Shark Read online

  The taxonomy of fishes, cartilaginous and bony, remains a fluid and contentious science. New species are being discovered regularly and there is considerable scientific disagreement on many existing classifications. Such are the classification dynamics that a major reference work quoted in this book, Sharks and Rays of Australia (CSIRO, 1994), by P.R. Last and J.D. Stevens, has undergone significant updating in the relatively short period since its first publication; the second edition, published in 2009, formally names more than 100 new species (previously identified only by letters of the alphabet), and includes 26 new species discovered and named, major changes to the systematics of the dogfishes and skates, and significant new biological and species distribution information.

  Presently the elasmobranchs are classed in fourteen orders, which are broken down into families; each family comprises one or more genus; each genus has one or more species (often very many species). The sharks can be further distinguished into two groups, the Galeomorphs (the first four orders in the table at the end of this chapter) and the Squaleomorphs, with the former considered to be more advanced in an evolutionary sense. The classifications in the table follow Leonard Compagno, the distinguished shark taxonomy expert, currently the Curator of Fish Collections, Iziko South African Museum, Cape Town.21

  As a link between this chapter and the next, it is useful to be aware of the study of traditional or non-scientific classifications, known as ethnotaxonomy, or folk taxonomy. To take one example, a 2007 survey of Brazilian fishing communities found that cetaceans—whales and dolphins—were variously described as ‘fish’, ‘mammal’, ‘not-fish’, ‘fish-mammals’ or ‘like sharks’ family’, the latter owing to convergent evolution and the resultant physical similarities between dolphins and some sharks. Interestingly, ‘some fishers mentioned that despite having watched on TV programs the information that cetaceans are in fact mammals, they continue referring to whales and dolphins as fishes because they have learned it from the elders’.22 Physical similarities aside, however, elasmobranchs are very different to both cetaceans and telecosts, as a closer look at their physiology and biology will show.

  Group name Common name—Subclass Elasmobranchii Order Families

  Galeomorphs Mackerel sharks Lamniformes (Lamna = ancient Greek serpent-monster, formes = Latin ‘shape’) Mackerel (mako, great white, porbeagle, salmon)

  Thresher Grey nurse (sand tiger/Raggedtooth)

  Basking

  Goblin

  Megamouth

  Crocodile

  Ground sharks Carcharhiniformes (Greek shark = karcharias) Hammerhead

  Hound

  Barbeled hound

  Cat

  False cat

  Finback cat

  Weasel

  Requiem/whaler (blue, bull, tiger, reef)

  Bullhead sharks Heterodontiformes (Heterodont = more than one type of teeth) Single family (nine species)

  Carpet sharks Orectolobiformes (Greek = long lobes) Collared carpet

  Long-tailed carpet

  Blind

  Wobbegong

  Zebra

  Nurse

  Whale

  Squaleomorphs Dogfish sharks Squaliformes (Latin squalidus = pale rough skin) Dogfish

  Rough

  Gulper

  Sleeper

  Lantern

  Kitefin

  Bramble

  Sawsharks Pristiophoriformes (Greek pristis = saw) Single family (approximately nine species)

  Group name Common name—Subclass Elasmobranchii Order Families

  Angel sharks Squatiniformes (Latin squatina = shark, skate) Single family (approximately 19 species)

  Cow and frilled sharks Hexanchiformes (Greek exa + ankos = six gills) Frilled shark Cow (sixgill and sevengill sharks)

  Batoids Sawfishes Pristiformes (Greek pristis = saw) Single family (seven species)

  Wedgefishes Rhiniformes (Greek rhinos = nose) Single family (approximately seven species)

  Guitarfishes Rhinobatiformes Greek rhinos = nose, batis = ray) Three families (guitarfishes, thornbacks, panrays)

  Electric rays Torpediniformes (from the Latin root for torpid) Four families (numbfishes, sleeper rays, coffin rays, torpedo rays)

