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The Great White Shark - Essay

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The Great White SharkContents:Introductory TableThe Ancestral Tale of Sharks

- Shark Family TreeWhat Makes a Shark? Identifying Features:

- Skeleton- Skin- Teeth- Fins- Gills- Sink or Swim- Muscles- Temperature Control

Shark Senses:- Smell- Sight- Hearing & Balance- Touch- Taste- Electrical- Ampullae of Lorenzini Pores- Pressure Differences/ Lateral Lines

Shark Reproduction:- Male & Female Sharks- Courtship & Finding a Mate- Fertilization- Birth- Why are shark numbers decreasing?

GlossarySources

Il grande squalo biancoIndice:Tabella introduttivaIl racconto ancestrale degli squali- L’albero genealogico dello squaloCaratteristiche identificative dello squalo:

- Scheletro- Pelle- Denti- Pinne- Branchie- Nuotare pena la morte- Muscoli- Controllo della temperatura

I sensi dello squalo:- Odorato- Vista- Udito & equilibrio- Tatto- Gusto- Elettricità- Pori ampolle di Lorenzini - Differenze di pressione/ Linee laterali

La riproduzione dello squalo:- Maschi & Femmine- Corteggiamento & identificazione del compagno- Fecondazione- Nascita- Perché il numero degli squali è in diminuzione?

GlossarioFonti

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Great White Shark

Shark Name Great White Shark

Scientific Name Carcharodon carcharias

Species Authority  Linnaeus, 1758

Shark Order Lamniformes - Mackerel sharks

Family Lamnidae - White, Mako, and Mackerel sharks (Müller & Henle, 1838)

Region Med

Appearance

Fusiform, conical snout, short gill slits (do not encircle head). First dorsal fin is large, second dorsal and anal fins are minute, caudal fin crescent shaped. Dorsal surface blue-grey to grey-brown, often bronzy, ventral surface white, boundary between two tones is abrupt. Small, dark spots may be present on flanks posterior to last gill slit. Most have a black oval blotch on axil of pectoral fin. Teeth are large, erect, triangular and serrated. Denticles are minute, and tightly packed, with three ridges and flat blades. Skin is smooth compared to many species.

SizeAverage: ~ 370-490 cm (max ~680cm)Mature: ~350cm (M), ~450cm (F) Maximum published weight: 3,400kgMaximum reported age: 36 years

Distribution Mostly temperate seas, although individuals have occurred in tropical waters. Occasional movement to cold, boreal waters. Western Atlantic: Newfoundland to Florida, northern Gulf of Mexico, Bahamas, Cuba, Brazil to Argentina; Eastern Atlantic: France to South Africa, including the Mediterranean; Indian Ocean: Red Sea, South Africa, Seychelles Islands, Reunion, Mauritius; Western Pacific: Siberia to New Zealand, Marshall Islands; Central Pacific: Hawaiian Islands; Eastern Pacific: Alaska to the Gulf of California, Panama to Chile

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Habitat: Mainly coastal and offshore areas of continental and insular shelves, also recorded off oceanic islands. Often close to surf line, shallow bays and offshore continental islands. Depth range: surface to ~1,875 m. Temp range: 7-27 °C.

Biology

Reproduction:Ovoviviparous - embryos hatch in uteri, females give birth to live young. Embryos nourished through oophagy (ingestion of unfertilised eggs). Embryonic sharks swallow own teeth, perhaps to reutilise calcium/minerals. Possible that females reproduce biennually, gestation time unknown, possibly up to 1 year. Litter sizes: 2-10 embryos, unconfirmed reports of up to 14 embryos.

Diet:Principle prey: marine mammals and fishes (including other sharks and rays), occasionally turtles. Birds and sea otters commonly found injured from encounters with White Sharks, but rarely ingested.

Predators:Humans represent the greatest threat, plus occasional predation by Killer Whales and larger sharks.

