Shark Read online

  The whale shark epitomises this about sharks: the more research is conducted on them, the more questions remain to be answered.

  Heterodontiformes

  Classification

  • One genus in one family

  • Nine species

  Biology

  • No nictitating membrane

  • Large ridge above each eye

  • Nasoral grooves connect nostrils to mouth

  • Sharp front teeth, blunt crushing back teeth

  • Five gill slits

  • Two dorsal fins with large spines

  • Anal fin

  • Oviparous reproduction

  Habitat

  • Bottom dweller in shallow marine environments

  • Warm temperate and tropical distribution

  • Western and eastern Pacific Ocean, western Indian Ocean

  Port Jackson shark (Heterodontus portusjacksoni)

  Port Jackson is the body of water around which the city of Sydney is built. Soon after the First Fleet came ashore on 26 January 1788, one of the local sharks was described:

  . . . the skin is rough, and the colour, in general, brown, palest on the under parts: over the eyes on each side is a prominence, or long ridge, of about three inches; under the middle of which the eyes are placed: the teeth are very numerous, there being at least ten or eleven rows; the forward teeth are small and sharp, but as they are placed more backward, they become more blunt and larger, and several rows are quite flat at top, forming a kind of bony palate, somewhat like that of the Wolf-fish; differing, however, in shape, being more inclined to square than round, which they are in that fish: the under jaw is furnished much in the same manner as the upper: the breathing holes are five in number, as is usual in the genus: on the back are two fins, and before each stands a strong spine, much as in the Prickly Hound, or Dog, fish: it has also two pectoral, and two ventral fins; but besides these, there is likewise an anal fin, placed at a middle distance between the last and the tail: the tail itself, is as it were divided, the upper part much longer than the under . . . This was taken at Port Jackson, but to what size it may usually arrive cannot be determined; perhaps not to a great one, as the teeth appear very complete.64

  Illustration of a Port Jackson shark in The Voyage of Governor Phillip to Botany Bay, 1789. (Libraries Board of South Australia, Facsimile Editions No. 185, 1968)

  The Port Jackson shark reaches a maximum length of about 1.5 metres, although is generally smaller, and is the largest of the horn shark family, a small group of sharks also known as bullhead sharks and tabbigaws. Like so many other shark families, the horn sharks have their own characteristics and shark smarts. Their skin colouring, nocturnalism and defensive spines on each dorsal fin protect them from predators such as other sharks and seals. As described above, their front teeth are sharp, the back teeth pavement-like, ideal for grinding the shells of crustaceans. In summer adults migrate south from the east coast of mainland Australia towards the cooler waters of Tasmania. The adults mate in deep water then, as philopatric (literally, ‘territory-loving’) sharks, return in winter to the same sandy-floored caves along the mainland coast. They rest in groups in these caves during the day. The female wedges her large, hard-cased eggs into rock crevices, where they take about a year to hatch. Up to 100 or so young at a time can be found in shallow, protected nursery areas.

  Lamniformes

  The mackerel sharks include some of the fastest and most efficient of the ocean’s predators. Many species are similar in body characteristics to the Carcharhiniformes, which indicates an evolutionary design for large, muscular, torpedo-shaped sharks to catch and consume large, speedy prey. Even so, the mackerel shark family also contains surprises—a ‘playful’ shark, and the two species that might be considered the beauty and the beast of the shark family.

  Classification

  • Ten genera in eight families

  • Approximately 16 species

  Biology

  • No nictitating membrane

  • Mouth extends well behind from the eyes

  • Five gill slits

  • Large teeth

  • Two spineless dorsal fins

  • Anal fin

  • Large similarly sized caudal lobes

  • Ovoviviparous reproduction

  Habitat

  • Coastal and open ocean

  • Cold to tropical waters worldwide

  Goblin shark (Mitsukurina owstoni) (Plate 10)

