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  The skin is multi-functional:

  Attached directly beneath the tough skin are the red and white muscles which . . . transform the cartilaginous architecture of a shark into a fluid, graceful art. Perhaps in part because it is so difficult to move through water efficiently, sharks are very muscular animals. Something on the order of 85 per cent of a ‘typical’ shark’s body weight is muscle, compared with about 35 to 45 per cent for humans.5

  Red muscles, which require oxygen (aerobic), are used for cruising; white muscles, which do not require oxygen (anaerobic), are used to generate bursts of speed. A deepwater species such as the goblin shark, which doesn’t rely on speed, has flabby muscles. A select group—thresher, great white, porbeagle, two species of mako and the salmon shark (Lamna ditropis)—benefit from an endothermic circulatory system in which the blood’s heat does not leave the body through the gills but is retained through a counter-current heat exchange system, the rete mirabile. Thus these cold-blooded fishes are intermittently warm-blooded, their body temperature higher than that of the water surrounding them, which enables the muscles to generate more energy. This is a considerable asset to a pursuit predator in cool water.

  Sharks have five types of fins, paired or unpaired, each of which has a highly specific function associated with locomotion. The first dorsal fin, so instantly evocative, acts like an upside-down boat keel, keeping the animal stable and upright in the water. It can be remarkably flexible. High-speed film of great white sharks demonstrates that the animals ‘warp and buckle this fin at will’ to control their movements.6

  The paired pectoral fins are also mobile and provide uplift in the water to counterbalance the downward thrust resulting from tail propulsion. Critical for manoeuvrability, in fast-swimming pelagic species the leading edges of these fins are narrow, with convex tops and flattened undersides. This increases water flow speed over the fins. Open-ocean migratory species such as the blue shark have huge pectoral fins, which allow them to glide great distances along the ocean currents, expending minimal energy.

  The paired pelvic and anal fins serve to adjust water flows around the shark’s body and increase stability. Many shark species have a second, usually smaller dorsal fin which also contributes to the animal’s stability. Some species have dorsal fin spines, often poisonous, to deter would-be predators. In male sharks the pelvic fins support the claspers, the reproductive organs.

  The tail—the caudal fin, comprising evenly or unevenly sized upper and lower lobes—is the shark’s main propulsion system. Shark tails vary tremendously in shape, size and function, from the great scythes of thresher sharks to the narrow whips of stingrays. Many sharks have heterocercal (nonlunate) caudal fins. This means that the caudal fin has unequal lobes and the vertebral column turns upward into the larger lobe. Some, such as mackerel and basking sharks, have equally lobed homocercal (lunate) fins. In skates and rays, the caudal fins are neither heterocercal nor homocercal, but either drastically reduced in size or absent altogether.

  The skates and rays are the result of a logical but nonetheless remarkable evolutionary development. Their platelike shapes are not single discs but greatly modified pectoral fins adapted to life on the seafloor (with a few exceptions such as the pelagic manta ray). The expanse of the disc enables the fish to glide just above the substrate with minimal energy expenditure. Oscillatory propulsion is the term used when the pectoral fins are flapped up and down like the wings of a bird. This is how the giant eagle and manta rays ‘fly’ through the open ocean. Some skates have pelvic fins modified into leg-like appendages, which punt the animal into motion off the floor; it then glides.

  Mouth region

  While breathing and biting may seem to have little in common, shark evolution determined otherwise. As described in the previous chapter, jaws evolved as a modification of the front pair of gills. The first primitive jaws were crushing structures, which became much more sophisticated once lined with teeth.

  Most sharks have five pairs of gills (a few species have six or seven) on either side of the pharynx. When water enters the moving animal’s mouth, it flows into the pharynx and out through the gill slits. These slits are screened by rows of filaments, on the surfaces of which are tiny growths called primary and secondary lamellae, so structured as to present a maximum surface area. As the water passes through the gills, these lamellae absorb its oxygen into the shark’s blood, at the same time releasing carbon dioxide from the blood.

