Shark Read online

  On the western seaboard of the Australian continent, there is extensive research into the tiger shark in Shark Bay (where Dampier described his specimen 300 years ago). The bay’s seagrass beds are major feeding grounds for dugongs, green turtles, dolphins, cormorants and sea snakes, all prey items of the tiger sharks that enter the bay. The program is associated with the Shark Bay Ecosystem Research Project, led by the Heithaus Lab of Florida International University.50 The tiger shark research had a specific aim:

  Sharks are top predators in many marine communities, yet no studies have quantified the habitat use of large predatory sharks or determined the factors that might influence shark spatial distributions. We used acoustic telemetry and animal-borne video cameras (Crittercam) to test the hypothesis that tiger shark (Galeocerdo cuvier) habitat use is determined by the availability of their prey. We also used Crittercam to conduct the first investigation of foraging behavior of tiger sharks . . . Although there was individual variation in habitat use, tiger sharks preferred shallow seagrass habitats, where their prey is most abundant. Despite multiple encounters with potential prey, sharks rarely engaged in prolonged high-speed chases, and did not attack prey that were vigilant. We propose that the tiger sharks’ foraging tactic is one of stealth, and sharks rely upon close approaches to prey in order to be successful. This study shows that using appropriate analysis techniques and a variety of field methods it is possible to elucidate the factors influencing habitat use and gain insights into the foraging behavior of elusive top predators.51

  The researchers are also investigating to what extent the presence of tiger sharks modifies the behaviour of the bay’s marine inhabitants on which they prey.

  Orectolobiformes (Plate 7)

  These are the carpet sharks, the common name deriving from their camouflage-like patterned colouring and multiple tassels (dermal lobes), which can make some resemble fabulous underwater Persian carpets. The order of carpet sharks is tremendously varied, from the slender metre-long epaulette shark to the gigantic whale shark.

  Classification

  • Fourteen genera in seven families

  • 31 species

  Biology

  • Nasal barbels

  • Nasoral grooves connecting mouth to nostrils

  • Five gill slits

  • Two spineless dorsal fins

  • Elongated upper caudal lobe

  • Striking skin patterning

  • Oviparous or ovoviviparous reproduction

  Habitat

  • Warm temperate and tropical waters

  • No freshwater species

  • Most common in the tropical Indo-West Pacific

  • Mostly bottom dwellers

  • Mostly nocturnal

  Blind sharks

  These small sharks use their barbels to seek out prey through electroreception, with their large spiracles assisting in breathing as they forage in shallow, particle-filled water. There are two endemic Australian species. The fairly common blind shark (Brachaelurus waddi) is found along the coasts of southern Quensland and New South Wales, generally in shallow waters among reefs and seagrass beds. It grows to about 1.2 metres but is generally smaller. The rare Colcloughs shark (Brachaelurus colcloughi), also known as the blue-grey carpet shark, is known only from the Queensland coast and grows to just 75 centimetres. When out of water, blind sharks try to protect their eyes by closing them, hence the name.

  Shy sharks

  Shy sharks are endemic to the waters of South Africa. In 2006, a new species, the Natal shy shark (Haploblepharus kistnasamyi), was discovered off the coast of KwaZulu-Natal. The term ‘discovered’ is relative here, and provides a working example of how classifications develop and change. This little shark (maximum length about 50 centimetres) had previously been considered a ‘colour variant’ of the puffadder shy shark (Haploblepharus edwardsii) endemic to waters further south along the Cape coast, until South African ichthyologists Leonard Compagno and Brett Human wrote it up in the systematics journal Zootaxa as a new species, because of its ‘stockier build, less depressed head and trunk and more depressed caudal peduncle’.52 The colour variation is described as

  Southeastern Cape form: sandy brown with 7 reddish-brown saddles bordered by black, and numerous small, dark brown and white spots between saddles; white below. Natal form: body cream in color with darker brown saddles and irregular white spots; white below.53

  Shy sharks eat a variety of prey including small teleosts such as anchovies, squid, crabs, shrimps and polychaetes. Their small mouths contain about 30 rows of teeth in each jaw. In turn they are preyed upon by larger sharks, seals and black-backed kelp gulls. They are also taken by recreational anglers. Like blind sharks, shy sharks protect their eyes when taken from water, but they do so by wrapping their tails across their eyes.