Behaviour:Movement on localised, regional and intercontinental scales. Capable of short, high-speed pursuits and launching clear from the surface. Patterns in movement and abundance may be linked to seasonal variations in surface temperature. Some swimming modes interpreted as avoidance of conspecifics and maintenance of individual space. Sometimes use displays in order to discourage other sharks – most commonly “tail slap” (caudal fin out of the water and slapping the surface, propelling water usually in the direction of a second shark).

Conservation Status Vulnerable (VU) - Low reproductive potential, vulnerable to target bycatch fisheries. Protected in some parts of range, but enforcement is weak. Global status of Endangered may be found with further data. Overall population unknown, regional estimates suggest decline in abundance and average size. Placed on CITES Appendix II 2004.

Threats These sharks are sparsely distributed and have slow reproduction rates, factors making the population particularly vulnerable and slow to recover from depleted numbers (2). Although the population size is difficult to assess, evidence suggests that numbers have declined in several areas by up to 90 percent over the last 40 to 100 years (8) (13). Sharks caught either accidentally as bycatch or deliberately targeted are sold for their flesh, skins, oil and fins for shark-fin soup (8). The teeth and jaws of great white sharks are particularly valuable; a recently recovered specimen was valued at US$ 50,000 (8). Game fishing has increased in popularity recently and the great white shark is something of a holy grail for enthusiasts due to its great size, powerful resistance to capture, and reputation as the most dangerous fish in the sea (3) (7). Unfortunately, its inquisitive nature and tendency to investigate human activities, as well as to scavenge from fishing gear, makes this shark vulnerable to capture (3). This species is often found close to human settlements and habitat degradation, depletion of prey species, negative attitudes towards the shark, and shark fences to protect bathers further affect population numbers (3) (8). The great white shark is viewed with fear throughout much of its range, making conservation efforts difficult to initiate, and unwarranted, media-fanned campaigns to kill great

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whites have even occasionally occurred, following shark attacks or in anticipation of such attacks

Conservation The great white shark is protected in South Africa, Namibia, Australia, the USA and Malta (8) (13). The recent surge of interest in shark dives and ecotourism, especially in South Africa, southern Australia, and Guadalupe Island, Mexico, may provide a substantial local income and an important method of education (12). With effective legislation and policing, this tourist trade may well be a vital method of saving the species despite the complex issues involved (12). Vital research into this misunderstood fish is being carried out in countries such as Australia, Mexico, New Zealand, South Africa and the USA (8), and the FAO (Food and Agriculture Organisation of the UN) has prepared an International Plan of Action for the Conservation and Management of Sharks (IPOA-SHARKS) (14). Indeed, recent scientific findings that great whites regularly undergo long-distance, trans-boundary movements only highlight the need for international protective measures, with national legislation being no guarantee of survival of the species (8). However, further information gained from ongoing studies into their movements and the specific habitats the sharks utilise will hopefully provide the basis for designing appropriate protection measures to aid the survival of this remarkable shark around the world.

Notes

Importance to Humans: High capture due to value of jaws and teeth. Some preserved by freezing/taxidermy for public display/trophies. Flesh, skin, liver, carcass and fins used for consumption, leather, oil, fishmeal and shark-fin soup respectively. 

Danger to Humans: Due to its size, power and feeding behaviour, this species is credited with more fatal incidents than many others.

ReferenceIUCN Red List of Threatened SpeciesFlorida Museum of Natural HistoryReefQuest Centre for Shark Research

Author's Name Rachel Turner

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The Ancestral Tale of SharksWritten by Emma-Louise Nicholls

Sharks have either shared or owned solely the position at the top of the food chain for over 400 million years! The species that briefly joined them as apex predators have consisted of massive marine reptiles such as Mosasaurs and Plesiosaurs, but sharks are the only group that have survived to modern day. So what is it that makes sharks so special that they can survive global catastrophes when other top predators cannot? The answer lies in the fossil record and to fully understand these magnificent emperors of the ocean, we shall travel back in geological time, to a period called the Devonian.