  Many shark species, particularly the bottom-dwelling rays and skates that are frequently motionless with their mouths on the sea floor, have two further gill-like openings behind the eyes. These are the spiracles, which pump in water independently of the mouth. The oxygen so absorbed is transported directly to the eyes and brain. In fast-swimming sharks, which have more water continuously entering their mouths and passing through their gills, the spiracles are either reduced in size or absent. The process of taking in water while moving is known as ram ventilation. The taking in of water while motionless is known as respiratory pumping, and is something that fast-swimming pelagic species are unable to do. This is why they must keep moving in order to obtain oxygen. The heart is located directly beneath the gills, ensuring the most efficient passage of oxygenated and deoxygenated blood between it and the gills.

  Physiologically the snout is not part of the mouth but, like the gills, has a function intimately associated with the mouth. There was a longstanding belief that because the mouth is underslung, a shark had to turn sideways or even upside down in order to grab prey. In fact, the fusiform sharks take their prey front-on. The jaws, being loosely attached to the skull, are protrusible; with the upper jaw ‘free to slide along a groove in the cranium’,7 initiated by the cartilaginous snout flexing up like a drawbridge. For example, ‘the lemon shark is an active hunter capable of rapid acceleration. When approaching its target at speed, it brakes with its pectoral fins, raises its snout, drops its lower jaw, protrudes its upper jaw and teeth, and then jabs forward several times to get a good grip’.8 And with the porbeagle,

  . . . the opening of the mouth causes a contraction of the front tissues of the lower jaw which makes the first rows of teeth protrude to the exterior, turning the mouth into a kind of hooked trap . . . Next, the top jaw is lowered, bringing into operation the teeth designed to lacerate the prey.9

  Not only does the speed of the initial jaw protrusion provide an advantage to the predator in striking its prey; the shark is then able to protrude its upper jaw repeatedly while chasing the prey or group feeding. Furthermore the complexity of the jaw movements are such as to ‘reorient the teeth for more effective biting and [to] permit a variety of functional novelties (e.g. chiseling, gouging, excavation)’.10

  The business end of a shark is its mouth, and the teeth in it—even more than a dorsal fin slicing water—are instrumental in feeding our negative attitudes towards sharks (one tragic result of which was the targeting of the harmless grey nurse shark in New South Wales waters, throughout much of the twentieth century, because of its ferocious-looking teeth; the species is now faced with local extinction). But, whether fearing sharks’ teeth or not, it is hard not to admire such jaw-dropping masterpieces of nature. There are two ways of considering sharks’ teeth. The first is by their physical form, the second by their manner of production.

  The jaws and teeth of a Greenland shark. In recent years research into this species has developed rapidly, through the work of the Canada-based Greenland Shark and Elasmobranch Education and Research Group. (Dr Chris Harvey-Clark, 2008)

  Because sharks come in all shapes and sizes, and fill so many different niches in the marine biota, as a class they have developed an amazing variety of teeth types. There are four type categories: biters and shakers; crushers; grazers; a combination of any or all of these. Depending on their function they pierce, impale, seize, slice, clutch, clip, nibble and crush. They can be saw-edged, serrated, asymmetrical, broad, pavement-like, plated or lanceolate (lance-like). They can have
prickles or knobs, be comb-like, awl-like, spine-like, thorn-like, cockscomb-shaped, inwardly curving, spindle-shaped or blade-like. And as if that wasn’t enough, some are even ‘bendable’: the teeth of the bamboo shark (Chiloseyllium spp.) are normally erect and pointed for grabbing slippery prey such as squid, but can be folded flat in order to crush the shells of crabs, a major part of their diet, after which they spring erect again.

