first time in a Western Australian river, although specimens had previously been taken from Queensland rivers. The speartooth is estimated to grow to two or three metres and takes its name from the fact that some of its anterior teeth have ‘cutting edges confined to slightly expanded spear-like tips’.17
The description of the Irrawaddy River shark (Glyphis siamensis) is based on a single specimen found in Burma. The Ganges shark (Glyphis gangeticus) takes its name from the Ganges–Hooghly River system in Bengal. It is rare, but popularly considered to be plentiful because of the frequency of shark attacks in those rivers—attacks which most experts consider to be perpetrated by the unrelated bull shark.
The Australian species of river sharks for many years had the tantalisingly uncertain names Glyphis species A, B and C. The description of Glyphis B had been based on a single specimen, until in 1998 a team from the IUCN Shark Specialist Group, working with local fishermen, rediscovered the species in a few rivers and named it the Borneo river shark. More recently Glyphis C has been renamed the northern river shark, Glyphis garricki, a species restricted to some freshwater systems of northern and Western Australia and New Guinea. This followed research by the CSIRO’S Wealth from Oceans Flagship project.
Why are these sharks so rare? It would be tempting to reply that sharks are not adapted to live in freshwater systems, so river sharks have always been rare; but some rays and sawfishes are exclusively freshwater, and the bull shark, a large predator, is equally at home in the sea and as far inland as it can get. The more accurate response is depressing: ‘Once prominent in tropical river systems, fishing pressure, pollution and habitat destruction through overdevelopment have greatly reduced the natural populations of freshwater sharks and rays’.18 These sharks’ slow growth rates, low reproductive levels and adaptation to an exclusive environment over a great period of time mean that even minor changes to that environment can fatally disrupt a population.
It may be that river sharks are adapted to fresh water with particularly low oxygen levels, a deterrent to bull sharks—which would prey upon both juveniles and adults—entering such rivers.
Reef sharks (Plate 4)
In stark contrast to river sharks, the four species of reef shark have been well studied. Coral reefs and atolls, in sunlit nutrient-rich waters, support a great variety of living organisms and there is no shortage of prey for local predators. So why are there so few shark species? The fact is that reef sharks are numerically abundant, but each species has evolved to occupy a specific ecological niche, thus minimising intraspecies competition.
The whitetip reef shark (Triaenodon obesus) grows to about 1.7 metres in length and weighs about 45 kilograms. It is widespread in the central and western Pacific Ocean, with populations also in Asia and Africa. It is lean, with
a flattish head and a broad snout, and prominent white tips on its dorsal and upper caudal fins. It has small, smooth-edged, thin, sharp teeth in the bottom jaw. The whitetip reef shark is generally described as ‘sluggish’ and during the day can be found resting in sandy caves or under coral overhangs. Unlike the pursuit predator, the whitetip reef shark doesn’t rely on speed. It uses its flattish head to search out the bony fishes, crabs and octopuses which hide in coral reef cracks and holes. The shark thrusts its head into its prey’s hiding place, driving itself inward by twisting and turning its body. It dislodges rocks, breaks off pieces of coral (frequently cutting itself in the process) and sometimes squirms right through the gap where the prey had been.
A common, gregarious species which lives in clear, shallow waters and is not aggressive towards humans, the whitetip reef shark is one of the most studied of sharks. One American study of nine whitetip reef sharks at a Cocos Islands reef managed to capture their mating behaviour on film, and the resulting footage became the first to show males who were unsuccessful with the female of their choice ejaculating into free water and swimming off with mouth agape. This study also offers the first detailed visible evidence of the size and precise functioning of the male siphon sacs.
Reproductive behaviour of these sharks is unusual. Multiple males attempt to copulate with one female, who uses a range of avoidance techniques, such as tucking in a pelvic fin to avoid it being grabbed by a male, and bending her body to keep her cloaca away from the claspers—strategies thought to indicate deliberate mate selection on the part of the female. Group courtship is common in some other species of sharks but had not been seen in whitetip reef sharks. Once a female has accepted a male, the act itself appears awkward, but also graceful:
The two sharks tumbled in copula down an underwater cliff face, with the male showing rapid caudal maneuvering that brought the pair to rest in a vertical head-down orientation on a rocky wall. Once they came to this position, the male ceased his swimming movements and began rhythmic but variable thrusting of his pelvic region towards the female. They remained there for several seconds before falling and coming to rest twice more, each time maintaining parallel, head-down positioning . . . At least four peripheral males with mature claspers circled the mating pair throughout the event. They nudged the female’s free pectoral fin with their rostrums several times, but did not grasp it . . .19