  According to Richard Ellis, the famed US marine artist and shark expert,

  This seems to me the strangest of all the sharks. It looks like some kind of prehistoric survivor, an experiment in shark design that doesn’t seem to work. And yet, by definition, it does work. Triceratops, the dinosaur with three horns, is long gone, as are Pteranodon and hundreds of other ‘impossible’ animals. There is little that can be said about this mysterious shark, because so little is known about it. And yet, we have the most curious, incontrovertible fact of all: Mitsukurina lives.65

  The goblin shark is totally unlike other big mackerel sharks, with their classic torpedo shapes and vivid presence in the popular culture as ugly maneaters. Yet the word ‘goblin’ itself conjures up an image of evil ugliness. A specimen was caught by a fisherman off Japan and subsequently described in 1898 by ichthyologist (and eugenics proponent) David Starr Jordan. These sharks still appear to be most common off Japan, although their patchy distribution is worldwide in deep waters below 200 metres.

  In its fundamental body plan the goblin shark is, more or less, ‘sharklike’, except for three strikingly unusual characteristics. The first is that it has a forwardly protruding, ledge-like snout; the second that its jaw is as mobile as a limb; the third that it is pinkish in colour, with blueish fins. It grows to almost four metres. The goblin shark’s rarity means that not much is known about the species (the only known one of its kind), but its unusual colouring comes from the blood vessels which are visible through its fine, translucent skin. Its fins and flesh are soft, its wavy tail elongated, all of which suggest a slow swimmer. What is certain is that the snout, crammed with ampullae of Lorenzini, is a prey detector. Scientists, however, are rethinking the long-held assumption that the goblin shark is exclusively a bottom dweller, using its snout to seek out prey hiding in the substrate. It may also be a mid-level feeder, but whatever the case, its feeding mechanism is exceptional:

  The jaws are modified for rapid projection to aid in the capture of prey. The jaw is thrust forward by a double set of ligaments at the mandibular joints. When the jaws are retracted the ligaments are stretched and they become relaxed when the jaw is projected forward. The jaws are usually held tightly [in the head] while swimming and function like a catapult when the animal wants to feed. Its slender narrow teeth suggest it mainly feeds on soft body prey including shrimps, pelagic octopus, fish, and squid. It is also thought to feed on crabs. The posterior teeth are specialized for crushing.66

  The goblin shark also has a large basihyal and expandable pharynx, suggesting that prey caught in its needle-sharp teeth are then sucked into the stomach. The goblin shark’s oil-filled liver contributes almost 25 per cent of its body weight. This means that it has near-neutral buoyancy, allowing it to drift, waiting for prey to swim past. It is speculated that the goblin shark might also partake in nightly vertical migration, moving up and down with the fishes and cephalopods upon which it preys.

  Crocodile shark (Pseudocarcharias kamoharai)

  This metre-long oceanic shark has the distinction of being the smallest of the lamnids. It is found worldwide in tropical waters to depths of about 600 metres, with occasional specimens found inshore. The body is slender and firm with a strongly forked asymmetrical tail. Its most noticeable features are its huge eyes and elongated gills slits extending to the top of the head. Not much is known about the crocodile shark, which is thought to be a vertical migrator. The skin is brown, sometimes with white speckles.

  Mako sharks (Plate 1
1)

  Mako is the Māori word for shark. There are two known species of mako shark, the shortfin (Isurus oxyrinchus), and the rarer longfin (Isurus paucus). Superlatives abound in describing the shortfin mako, which outclasses every other shark in its combination of beauty, elegance, power and speed. At a maximum length of about four metres and a weight of some 500 kilograms, this shark’s most distinguishing features are its big, pointed snout above permanently exposed multiple rows of hooked teeth, huge, glistening black eyes and brilliant two-tone colouring of indigo and white. A major pelagic species—though it does come inshore—the shortfin mako’s range is truly global in warm waters, comparable to that of the blue shark.

