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  Megamouth shark (Megachasma pelagios)

  This shark was unknown to science until 1976, when one became entangled in the anchor line of a US Navy ship off Hawaii. The process of describing and classifying the specimen took seven years, but controversy remains as to its precise relationship to not only other mackerel sharks but also to the basking shark which, like the megamouth, is a filter feeder. By the end of 2006, nearly 40 specimens had been sighted or captured, the majority in open ocean waters off Japan (home to those other rarities, the frilled and goblin sharks) and the Philippines, although a few have been recorded off the west coast of the United States and individual specimens recorded as far afield as Western Australia, South Africa, Mexico, Senegal, Brazil and Ecuador. Each megamouth is known by its chronological number, surely a unique occurrence in the animal kingdom.

  The megamouth is, therefore, rightly described as the rarest known shark, and almost everything about its behaviour must be inferred. The largest specimen caught was 5.49 metres long. Another weighed over 1000 kilograms. It has small dorsal fins, a long tail to propel the body slowly through the water and large, tapering pectoral fins. The head is huge, wide and bulbous, in order to support the great terminal mouth. Each jaw contains about 50 rows of teeth, although only a few rows are functional at any one time. The inside of the mouth is silvery, the tongue dark purple. The purpose of a distinct white stripe above the mouth is not clear—perhaps it has some function in feeding, or as a distinguishing mark. The shark’s skin is dark, turning black when exposed to air.

  The skin itself is soft, as are the muscles and connective tissue, and the cartilage has ‘poor calcification’.72 (There is no better evidence for these attributes than to look at a photograph of a dead megamouth. Typically, the whole facial structure appears to have collapsed, the jaws themselves sagged and slewed. Despite the somewhat unappealing, ‘flabby’ nature of megamouth flesh, a number of the specimens caught in the Philippines were eaten.) The megamouth has protrusible jaws: it swims through clouds of plankton, shrimp and other tiny prey, sucking them in, then retracts its jaws and closes the mouth in order to expel water through its gills. A live specimen caught off the west coast of the United States was tracked for a sufficient period of time to establish that it stayed at a consistent depth of 150 metres during the day, rising to fifteen metres below the surface during the night. This indicates that the megamouth follows its prey, as krill undertake their nightly vertical migration towards the ocean’s surface. Many of the observed specimens bear scars which suggest that while they are feeding, they in turn are being preyed upon by the vertically migrating cookie-cutter shark.

  Thresher shark

  The thresher shark is most distinguished by its tail. The length of the upcurved tail equals the length of the rest of its body, so that a thresher with a head and trunk length of three metres becomes a six-metre-long shark, although it weighs considerably less than other sharks of similar length (a thintail thresher has been measured at 7.6 metres and weighing 348 kilograms).

  There are three species, widespread globally. The most common and largest, the thintail (Alopias vulpinus) has a vast range, inhabiting temperate waters worldwide. It is primarily an open-ocean shark, but is also found close to shore, possibly as part of its migratory or feeding patterns. The body shape—minus tail—is not dissimilar to that of the rotund porbeagle. The first dorsal fin is quite large; the second is tiny, its function being taken over by the enormous upper caudal lobe. The pectoral fins are fairly long and pointed at the tips. The dermal denticles are very small and closely overlapping. The tail is described as being like a scythe, narrowing to almost whiplike. It is a body plan which suggests great agility: a light yet muscular shark benefiting from the propulsion generated by such a huge caudal fin.

  Underneath large eyes the thresher’s mouth is quite small, as are its teeth, which are bladelike, curved, pointed, unserrated and extremely sharp. Its prey is thought to be schools of small teleosts such as mackerel and shad. Conventional wisdom is that the thresher uses its tail to herd and round up and then stun (or otherwise injure) these schoolfish, before feeding. Cooperative hunting by two or more threshers has been observed but despite its numerical abundance, and the fact that it is regularly in contact with humans through its persistence in harrying the catches of commercial fishing boats (especially mackerel), there remains some doubt over the precise functioning of the tail. There are not many recorded eyewitness accounts of the herding and stunning of prey.