  Control of bending is built into the tooth, which has a thick root and a relatively small cusp sticking out. This configuration creates a lever that resists bending initially but flattens quickly once the tooth is pushed down hard enough.11

  Being a predator, but lacking limbs and claws to help it seize and hold prey, a shark must have teeth that are always in prime condition. This is achieved by the teeth being continually shed and replaced. How is this done? First, shark teeth are not rooted in the jawbone but merely anchored to the skin by connective tissue. Second, while a shark has one or more functioning upright rows of teeth at any one time, replacement rows are developing in membranes on the inside of the jaw, deep in the mouth. As a row, or set, grows, it is pushed forward in the mouth by those developing behind it, until the upright functioning row (or rows) of teeth are ejected and replaced by brand new ones. Mechanically it is a conveyor belt arrangement. Replacement rates vary according to species. In some species new teeth appear as regularly as every eight to ten days. Some species of dogfish eject an entire set at once, while the cookie-cutter shark will sometimes swallow its entire set of teeth with a plug of flesh it has gouged from its prey. At any one time, functioning teeth in a shark’s mouth number from a few dozen to many hundreds, which means that some species may shed 20 000 or more teeth over their lifetime. Of all their many attributes, that sharks throughout their lives have marching teeth in mobile jaws is surely cause for admiration.

  It is speculated that shark teeth could even play a role in determining the edibility of potential food, since the greatest concentration of taste buds in a shark’s mouth is in the lining right near the teeth. In other functions, a male shark uses its teeth to get a firm hold of the female’s pectoral fin during mating; and territorial grey reef sharks are known to follow an agonistic display with a slashing teeth attack to drive off that threat.

  Internal parts and digestive processes

  A cartilaginous structure, the basihyal, is anchored to the floor of the mouth. It is erroneously likened to a tongue. In most species the basihyal is small and functionless, but some, such as the goblin shark (Mitsukurina owstoni), use it in conjunction with the muscles of the pharynx as a powerful prey-inhaling vacuum. The pharynx leads to the oesophagus which opens into the bag-like stomach. The pancreas secretes powerful digestive enzymes (mainly pepsin) and hydrochloric acid into the stomach to rapidly break down food. For reasons not fully understood sharks have an ability to retain food in their stomachs for long periods without breaking it down. Famously, the 1935 Shark Arm Case in New South Wales involved a large tiger shark which, two weeks after being caught and put on view in the Coogee Aquarium, spewed up a human arm with a piece of rope knotted to it, the arm having been sawn from the body. The arm was so well preserved that detectives were able to use a tattoo on the skin, of two boxers, to determine the identity of the murder victim.

  The tiger shark and the severed, tattooed arm of the famous ‘Shark Arm Case’. (From Shark Attack by V.M. Coppleson, Angus & Robertson, 1958)

  The stomach forms a double bend into the intestine, which is short and comprises spiral valves, their shape enabling greater surface contact with the food for the absorption of nutrients. Like many predators, sharks have symbiotic or parasitic relationships with a considerable variety of internal living organisms. Each of the shark’s intestinal spirals may house a different species of tapeworm. Other parasites live in the stomach and feed off what the shark has consumed. Waste is excreted through the cloaca.

  The ability of sharks to disgorge material in their stomach by everting the stomach bag is called voluntary stomach rinsing. The process is not often seen in action, as in this description:

  A lemon shark, N. brevirostris, was seen to evert its stomach naturally, where it hung limply out of one side of the mouth before head-shaking released an oily ‘scum’ from the surface of the stomach, which was then retracted slowly through the mouth and swallowed. The whole episode lasted 25–30 seconds . . . This process serves to remove parasites, indigestible material, toxic food, and . . . accumulated toxic metabolites.12

  The largest internal shark organ is the liver, consisting of a left and a right lobe, and a much smaller median lobe, positioned beneath the oesophagus and stomach. Proportionately it is about twenty times larger than a human liver, and in some species can exceed one fifth of the total body weight of a shark. It is filled with squalene oil. Chemically squalene is an unsaturated hydrocarbon and also a triterpene, from which steroids derive. It is high in vitamins A and D. (The human body produces squalene as a skin lubricant.) It helps to fuel the shark’s muscles and, because oil is lighter than water, provides the buoyancy that a shark would otherwise lack, having no swim bladder.