  Wobbegongs (Plate 8)

  The Aboriginal name of this flattened shark identifies it with Australian waters, although it has been found as far north as Japan. ‘Wobbegong’ is thought to translate either as ‘living rock’ or ‘shaggy beard’. It is a shallow-water, bottom-dwelling predator, almost invisible in rocky, algae-covered coastal terrain. While a few species are quite small, the larger ‘wobbies’ grow to almost three metres, and weigh about 70 kilograms.

  The wobbegong has a broad, flattened head with groups of sensitive dermal lobes protruding from the sides of the head, in front of the eyes and the lips. These help it to detect prey in murky water. Large spiracles behind the eyes enable it to breathe while motionless, and to avoid drawing in silted water. The heavy body tends to be the same width as the head, narrowing abruptly at the pelvic fins, above which is the first dorsal fin. The tail is narrow with a low caudal keel. It is not the body plan of a fast pursuit predator but the camouflaged wobbegong is perfectly suited to ambushing prey that comes within range. The wobbegong’s terminal mouth and powerful protruding jaws contain wickedly sharp, long teeth. It lunges and grabs its prey, which is then sucked in by its pharyngeal muscles. The wobbegong generally waits for the prey to die, before swallowing the victim whole. It is a nocturnal feeder with a preference for octopus, teleosts and crabs. It also stalks its prey by ‘walking’ on its pectoral and pelvic fins.

  The large spotted wobbegong (Orectolobus maculatus), combining shades of yellow, green and brown, is intricately patterned with leaf shapes and circles. Reports have it growing to three metres. The ornate wobbegong (Orectolobus ornatus) is described as ‘extremely ornate and variegated; yellowish brown to grayish brown with darker, corrugated saddles; each saddle, and the many paler bluish to whitish patches, bordered with lines of small, black spots; a prominent white spot behind each spiracle; pale ventrally’.54

  Wobbegongs are ovoviviparous, the embryos receiving nourishment from the yolk and also secretions from the mother’s uterine wall. Gestation is about eleven months, with litters varying considerably in size, from fewer than ten to more than 30. It is thought that these differences may be linked to migratory movements.55

  In an interesting example of the fluid and dynamic nature of scientific classifications, in 2006 the banded wobbegong was successfully redescribed as being not one but two species, Orectolobus ornatus and Orectolobus halei—the smaller ornatus previously being classified as the juvenile halei. The famous Australian ichthyologist Gilbert Whitely had proposed O. halei as a distinct species as early as 1941, but a subsequent lack of specimens placed it in synonymy with O. ornatus. Charlie Huveneers successfully argued his case for a new description on this comparison:

  Orectolobus ornatus differs from O. halei in colour pattern, a smaller adult size, fewer dermal lobes at the posterior preorbital group, lower vertebral and spiral valve counts, and the absence of supraorbital knob. Morphometrically, O. ornatus has a longer pelvic fin to anal fin interspace, smaller pectoral fins, smaller head dimensions, and relatively smaller claspers in mature specimens.56

  Despite being a common and easily studied shark, found in good numbers around most of Australia’s e
normous coastline, as recently as March 2008 two new wobbegong species were discovered, bringing the total known species to nine. The two newly described species were at first thought to be juveniles of known species. One, the dwarf spotted wobbegong (Orectolobus parvimaculatus) grows to about one metre, far smaller than the two-metre spotted wobbegong.