  Stingrays Myliobatiformes Nine families (stingarees, giant stingarees, sixgill stingrays, river stingrays, whiptail stingrays, butterfly rays, eagle rays, cownose rays, devil rays)

  Skates Rajiformes (Latin raja = ray) Three families (skates, softnose skates, legskates)

  Common name—Subclass Holocephali Order Families

  Chimaeras Chimaeriformes (Latin = disparate body parts) Ratfish

  Spookfish

  Elephantfish

  Rabbitfish

  Ghostshark

  5

  SHARK BIOLOGY

  Form and Function

  Most people—at least since the movie Jaws—assume the creatures are solitary, stupid, antisocial brutes . . . Our research demonstrates that white sharks are intelligent, curious, oddly skittish creatures, whose social interactions and foraging behavior are more complex and sophisticated than anyone had imagined.1

  Cartilage and skeleton

  The enduring and incorrect assumption that cartilaginous sharks are primitive, because of their great evolutionary age, also implies that the ‘younger’ teleosts, having bony skeletons, are more advanced life forms. The reality is that biological processes result in form-according-to-function diversity, and with fishes this is clearly evident in the endoskeleton. Furthermore, marine animals with bony skeletons—acanthids, placoderms—appeared in the fossil record before sharks arose.

  The fossil record shows that from about 230 million years ago some marine fishes began to absorb and internally deposit hard calcium phosphate—bone—until eventually most fishes adapted to this form of skeleton (and ensured that land mammals have bony skeletons, having evolved from amphibious teleosts). Why did less than five per cent of the fishes not adapt to bone? One way of considering this is strictly in terms of function: evolution ensured that cartilaginous skeletons enabled mineralising calcium and dentine to be deposited elsewhere, specifically into mass production of predatory teeth. And perhaps there is an evolutionary parallel across the plains of Africa, where the vast majority of mammal species, herbivores, are preyed upon by a few carnivores.

  So it is that biologically sharks are primarily distinguished from other fishes by having cartilage as their internal supporting architecture. Cartilage is ideal for pursuit predators requiring speed because it is light and flexible. And because cartilage does not provide a sufficiently rigid internal support base for muscular attachment, shark muscles also attach directly to the inner dermis, which means a larger area of support and, therefore, more muscle for speed and attack.

  There are three fundamental differences between bone and cartilage. First, bone tissue is a combination of hard mineralised calcium, magnesium and phosphate (known collectively as hydroxyapatite) and flexible collagen fibres (collagen is the main protein in connective tissue). Cartilage contains collagen but lacks the mineralising elements. Second, unlike bone, cartilage has no blood vessels. Third, unlike bone, cartilage does not have blood-producing marrow cells. In the elasmobranchs, hematopoiesis (blood production) takes place in the spleen, in tissues around the gonads and, in some species, in Leydig’s organ associated with the oesophagus.

  Cartilage’s basic building blocks are cells called chondrocytes, which produce and are eventually surrounded by an extracellular matrix of collagen, glycoproteins and complex carbohydrates. Water is also present in cartilage. Lacking a blood supply, chondrocytes are fed by diffusion. There are three types of cartilage, hyaline (gristle), fibrous (hard) and elastic (flexible). Elasmobranch cartilage is mostly hyaline.

  The chondrocyte equivalents in bone formation are osteoblast cells which, because of their mineralising abilities, grow bone rather than cartilage. Despite these diferences the relationship between cartilage and bone is an intimate one. Juvenile mammal bones are mostly
cartilage. Calcium in the mother’s milk is deposited as calcium salts and the cartilage is gradually replaced by bone. In sharks, the vertebral centra become partially calcified by the calcium salts contained in seawater, as do the sharks’ denticles, jaw, braincase, gill arches and fin supports. (Like tree trunks, calcified centra have annual growth rings, by which age can be determined.)