There are four periods that make up the entire history of planet Earth. The first period, the Precambrian, began with the evolution of the Earth 4.6 billion years ago. It is categorised by a complete lack of animal life. The second period, the Paleozoic, spans 545 to 250 million years ago and saw the evolution of life from single celled organisms to bony fish and sharks. The third period was the Mesozoic, famous for the reign of the dinosaurs, and which lasted from 250 to 65 million years ago. The most recent period, the Cenozoic, spans from 65 million years ago to modern day.

Hammerheads were the last of the modern shark groups to evolve, and did so in the Cenozoic. It is actually very difficult to say for certain when they first appeared as their teeth are easily confused with those of another shark, but their evolution date is estimated at between 50 and 35 million years ago. Following the fall of the great marine reptiles at the end of the Mesozoic, only modern sharks and toothed whales (such as the Killer Whale) remain at the top of the food chain, but they do so with diversity, adaptability, global domination and above all, beauty.

Shark evolution is an exciting and complex story that includes two major radiations and the miraculous survival of five mass extinctions. But the question is: will we drive them to become part of the sixth mass extinction? The one occurring right now. The one threatening to wipe out more species than any of the previous five. The one being caused by humankind.

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Shark Family Tree

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What Makes A Shark?

Identifying featuresSharks have inhabited our oceans for over four hundred million years. They occupy almost every marine ecosystem on earth and some can survive in freshwater, although no species lives there exclusively (1). Sharks comprise about 7% of living fish species (over 450 species are recognised as sharks) (2). Sharks come in a huge amount of sizes and are adapted to eat many different things. 

The main characteristics that define a shark are:

Skeleton 

Bony fish such as tuna and mackerel have a skeleton that is made out of bone just like a human skeleton.  A shark skeleton is made entirely of cartilage (gristle).  The same material as human noses and ears are made of.  It is lighter than bone, helping the shark stay afloat.  It is also more bendy than bone and grows as the shark does (1).  

Skin 

Most bony fish have flat scales covering their body, which grow with the fish. The skin of sharks is much rougher than that of other fish as it is covered with millions of tiny teeth called dermal denticles (2).  These denticles point backwards and help the shark to swim faster by reducing the water turbulence.  As a shark grows the denticles are shed and replaced with slightly larger ones.  Some denticles are so big that the shark uses them as defensive spines or shields.

Shark facts: 

The hydrodynamic efficiency of shark skin has been copied

by swimming costume manufacturers, such costumes are said to reduce swimmers

times.Shark skin can be very rough

and it has been used for sandpaper in the past.

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Teeth A shark’s teeth are larger versions of the denticles covering a shark’s body.  Teeth are arranged in rows, which slowly move forward from the back of the jaw (a bit like a conveyor belt).  As the front rows of teeth wear down or fall out new rows move from behind to replace them.  These replacements happen about every two weeks (2). 

 

Different sharks have different shaped teeth depending on what food they eat.  Sharks that eat shellfish and crabs have flat crushing teeth. Sharks that eat fish have pointed teeth and those that sometimes eat seals and sea lions have razor sharp teeth.  Filter feeding sharks (Whale (Rhincodon typus), Basking (Cetorhinus maximus) and Megamouth (Megachasma pelagios) have greatly reduced teeth. They use gill rakers to strain plankton from the water (1,2).

Fins 

Bony fish and sharks have fins that help them swim properly. Bony fish have fins that fold down when they are not being used. Sharks have five pairs of rigid fins, which can't fold down.

The big triangular dorsal fin of the shark is the first part you see if the shark is at the surface. This fin helps the shark to balance itself. A shark’s tail is called the caudal fin and this fin varies in shape between different sharks. Sharks move their caudal fin from side to side to push themselves forward. They steer with their pectoral (side) fins. 

Shark facts: 

Shark teeth have been for knifes, jewellery and tools in

the past (3).

 Shark facts: 

Shortfin Mako Sharks (Isurus oxyrinchus) are the fastest sharks and they can move at speeds

of up to 69kph

  Sharks can't swim backwards like some fish can.