  Such a significant appendage could have taken the thresher into a pelagic niche not occupied by the larger, heavier and faster elasmobranch predators, namely, that it takes smaller, darting prey. A comparison can be made between the thresher and one of the planet’s terrestrial predators: ‘The cheetah’s long muscular tail works as a rudder, stabilizing and acting as a counter balance to its body weight. This allows sudden sharp turns during high speed chases.’73 Unlike the larger cats, the cheetah specialises in small, darting prey and it may be that the thresher’s whippy tail gives it exceptional manoeuvrability. Its scientific name vulpinus is ‘fox’, an animal known not only for its bushy tail but also its considerable degree of intelligence. Perhaps this unscientific but pragmatic, empirical account of catching threshers gives some indication of their all-round energies and, possibly, fox-like cunning:

  Fighting thresher sharks can be very challenging. You have to endure pandemonium when hooking these sharks. If you catch one, you will know what I mean. It can drag you in any direction it wants with its tremendous speed and power, it jumps, or dives deep in the water, it never gets tired fighting with you, and even if you think you are winning, it goes around the pilings of the pier and cuts your line. Its long tail is another challenge. If the tail tangles with your line, the game is over, you are finished . . . But who cares even if you lose. Thresher sharks are one of the finest game fish to fight with . . . Thresher sharks migrate together, so when someone hooks one, you have a chance to catch it too. On the 23rd of May, 1998, we hooked 9–10 thresher sharks at Santa Monica Pier. It was almost new moon, high tide was around 8:30 a.m. Waves were calm and it was a kind of condition that we call thresher water . . . the first action, we saw a thresher splashing water with its tail . . . As soon as I saw my rod was moving, I grabbed it and waited until the fish start running . . . The fish willingly came close to the pier and splashed one time and start running again . . . I finally pulled the fish into a close range where my friends were able to gaff the fish. It was an 8 and half foot, and probably 80–100 lb shark. I thanked my fishing buddies and told them let’s hook more. Threshers continued to grab our live mackerel. But, somehow, we lost every one of them except mine. Threshers won 9–1. But, we were all happy fighting with this amazing fish.74

  The two other species are the bigeye thresher (Alopias superciliosus), primarily distinguished by its huge eyes, and the pelagic thresher (A. pelagicus), the smallest of the three species at about four metres.

  Great white shark (Carcharodon carcharias) (Plate 13)

  It is no coincidence that the explosion in scientific elasmobranch research coincided with the cultural branding in the 1970s of the great white shark as the sum of all our fears. Through its dramatic fictional distortion in print and on the silver screen, this most awe-inspiring of apex predators became a catalyst for an urgent reassessment of how we treat the living oceans.

  The great white, also known as the ‘white pointer’ and ‘white shark’, has a worldwide distribution in temperate coastal waters, although it also travels into cooler northerly and southerly waters. As a migratory species which crisscrosses ocean basins, it is in no way exclusively ‘coastal’. Some observers believe that the tiger shark is its apex predator equivalent in tropical waters, and that their preference for different water temperatures ensures minimal competition between the two species.

  Despite the amount of research carried out over the past forty or so years, there is still much to learn about the great white’s biology and
behaviour. There are two reasons for this: first, great white populations have a habit of disappearing and satellite tagging as yet has not found out why; and second, because it is possibly the shark least capable of being kept in captivity. Great whites are considered rare, their numbers uncertain, although their aggregations at certain locations off the coasts of Australia, South Africa, the United States and Mexico have enabled considerable research into them. In 2006, Columba—a 3.5-metre, 500-kilogram great white—was satellite-tagged by a CSIRO team led by Australia’s foremost great white shark expert, Barry Bruce. The team’s research revealed that the great whites

  . . . move from one site to another, sometimes thousands of kilometres apart, usually at a speed of about 3 km/h. They can stop for periods of months in the same location . . . Evidence gathered so far suggests there are biological hotspots that may dictate the sharks’ movements. These include seal colonies in places such as Port Lincoln, islands in the Great Australian Bight and near Esperance; schooling fish such as snapper, which frequent Spencer Gulf and Gulf St Vincent, and schools of gummy shark – also in the Bight – at certain times of the year. Scientists now believe sharks instinctively know where these hotspots are and travel directly to them using ‘set corridors’.75