  A requirement for living in water is that there is osmotic balance—equal concentrations of water and soluble substances—between the animal and its surrounding medium. Marine teleosts have far less salts in their bodies than seawater and therefore are constantly losing water through their skin and gills. To compensate, they replace this diffused body water by drinking almost constantly. The unwanted salt consumed in this way is excreted through salt glands near the gills. Elasmobranchs maintain the balance by a reverse biological process, involving their kidneys. Most of their nitrogenous waste is retained and converted into urea in the blood. Shark blood is on average about 2.5 per cent urea—almost 100 times that of teleosts. To prevent such high levels of urea damaging the shark’s proteins, its blood also contains high levels of an organic compound, trimethylamine oxide, balancing out the urea and keeping the proteins stable.

  Reproduction

  Sharks reproduce by copulation. There is much to learn about this aspect of shark biology. Males have a pair of claspers, known as intromitent organs, between the pelvic fins. The male usually grasps the female’s pectoral fin with its teeth in order to ensure that they remain together during copulation. Shark species mate in a variety of positions, including side-by-side, or with the male wound around the female’s midriff. Male sperm passes from the testes along the Wolfian ducts to the seminal vesicle for storage. Siphon sacs near the claspers pump seawater into grooves in the claspers, the water transporting the semen into the female’s cloaca and to her paired oviducts, her ovaries having released mature ova into them.

  The gestation period after fertilisation is slow and varies between species, from about nine months to as long as 24 months. Birth numbers also vary considerably, from a single pup in some species to 300 in others. There are three methods of shark gestation and birth:

  • Oviparous: after fertilisation a tough sac grows around the eggs before the female releases them through the cloaca. This egg sac is often known as a mermaid’s purse and can be found in a variety of shapes, some of which have tendrils to help anchor the sac to the sea bed or rocks.

  • Viviparous: the eggs hatch inside the mother and the young are nourished by a yolk-sac placenta until their live birth.

  • Ovoviparous: the term traditionally used to describe the development of young inside eggs retained in the mother’s body. They are nourished by the yolk sac. This term is being replaced in the literature with more specific descriptions relating to viviparity.

  Egg cases of the Antarctic starry skate (Amblyraja georgiana). (M. Francis/NIWA)

  Newborn sharks are immediately able to feed and there is no parental care. Indeed, the newborn can become prey to bigger sharks, including their own kind, which is why many species use pupping grounds that are either far from where adults of their species live, or in shallow waters that adults don’t enter.

  6
r />   CREATURES OF EXTREMES

  Descriptions of Sharks, Skates, Rays and Chimaeras

  The squaliform sharks are creatures of extremes: in size they range from the puny to the downright gigantic, they inhabit a wide range of depths, from sundappled shallows to the chill blackness of the abyss, and their taxonomy is a veritable morass of contention and tentative revision’.1

  This quotation is no exaggeration and it does not always appear logical how and why shark species, genera, families and orders are arranged. This reflects the highly complex and mysterious nature of neoselachian evolution and the more fundamental reality that appearances can be deceptive. The literature does sometimes arrange sharks by categories other than their scientific orders, for example by size, geographic distribution, or whether inshore or pelagic. This extended chapter describes a representative selection of sharks, skates, rays and chimaeras arranged by their scientific orders.

  THE SQUALEOMORPHS: DOGFISHES, SAWSHARKS, ANGEL SHARKS, COW SHARKS, FRILLED SHARKS

  Squaliformes

  This order of sharks, commonly called the squaloids or dogfish sharks, has the following characteristics:

  Classification

  • Approximately 22 genera in seven families

  • Approximately 107 species

  Biology

  • No nictitating membrane

  • Five gill slits

  • Two dorsal fins, often with spines

  • No anal fin

  • Bioluminescence in many species