  The skeleton of a porbeagle shark ( Lamna nasus). (Dr Steven Campana, Bedford Institute of Oceanography, Canada)

  The cartilaginous shark skeleton is an uncomplicated structure, comprising a braincase, jaw supports, gill supports, spinal column and various fin supports. The braincase has a base, side walls and a partial roof, a rostrum to support the snout and shaped parts for the eye and nasal sockets and the otic capsules. At the rear of the braincase the brain’s highway, the spinal cord, exits through the foramen magnum (‘big hole’). The paired gill arches which support the gill filaments—most sharks have five pairs—are positioned at the rear of the braincase.

  Parts of a shark

  The spinal column consists of a flexible chain of linked vertebrae, more than 150 in some larger sharks. Each vertebra of the upper spinal column consists of a centrum through which the notochord runs, and a neural arch which protects the spinal cord. The arch is also designed so that muscles can attach to it. The caudal vertebrae bend upwards and so form the shark’s tail. Each caudal vertebra also has a haemal arch, through which the dorsal aorta delivers oxygenated blood throughout the body. Fin supports in the form of girdles and radials attach to the spine. The u-shaped girdles support paired fins, while the radials are jointed and radiate out as spokes, in the shape of the fin. Over millions of years the proportion of the fin supported by radials had decreased significantly, allowing for greater flexibility.

  Sensory processes

  While there remains much to be learned about shark brains, tests have long since shown some species to have substantial learning capacities, in their responses to shape, sound, scent and colour. And tests have shown that sharks can differentiate quickly between human-set decoy prey and the real thing. Physically the mako, porbeagle, great white and hammerhead have proportionally larger brains than many of the so-called higher mammals, and these brains are required to process considerable amounts of information collected in remarkably sophisticated ways.

  Sharks rely significantly on their sense of smell and ability to detect movement in the water around them, but many also have better eyesight than is commonly realised. Shark eyes are well adapted to water, a medium 80 times denser than air. They can see up to 25 metres in clear water and many species see in colour. Shark eyes have a visual streak across the retina that improves their vision of the underwater horizon. Not surprisingly, deepwater sharks tend to have big eyes, some with permanently dilated irises to allow maximum light penetration. Sharks’ eyes, along with those of nocturnal vertabrates, have a feature called the tapetum lucidum. This reflecting layer behind the retina consists of thousands of minute platelets silvered by guanine crystals and enhances vision in dim light by bouncing the available light back to the retina, thus amplifying the image. In brighter light, dark pigments cover this layer so the eye doesn’t take in too much light. Many shark species have a moveable eyelid, the nictitating membrane, to protect the eye from defensive actions of prey. Those which don’t, including the great white, roll their eyes back in their sockets, exposing a hard pad at the back of the eye. This is why close-up photographs of feeding great whites often show them to have black, glistening, ‘sightless’ eyes.

  Sharks also have a light-sensitive organ situated at the forefront of the brain, the epiphysis cerebri, more commonly known as the pineal gland or the mystical ‘third eye’. The skin above this gland is translucent, allowing light to penetrate and stimulate the hormones. One theory is that this enhanced ‘vision’ allows sharks to process information about the seasons—because of their varying degrees of light—which is of fundamental importance for migration and mating.

  Seven senses of a shark

  Sharks’ nostrils are properly called ‘nares’ because they play no role in breathing. They are generally set forward on the snout, often with grooves leading from the mouth to facilitate water flow into them. The nares themselves have flaps which direct water flow into the olfactory lamellae—paired sacs packed with sensory hairs and nerve fibres that are acutely sensitive to chemical stimuli. Information about these stimuli is then transmitted through olfactory bulbs to the brain’s large olfactory lobe. The size and complexity of this apparatus are clear indications of the importance of the sense of smell for the many species of shark which rely significantly on highly diluted chemical trails, both to seek prey and to communicate with their own kind.