  Unfortunately, the shortfin mako is also one of the most lucrative of the elasmobranchs. It is targeted commercially for its flesh, its teeth and its jaws, and for the satisfaction of mastering the planet’s ultimate fighting fish: ‘It can be an especially dangerous adversary when gaffed or tail roped, and there are countless stories of violent mako encounters at boat-side!’67

  The mako shark is supremely hydrodynamic. Behind the sharp snout and cylindrical forward body, the narrow dorsal fin is upright and prominent, the pectoral fins broad-based and tapering to pointed ends. Water flow over the tightly packed, small dermal denticles is further improved by the presence of ridged caudal keels in front of the large tail of equal-sized crescent-shaped lobes. The shortfin mako’s superbly engineered tail, together with its endothermic circulatory system (see Chapter 5), gives it tremendous powers of acceleration. It can shoot through the water—and sometimes high out of it—reportedly at speeds in excess of 70 kilometres per hour.

  The shortfin mako targets fast-moving prey, including tuna, bluefish, other sharks, and probably dolphins and porpoises. There is at least one known record of a swordfish rostrum being found in a shortfin mako’s stomach.

  Makos are solitary and migratory and may follow warm currents as part of their lifecycle, but their movements and blue-water habitat make them difficult to study. Interestingly, one American research program that tagged a mako off North Carolina in 1984 retrieved the shark nearly thirteen years later; it had been caught by a commercial fishing vessel off South Carolina, implying philopatric behaviour. The southern California bight on the west coast of the United States is known to be a shortfin mako pupping and nursery ground. The ovoviviparous young, averaging about fifteen in a litter, can exceed 70 centimetres and weigh more than ten kilograms.

  Estimating an animal’s lifespan is critically important in understanding the dynamics of its population. This is especially important with the shortfin mako, given the lack of knowledge about it and its vulnerability to commercial exploitation. One program designed to estimate the lifespan of shortfin mako sharks used what is known as ‘bomb dating’. In the 1950s and 1960s atmospheric thermonuclear testing released enormous amounts of radiocarbon into the atmosphere. This eventually settled in the oceans, entering hard organic structures such as corals, teleost earbones and calcified elasmobranch vertebrae. Researchers are able to use these radiocarbon deposits to calculate the age of an individual organism. The California State University program employs this technique with great precision: ‘The [shortfin mako vertebrae] cores will be weighed to the nearest 0.1 mg and stored in sterile plastic cryo vials in preparation for radiocarbon assay by Accelerator Mass Spectrometry (AMS) at the Center for Mass Spectrometry, Lawrence Livermore National Laboratory, Livermore, California’.68 The program selected 54 vertebrae collected between 1950 and 1984. Results confirm that the shortfin mako lives for at least 31 years.

  Porbeagle (Lamna nasus)

  This unusual Cornish name is a portmanteau word, combining porpoise and beagle. (It was coined, however, long before Lewis Carroll invented the word ‘portmanteau’.) The porbeagle is frequently mistaken for a mako shark. Furthermore, it is also often confused with its close relation the salmon shark (Lamna ditropis). There are discrete porbeagle populations in the western and eastern Atlantic Ocean, and in the southern hemisphere. The Atlantic porbeagle reaches a maximum size of about 3.5 metres and weight of 230 kilograms. Adults mate in autumn or winter and gestation is about nine months, with a litter of about four young live-born. And, ‘the little data that exists for the Southern Hemisphere populations indicates that they may be out of phase with those of the Northern Hemisphere, giving birth off New Zealand and Australia in winter’.69 Despite the confusion between the two, the porbeagle is stouter than the mako and has straighter teeth. It has a white splotch at the rear base of its dorsal fin and the blue of its upper body is greyer than the intense blue of the mako. The porbeagle is unique in that it has a second caudal keel, a ridge on the upper part of the lower tail lobe. Also, it is not exclusively a temperate-water shark, being found as far north as the Bering Straits in the Pacific and northern Scandinavia and Russia in the Atlantic.

  L. ditropis, the salmon shark, is found only the North Pacific and there is presumably a good reason why the two species do not, apparently, co-exist in the same waters. Both prey heavily on schoolfish and are likely to have their seasonal movements dictated by their prey’s migration. Unsurprisingly the principal diet of L. ditropis is Pacific salmon. Like other mackerel sharks, however, both species are opportunistic apex predators although neither is as fast as the mako, which again reduces the level of direct competition. Both the porbeagle and the salmon shark have adapted to their cooler pelagic habitats by evolving an ability to raise their body temperatures as much as fifteen degrees Celsius above the ambient water temperature.