  The great white is distinctively two-toned, with a grey or blue-grey upper half and white lower half. Its dermal denticles are tightly packed, so its skin is less rough than that of most sharks. The maximum length recorded is 6.4 metres, with a weight of 2500 kilograms. The great white weighs so much more than other large neoselachian predators that descriptive terms such as ‘fusiform’ and ‘stout’ need to be considered in context. The dorsal fin is broad and upright, the pectoral fins huge, the lobes of the tail almost equal in size. Aerobic (‘red’) muscles attached to the tail run almost the length of the body. The huge fins and tail, together with the shark’s sheer muscular bulk, allow it to leap well clear of the surface as part of its feeding strategy. Enormous energy is required to propel thousands of kilograms at great speed vertically out of the density of seawater.

  The great white’s teeth are large, upright, serrated and triangular, ideal for disabling prey. Its large, black eyes roll back into their sockets at the moment of impact, to protect them from injury. The huge jaws lunge forward in a tremendously wide gape and inflict a crushing bite. As a great white matures its prey base becomes increasingly rich in fats, which contribute to its physical prowess. Marine mammals such as seals, sealions and walruses are a primary food source. Whale carcasses are scavenged for their blubber. Marine mammals live in temperate and cold waters; the great white’s ability to regulate and raise its body temperature enables it to hunt in a range of water temperatures. Research into great white predation describes a five-stage process: detection, identification, approach, subjugation and consumption. In other words, great whites typically lurk around rocky pinniped colonies and ambush individuals as they leave or approach the colony. The great white is the only shark known to pop its head above the surface, looking for or scenting prey, but its favourite method of attack is from below. The shark’s acute senses of vision and scent first detect and lock on to its selected victim. The shark stalks the prey that is swimming near the surface, then hurtles up at it, prey and predator erupting from the water in a cloud of spray. Larger prey such as elephant seals, which grow to about five metres and weigh over 2000 kilograms, tend to be bitten towards the rump, with the shark then backing off while the victim bleeds to death. Seals and sea lions are grabbed whole and held under the water until they die.

  Great whites live seasonally in the vicinity of marine mammal colonies, where they appear to have semi-social interactions. They undertake solo migrations associated with breeding, although researchers have not yet been able to observe mating behaviour. Pregnant females travel to pupping grounds, where litter sizes vary from just two newborn to nearly twenty, the pups being about a metre in length.

  Basking shark (Cetorhinus maximus) (Plate 14)

  The basking shark is a megaplanktivore obtaining nutrition by obligate ram filtering in both normal and reverse diel vertical migration patterns. In other words, it’s a passive filterfeeder and a bloody big fish, reaching a maximum length of about twelve metres and weighing up to 4000 kilograms. It is the only species in its genus. It is a widespread coastal-pelagic shark in temperate and Arctic waters but although the basking shark spends considerable time in inshore waters and is relatively common, not much is known about its behaviour and almost nothing about its reproduction. Nor do researchers know where this migratory shark spends its winter. It can be defined as an apex predator, but it is at the top of an extremely short food chain: phytoplankton, zooplankton, vertebrate.

  While the basking shark has the lamnid characteristic of a large tail of almost equally sized lobes, this is its only physical similarity to its highly mobile predatory relatives. The mouth below the conical snout is gigantic and filled with hundreds of tiny hooked teeth. Just as distinguishing—and unique—are the five gill slits that virtually encircle the head. These large gill slits contain gill rakers which are the shark’s primary means of capturing its tiny plankton prey. Its eyes are quite small, its olfactory sense highly developed. The pectoral fins are large, and the predominant skin colour is grey, with a pale underside. Like the goblin shark, the basking shark’s liver weighs almost 25 per cent of its body weight, which creates near-neutral buoyancy.

  The basking shark does not actively pump water into its mouth in order to feed but simply swims (slowly) mouth agape through zooplankton-rich patches of water. Researchers estimate that for this feeding method to sustain its bulk, an adult shark must strain some 2000 tonnes of water per hour during feeding. To survive, therefore, the basking shark must be able first to locate the rich patches of zooplankton, then to capture efficiently the tiny animals they contain.