  Less rigorous on a scientific level, but no less interesting, is this observation from Julian Rocks, a popular dive spot located off Byron Bay:

  Crayfish form commensal relationships with wobbegongs. Wobbegongs eat octopus, and octopus eat crayfish. By living in close proximity to the wobbegong, the crayfish serves the wobbegong as bait and in turn receives protection.57

  Divers interact continually with wobbegongs, which are not aggressive, but bite people more often than any other Australian shark species, perhaps because they seem docile and unsharklike, so are harassed or, being shallow-water dwellers, are trodden on. When disturbed they do, however, issue an unsubtle warning:

  The most spectacular snap I ever witnessed was at Byron Bay. Diving with local guide, Pete Murphy, at Julian Rocks, he was giving us a guided tour around Cod Hole. Swimming over a gutter we spotted a monster 3m long spotted wobbegong, with a head close to a metre wide. Pete decided he was going to pat this massive shark, so I just sat back to watch. As Pete reached out with his hand the wobby quickly turned towards him, opened its mouth wide and snapped its jaw shut just millimetres from Pete’s mask. This shark was big enough to have engulfed not only Pete’s head, but half of his torso. The wobby’s warning snap had the desired effect, with a very pale looking Pete quickly backing away.58

  Nurse sharks

  This shark family of just two genera is widely distributed in warm shallow waters from Africa’s east coast across to the central Pacific. The tawny (or tawny nurse) shark (Nebrius ferrugineus) reaches a maximum length of more than three metres and is a fairly distinctive shark, being uniformly tan-coloured with large, pointed dorsal fins set well back, large, sickle-shaped pectoral fins and a long upper caudal lobe. It has a large head, small eyes set back from a small underslung mouth containing small, multicuspid teeth and a pair of barbels connected to the nostrils, the latter being connected to the mouth by nasoral grooves. What is such a small feeding apparatus doing on a creature of this size? The tawny nurse has developed a method of feeding similar to that of the wobbegong. It has powerful gill and throat muscles, which it uses to suck in prey, using its pharynx as a powerful vacuum. If the barbels detect prey hiding in a crevice, the shark will attempt to hoover out the prey.

  The tawny shark is a nocturnal hunter, preferring shallow waters associated with reefs, channels and sandy or seagrass bottoms. During the day, groups of nurses sleep together under ledges and overhangs. They are commonly seen like this by recreational reef divers, leading to a mistaken belief that they are harmless:

  Due to their apparently docile nature, Nurse Sharks have gained a reputation for being ‘harmless’. They are not. If touched, grabbed, poked, or otherwise molested, Nurse Sharks often turn on their human tormentors with astonishing speed. Clamping on with their short, powerful jaws and multi-cusped teeth, Nurse Sharks typically roll over and over in the water—like a crocodile in a ‘death roll’—causing nasty bruises and sometimes massive tissue damage. Rescuers often must carry the ‘victim’ out of the water with the shark still attached, the animal stubbornly refusing to let go until it suffocates or is killed. It is therefore always best to let a ‘sleeping’ Nurse Shark lie.59

  The nurse shark also defends itself by reversing its inhalatory capability, that is, by ejecting powerful streams of water at the source of the threat. Not surprisingly, this has given rise to the common name ‘spitting shark’. It is also called the sleepy shark, for its habit of aggregating in resting groups—stacked one atop another—in caves. The tawny shark is ovoviviparous, giving birth to small litters of fewer than ten, the newborn being an impressive 40 centimetres long.

  Zebra shark (Stegostoma fasciatum) (Plate 9)

  This species is noteworthy on a number of accounts. It is one of the few large carpet sharks, possibly growing to a maximum length of about 3.5 metres, although adults are generally about 2.5 metres. Despite its size, it is harmless to humans. Juveniles have light stripes and blotches on a dark skin which morph into dots on a paler skin in adulthood. Furthermore, while zebra shark is its common name in Australian waters elsewhere it is known as the leopard shark (Triakis semifasciata)(it is widespread across coastal Indo-Western Pacific). There is a ground shark commonly known as the leopard shark, but it is quite different, physiologically and behaviourally, to S. fasciatum and is restricted to the coastal waters of the west coast of the United States.