 

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Gills Sharks and rays have five to seven gill slits. As a shark swims forward, water passes over its gills.  Oxygen in the water is absorbed into tiny blood vessels near the gill. The blood carries the oxygen around the body (1). Some sharks can pump water across their gills. The shark opens its mouth to drawn water in then shuts its mouth. Water is squeezed over the gills as more water is sucked into the mouth. Valves stop water going down the throat, small gill flaps stop water coming in through the gill slits.  To aid the water flow sharks swim forward all the time, if they are trapped for any reason they cannot breathe properly.

Sharks that live in the open ocean depend on swimming forward for their buoyancy as well as to breathe, if they cannot swim they sink and suffocate.

Wobbegong and other bottom living sharks do not have to keep swimming, they breathe in through their spiracle holes and out through their gills slits just like skates and rays.

Sink or Swim A shark’s body tissues are denser and heavier than water so it is natural for a shark to slowly sink to the seabed.  Sharks have various buoyancy aids to stop this happening. Sharks reduce sinking by having a big oily liver and light, cartilage skeleton.  Some sharks can stop swimming as they continue to actively move water over their gills. Sharks like the Whitetip Reef Shark have been seen resting on the seabed near reefs.  Unlike bony fish sharks do not have a swim bladder, however, some sharks gulp air into the gut as a very primitive swim bladder.

  

 

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Muscles Sharks have two kinds of skeletal muscle (muscle that moves the skeleton). The first is a thin layer of red muscle, which lies just under their skin. This red muscle has a good blood supply and helps the shark to swim slowly for a long time. Red muscle works by breaking down the fat in the shark’s body.  The second type is white muscle, found under the red muscle.  White muscle has a poor blood supply and works by using the energy from the breakdown of glycogen (sugars). Sharks use their white muscle to make short fast sprints when catching prey or moving away from danger.

Temperature Control 

Most fish are cold blooded; their body temperature is the same as the surrounding water.  Mako, White (Carcharodon carcharias), Porbeagle (Lamna nasus) and thresher sharks are all warm blooded and can control their own body temperature, like birds and mammals. Their body temperature can sometimes be about 10°C above the surrounding water temperature.

Sharks use these amazing adaptations to dominate the world’s oceans.  They are now threatened by human activities such as overfishing and accidental by-catch. 

  

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Shark Senses

Sharks are advanced animals with highly developed senses

Sharks have a wider range of senses than humans. Humans have senses which enable them to register sight, smell, sound, taste and touch. Sharks however have senses that we do not! This includes the same five as humans, as well as the ability to sense electrical currents and pressure changes. 

These senses are what make the shark such a successful animal: helping them navigate the wide oceans, finding food or mates, and enabling them to survive in such a diverse array of habitats.

Smell

Sharks do not have nostrils in the sense that humans do, as the nasal passage is not connected to any breathing apparatus. The external openings that enable sharks to sense scent in water are called nares. Nares are small sacs that water can move into and out of, passing over smell receptors.

Smell is essential to sharks in enabling them to find food or mates. In the Great White Shark, over 14% of the brain is used purely to process smells. Sharks have one nare on either side of their heads, which allows them to smell a wide area of water; this is most pronounced in Hammerhead sharks. The Hammerhead in particular can then use these two nares to decide on which side of its head an interesting scent is stronger, and follow it back to the source of the smell. Experiments have shown that sharks respond most strongly to odours produced by injured or distressed prey.  The same prey if healthy and uninjured does not produce such a strong response from the shark.

Some scientists believe that sharks are able to use air-borne smells as well as those distributed by the water in the sea. Great White Sharks are often seen with their heads above the water, apparently spy-hopping (or deliberately using vision above the surface of the sea to inspect prey). This behaviour is well known in marine mammals such as Killer Whales, but not in fish. It has been suggested recently however that the Great White Shark may be using its nose and not its eyes above the water to better smell its close environment since its sense of smell is so refined. Sharks respond to one part of blood to one million parts of water – the equivalent to one teaspoon in an average swimming pool!