  The gill rakers are more than a humble ‘sieve’. To ensure they are in optimal working order, they are shed and replaced throughout the animal’s life. It was once thought that the basking shark shed and regrew its rakers during the winter, while it hibernated, but it is likely that the shark continues to feed in winter, albeit in much deeper waters. The mechanics of the gill rakers are as fascinating as the very idea of microscopic creatures creating and sustaining a giant:

  The incoming current is strained for planktonic food (e.g., fish eggs, copepods, cirripedes and decapod larvae) by a thousand or more gill rakers, each about 10 centimetres (4 inches) long, that are borne on the hoop-like gill arches in the wall of the gullet. When the mouth is opened, these rakers are erected by muscle strands connecting their bases with the branchial cartilages. Upon closure of the mouth, elastic fibres return them to a resting position, lying flat on the gill arches. Mucus secreted by the epithelium at the base of the rakers traps planktonic organisms and, when the mouth closes, these are probably squeezed into the mouth cavity by the collapsing rakers. Teeth are still present, but they are of very reduced size and represent simple modifications of the placoid scales present in the skin, with lateral expansions that appear to be vestigial cusps. During feeding, respiration continues simultaneously and automatically.76

  It is thought that when these sharks gather to feed in plankton-rich waters during summer, the resulting social behaviour is oriented towards mate selection. Mate selection may also be the reason behind one of the basking shark’s most unusual behaviours. This huge, slow creature is one of the few neoselachians that breaches spectacularly—a display which imbues it with both power and mystery. Tradition holds that the animal leaps from the water in order to dislodge parasites, but it is more likely that such impressive aerial displays are designed to attract mates.

  Long-term studies of basking sharks off the coast of Britain, particularly off Cornwall, have also linked mate selection to feeding grounds. Basking sharks sometimes gather in very large numbers, and demonstrate a tendency to swim nose-to-tail. Some suggest that traditional tales of sea serpents may have arisen from the sight of many
large dorsal fins, all in a row, playing follow-my-leader.

  Lead sharks undertook convoluted swimming paths similar to those seen in solitary, feeding basking sharks . . . Rearward sharks made identical adjustments in their swimming trajectories indicating they were following precisely the movements of the shark in front . . . Sharks engaged in following behaviour spent significant periods with their mouths closed, indicating feeding was secondary during this particular activity.77

  The apparent simplicity of an animal passively feeding off plankton has been reconsidered in the light of studies of the feeding patterns of the basking shark:

  The copepods [zooplankton] are thought to be able to sense the position of the chaetognaths [marine worms] during the day and adapt the amplitude and direction of their DVM [diel vertical movement] accordingly . . . this suggests that the reverse DVM of sharks . . . may be attributable to reverse DVM of copepod prey as a consequence of chaetognaths being present in surface waters during the day. These observations point to an intriguing example of how a plankton invertebrate predator indirectly affects the behavior of a fish megaplanktivore.78

  As with most other sharks, there is still much to learn about the second-largest fish. The inoffensive basking shark has never had an easy time of it, its sheer size giving it an aspect of horror. Their decomposing carcasses also diverge greatly from the living animal, adding to the unfortunate reputation of this mild-mannered shark. When dead,

  the entire gill apparatus falls away, taking with it the shark’s characteristic jaws, and leaving behind only its small cranium and its exposed backbone, which have the appearance of a small head and a long neck. The triangular dorsal fin also rots away, sometimes leaving behind the rays, which can look a little like a mane—especially when the fish’s skin also decays, allowing the underlying muscle fibers and connective tissue to break up into hairlike growth. Additionally, the end of the backbone only runs into the top fluke of the tail, which means that during decomposition the lower tail fluke falls off, leaving behind what looks like a long slender tail. The pectoral and sometimes the pelvic fins remain attached, but become distorted, so that they can (with a little imagination!) look like legs with feet and toes, and male sharks have a pair of leglike copulatory organs called claspers, which would yield a third pair of legs. Suddenly, the basking shark has become a hairy six-legged sea serpent!79