  Sharks do not have outer ears, but their inner ears play an important role in maintaining the animals’ balance and equilibrium as well as detecting sound. Small openings behind the sharks’ eyes allow seawater to flow through narrow tubes, the endolymphatic ducts, into the inner ears. The cavity of each inner ear contains three acutely sensitive fluid-filled cartilage tubes, and tiny calcium carbonate bones called otoliths, or ear stones. As the shark moves through the water, its brain picks up the signals of the fractional movement of these tubes and bones in their fluid-filled cavity. These signals tell the animal about its position in the water relative to gravity, as well as what its body is doing: accelerating or decelerating, pitching up or down, or moving from side to side. The otoliths also detect sound waves. Sharks are particularly receptive to irregular, low-frequency sound waves, such as those emitted by the erratic struggles of a fish or other creature in distress.

  Sharks were once thought to have an otic system running the entire length of their bodies. While this has been proved not to be the case, they do have vibration-sensitive canals running along each flank just below the skin. This is the lateral line system. Open pores on the skin’s surface allow seawater into tiny canals full of sensitive hairs which detect wave movements made by living things. The shark can then orientate itself in relation to the source of the wave movement. These canals also form complex patterns on the head.

  Pit organs are minute sensory organs which bear similarities to the cells of the lateral line system. They are widely scattered inside pores or grooves across the upper body, including in species-specific patterns, which may hold a clue to their as-yet mysterious function. Another feature of pit organs is that they appear to be protected by distinctively shaped dermal denticles. Over the past century numerous theories have been ascribed to them, including external taste buds, detection of salinity variation, monitoring of swimming speed, monitoring of tidal currents, and prey detection.

  Sharks, as predators, benefit from the fact that water is a good conductor of electricity. Even when a fish is motionless, or buried under sand, its muscles and heart emit electrical pulses. Clusters of visible pores on sharks’ heads and snouts (which can bear a resemblance to a three-day growth) lead to jelly-filled canals which contain electroreceptors. These are called the ampullae of Lorenzini, after Stephan Lorenzini, who was an assistant to Nicolaus Steno. They not only detect minute electrical fields, but possibly also function as an internal compass tuned to Earth’s electromagnetic fields.

  A few species of rays create their own electricity, and a number of deep-water shark species are capable of bioluminescence, the chemical generation of light. These sensory abilities are used for predatory or defensive purposes, and will be addressed when describing the individual species, the electric rays (Narcine spp.), the cookie-cutter shark (Isistius brasiliensis) and the megamouth shark (Megachasma pelagios).

  Movement

  Elasmobranch skin, the integument, differs from that of teleosts in that it does not have scales. A shark’s dermal denticles are individual growths rooted in the dermis, with enamel-tipped spines which break through the epidermis. Unlike scales, sharks’ dermal denticles once grown are ‘nonliving’ and are regularly shed and replaced.2 Their composition—dermal bone, dentine, ename
l—classifies them as ‘toothlike’, as their name suggests, although they are more formally known as placoid scales. Dermal denticles grow in a profusion of shapes and sizes: they can be rounded (snout); shield-like (belly); keeled (flanks); diamond-shaped (leading edge of fins); or tiny and quadrangular (pharynx). Backward-facing, their primary purpose is to ease the animal’s smooth passage through the water. Furthermore, ‘possible and probable derivatives of dermal denticles include thorns, rostral sawteeth, fin spines, stings, clasper spines, and gill raker denticles’.3

  The denticles’ arrangement may also have evolved to render the predator ‘hydrodynamically quiet’.4 In some species, the denticles have an obvious defensive function, being sharp or abrasive enough to inflict considerable pain and wounds. Large pelagic predators tend to have fewer and smaller denticles, being required less for defence than to facilitate rapid movement through water. Most ray species, and a few shark species, have reduced numbers of denticles and their skin is instead often covered with a layer of mucus. This protects the slow-moving or frequently motionless animals from marine parasites.

  Shark skin colouring and patterning are very varied, but usually designed for camouflage, in both predators and prey. Thus, the fast-swimming pelagic predators sport a range of countershades of blue-grey and white (hard to see from above, hard to see from below), while rock and reef bottom-dwelling predators such as the wobbegong have blotchy camouflage-patterned skins and seaweed-like tendrils.