  The zebra shark’s patterning is not its only distinguishing feature. ‘This distinctive species, with a very long, blade-like caudal fin, is unlikely to be confused with any other.’60 The tail is half the length of the body but is not raised, so is more like a tapering of the body proper rather than an appendage. The zebra shark has a transverse barbeled mouth and small teeth, each comprising three sharp points. Its eyes are small and it has large spiracles. The flanks are shaped by a pair of prominent ridges running from behind the head to the caudal fin. The pectoral fins are large and wide, the underside of the body flattened. It is a body plan designed for slow movement, gliding and motionlessness. Typically, the gills pump water while the animal rests during the day. At night, it feeds on snails, crustaceans and small fish on or near the shallow bottom, and its slenderness enables it to squirm into cracks and crevices in search of prey.

  Despite being common in warmer Australian waters, from the south coast of New South Wales around the Top End to the mid-coast of Western Australia, this shark, like so many others, is essentially a mystery. ‘Leopard sharks aggregate annually in shallow coastal waters of southern QLD over the austral summer and, to date, there is very little information on this species in the wild and the function and duration of this aggregation is unknown.’61 For this reason, the University of Queensland has instigated a research program into the ecology and population dynamics of leopard sharks, intended to determine likely adverse impacts on the species through fishing in the western Pacific, where it is taken commercially.

  The zebra shark is oviparous. The large dark egg cases, which measure about fifteen centimetres by eight centimetres, are covered in external fibres which anchor them to rocks and the sea floor.

  The whale shark (Rhincodon typus)

  This is the planet’s largest fish, growing to at least twelve metres and weighing 15 000 kilograms. (Its shape was memorably described by David Stead as being like ‘a prodigious tadpole’.62) The whale shark is a warm-water inhabitant, its range described as ‘worldwide’ in tropical and warm temperate waters, but so little is known about this shark there is no real knowledge of its habitat and numbers. In all likelihood, it is rare. It was discovered early in the nineteenth century, when a specimen was washed ashore in Table Bay, Cape Town; until the 1980s there had been just a few hundred reports of the whale shark, since when intensive studies have taken place.

  This huge animal has two other distinguishing features: its extraordinary skin patterning and cavernous gape. The skin’s basic colouring is blue–brown above with a white belly below. The upper body is covered with distinctive vertical and horizontal stripes which create a checkerboard effect. Inside each square is a creamy white blob. The mouth has up to 3000 tiny teeth. So gigantic is the shark’s maw that when it is fully opened the head seems to disappear altogether.

  The first dorsal fin is large and set well back towards the large semi-lunate tail. There are prominent ridges along the flanks. When the mouth is closed the head appears to be very flat, broadening into a bulky, powerful, streamlined body built for slow cruising. Long believed to be oviparous, it has now been established that the whale shark gives birth to live young—300 were counted in a female caught in Taiwanese waters. The frequency of reproduction and rate of gestation are unknown. The fi
ve gill slits are very large and contain spongy filtering screens designed to capture the tiniest of zooplankton (warmer waters being less rich in zooplankton than temperate waters which benefit from particularly plankton-rich cold upwellings). There is nothing discriminatory about its feeding:

  The bellows-like gill pouches make the whale shark a versatile filter-feeder, enabling it to consume a wide variety of planktonic crustaceans and even small to mid-sized fishes such as sardines, anchovies, and mackerels. Because of its ability to suck food into its mouth, the whale shark is not dependent upon forward motion to operate its filters and often assumes a vertical posture when feeding. It has been reported that whale sharks enhance the efficiency of vertical feeding by ‘bobbing’ up and down in 15 to 20-second cycles, pausing at the surface to allow food-bearing water to rush into their mouths and strain through their spongy gill plates.63

  Seasonal whale shark populations frequent numerous locations in the three major oceans. In Taiwanese and other Asian waters they are subject to commercial exploitation for their flesh and fins, whereas in Western Australia’s Ningaloo Marine Park they are protected and studied, while also generating considerable tourist income through dive companies. A whale shark identification programme, using photographs taken of each individual’s unique patterning, has generated a large database which is of considerable value in building up a life history profile of this species. Even so, much has to be learned about the largest and most docile of sharks (though it has been known to headbutt pestering boats). Why the dramatic skin patterning? Where do they breed? How long do they live? Why, alone among the carpet sharks, are they not bottom dwellers? Are they endangered?