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Sight

At a distance of 15m or less, vision is a shark’s dominant sense. However, shark eyes have to be able to cope with the distorting affect of light in water. Human eyes focus light in the air using a cornea and an oblong lens. In order to focus light that is diffracted by water, the shark only uses a lens; however this is modified to almost a perfectly spherical shape.

Typically, in ocean water, natural light can only penetrate to 100m deep (the photic zone), which shows that light cannot travel far in water. Sharks require visual adaptations to enable them to maximise the amount of light available in order to help them see. The iris controls the amount of light entering the eye, a band of muscle surrounding the pupil.  In low light the iris muscles contract and the pupil dilates to allow as much light as possible into the eye. In high light conditions, the pupil must become smaller, reducing the amount of light in the eye and the iris muscles relax. The human eye controls light in the same way, however regular bony fish cannot. At the back of the eye is the retina, which is made up of two types of cell: rods and cones. Rods enable detection of light and dark but cannot register finer detail. Rod cells help the shark detect movement. The ratio of rod to cone cells in humans is 4:1, but this ratio varies in sharks. For example, the Spiny Dogfish has a ratio of 50:1, the Sandbar Shark 13:1 and the Great White 4:1 (the same as a human). Cone cells are used to determine colour and fine detail. Old theories suggested that sharks had few cone cells at all and that shark vision had no colour and little detail. However, modern technology has been able to prove that this is not true. Shark eyes also possess an ability which we humans do not have: nocturnal vision. Like cats eyes, they have a tapetum lucidium which literally means “carpet of light.” This is a reflective layer of shiny cells that increase the amount of light available to the eye in low light conditions. If light levels increase, the tapetum lucidium can be covered with darker coloured cells to prevent too much light entering the eye. This enables sharks over ten times the sensitivity of a human to light!Sharks have two eyelids per eye however these do not always meet and a nictitating membrane can be used to protect the eyeball. However some sharks, such as the Great White, do not possess this, and have to roll their eyes to face inwards to protect them when attacking prey. This exposes the white sclerotic coat of the eyeball which is tough enough to protect the eye.

Examples of shark eyes: from left to right Galapagos (Carcharhinus brachyurus), gulper and swell shark; images Marc Dando.

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Hearing & Balance

Sharks inner ears are located on either side of the sharks head behind the eyes, and are only visible from the outside as two small holes. The ear senses sound by the vibration of tiny hair cells. Sound waves cause these hairs to vibrate and this is interpreted by the brain, providing sharks with their sense of hearing. In field studies scientists have seen that sharks are particularly attracted to sounds, such as struggling fish.

The shark ear is made up of three cartilage tubes filled with fluid and lined with hair cells. The three tubes are arranged so that movement can be sensed in each dimension of the 3D liquid environment. This allows the shark to sense its position and balance itself in the water. A small area in each ear is made up of calcium carbonate, which are called otoliths. These otoliths enable the shark to sense gravity and its orientation in the water i.e. which way to the surface or the seabed.

Inner ear of shark showing semi-circular canals: image Marc Dando

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Touch

Shark skin is roughly textured and unlike regular fish with overlapping smooth scales, shark skin is formed of tiny tooth like structures called dermodenticles. This makes the shark more streamlined in the water and offers a degree of protection from predators (shark skin has been used in the past as sandpaper owing to its toughness). However, despite this sharks have multiple nerve endings under their skin to detect touches. They can sense temperature changes, pressure and pain. Some sharks have barbells around their mouths to probe the sand for prey. 

The variety of barbels that sharks have from left to right: Australian Angel Shark (Squatina australis), Ornate Wobbegong (Orecolobus ornatus), Rusty Carpetshark (Parascyllium ferrugineum) and Shortnose Spurdog (Squalus megalops) images Marc Dando.

Another way sharks sense touch is through their teeth. It may appear that shark teeth are set firmly in the jaw, as human teeth are, and as they appear in dried shark jaws. However, in living sharks, each tooth can be rocked back and forth up to 10 degrees. The jaws of sharks are not fixed to the skull and the teeth are flexibly fixed to these jaws. The teeth contain numerous nerves which are pressure sensitive, and lacking hands to touch items, the shark uses its teeth to test objects.