  The porbeagle displays a social intelligence not expected in a shark:

  The Porbeagle is among the very few fishes that seem to exhibit play behavior. There have been a few, sporadic accounts—principally from off the Cornish coast—of Porbeagles playing with floating objects, both man-made and natural. Porbeagles have been reported rolling while swimming along the surface, repeatedly wrapping and unwrapping their snouts and forward portion of their bodies in kelp fronds, which often trail behind the shark like rubbery streamers. Sometimes a Porbeagle thus engaged was observed being chased by other Porbeagles . . . The repetitive nature of this behavior is highly characteristic of what in other mammals would unhesitatingly be termed play. But because the Porbeagle is a ‘mere’ fish, ethologists are reluctant to use that term. The Porbeagle, of course, doesn’t care about such semantic contention, and does what it does for its own reasons.70

  Grey nurse shark (Carcharias taurus) (Plate 12)

  Confusion surrounds this shark’s classification, because it has a number of interchangeable, local common names and there are a number of closely related species. Thus the grey nurse shark found in Australian waters ‘is also known scientifically with the synonyms Odontaspis taurus, Eugomphodus taurus and Carcharias arenarius’.71

  The Australian grey nurse shark is currently classed as one of four species in the family Odontaspididae (although there may be two more species). Its common names in other localities include the sand shark, sand tiger shark, ground shark, slender-tooth shark, ragged-toothed shark and spotted ragged-toothed shark. The group is also referred to as the snaggletoothed sharks.

  The grey nurse is a heavy-bodied mackerel shark with a somewhat humped back that can probably grow to over 3.5 metres in length and weighs in at over 150 kilograms. It possesses large, similar-sized dorsal fins, the first set well behind the pectoral fins. The pectoral and anal fins are also large, as is the heterocercal tail. Unlike the super-fast sharks, however, the grey nurse’s body does not taper significantly towards the tail and the overall body plan indicates a slow mover. The upper body, covered in loosely spaced dermal denticles, is grey–brown. Juveniles have reddish or brownish spots which eventually fade. The eyes are quite small and, unlike other large, predatory sharks, lack the protective nictitating eyelid. As its common names suggest, this is a toothy family: the mouth contains about 90 long, curved pointy teeth, their multiple rows exposed in a permanent gape underneath a pointed, upturned snout.
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  The grey nurse and its close relatives inhabit coastal waters in fairly well-defined regions: southern Brazil; the east coast of the United States; parts of west and north-west Africa; South Africa; parts of the Mediterranean; the Red Sea; and Australia north to China. Researchers are able to be so precise because although the grey nurse migrates as part of its life cycle, it has a set home range to which it returns. Preferred habitats are deep sandy gutters and caves, and a grey nurse cave may be likened to a terrestrial social mammal’s den. The grey nurse preys upon schoolfish as well as small sharks, rays and squid, and is considered to be one of the few types of shark which hunt cooperatively. Small groups of resting grey nurses have been seen drifting just above the sandy bottom, almost motionless.

  The physiology and appearance of these sharks render them doubly vulnerable to human interference. On average, a mother gives birth to just one pup per year. Uterine cannibalism results in a maximum of two pups. This very low rate of reproduction heightens the adverse effects of commercial bycatch and recreational angling, and means that juvenile population numbers may be unable to sustain the natural predation of larger sharks. Worse still, for decades this docile shark’s deceptively savage appearance meant it was considered to be a maneater. According to ichthyologist David Stead, throughout the first half of the twentieth century Australian newspapers inevitably reported shark attacks as being perpetrated by ‘grey nurse’ sharks. Well into the 1970s, the east coast grey nurse populations were targeted as a threat to swimmers. So many sharks were caught that in 1984 the New South Wales government became the first in the world to pass legislation protecting an elasmobranch, when it listed the grey nurse as a protected species. Today there are thought to be fewer than 500 individuals on the east coast, and the grey nurse is listed as Critically Endangered. The population along the coast of Western Australia is listed as Vulnerable.