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Taste

Sharks can detect the same basic flavours that humans can: bitter, salt, sweet and sour, due to the taste buds that line their mouth and gullet. However, it is only once an item is inside the shark’s mouth that the shark can decide whether it is suitable food and if the shark does not like the taste then the item will be spat out. Possibly this accounts for the high survival rate of shark victims, where sharks have accidentally bitten humans and then left without further incident. In these cases it is likely that the sharks sense of taste indicated that the human was not an adequate food source.

Perhaps the best evidence for this is the case of Californian Sea Otters. When otters die at sea, often the body is washed up on the shore and scientists have noticed that 9% of all the Californian Sea Otter carcasses had signs of Great White bite marks. However, no Sea Otter has ever been found in a Great Whites' stomach, so why weren’t the sharks eating the otters? The theory suggests that since Sea Otters are related to skunks and weasels, the otters taste and smell must be strongly unpleasant and so the Great Whites find them inedible.

Taste and smell are closely linked,as anyone who has tried eating whilst pinching their nose will confirm, and this is the same with sharks. The chemistry of smelling and tasting depends on dissolved material and these senses complement each other.

Electrical

All sharks have bioelectric receptors positioned on the head and snout area called the Ampullae of Lorenzini. These are small pores filled with jelly matter and tiny hairs which enable the shark to detect weak electrical fields, such as those generated by the Earth’s geomagnetic field or muscle contractions in prey.

The Earth’s geomagnetic field is thought to help sharks orientate themselves and navigate the world’s oceans, which may explain how they are able to migrate such immense distances so accurately.

The sharks can also use these ampullae to avoid predators and detect food, as every animal in water gives off a weak electrical field. For example, as a fish swims, muscles contract along its body which in turn create a weak electrical field in the water surrounding the fish. Sharks can use this sense to hunt for food when vision is difficult, such as at night or finding fish that are hidden under the sand.

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Ampullae of Lorenzini pores

Over the shark's snout and lower jaw are lots of small pores called 'Ampullae of Lorenzini'. The pores contain hair cells and jelly matter, allowing the shark to detect minute electrical currents in the water. Sharks use this sense to build an electrical 'map' of their surroundings allowing them to find prey and to avoid predators. Metal objects make electrical currents and attract sharks. This is why sometimes sharks can be seen attacking metal shark cages.

Location of Ampullae of Lorenzini pores image Marc Dando  Electric rays can give out currents up to 200 volts. The flathead of hammerhead sharks gives more space for these pores

and so they can pick up electric currents better than other sharks.

Pressure Differences/Lateral Line

The lateral line is a long tube like structure, just below the skin of the shark, this is filled with water and tiny hair cells that runs the length of the body. The lateral line has pore openings through the skin to the surrounding water, enabling special cells to sense pressure differences.

The shark is able to sense waves of pressure in the water, such as those created by a swimming fish, allowing the shark to avoid predators and locate prey. As the lateral line runs from snout to tail, the shark can assess the different intensities of pressure along its body. This enables a shark to create a pressure map of the water and sense the magnitude, distance and orientation of the source of pressure change.

There is no human equivalent of this sense, because air is not dense enough to feel any pressure differences.

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Shark Reproduction

Sharks have been around for millions of years and have a reputation of being ancient, or primitive animals. However, their reproductive strategies have evolved to be more advanced than those of bony fish and in some cases their strategies are as highly developed as mammals.

Bony fish versus sharksBony fish have a reproductive strategy where they invest their energy in producing large amounts of small eggs. The female fish release millions of eggs into the water and the male fish simply cast their sperm into the water near the eggs. Fertilization is left to chance, many individuals will be eaten but a few will survive to maturity to reproduce. Scientists know this strategy as ‘r-selection’. Sharks produce fewer much larger eggs and fertilization takes place internally.  This means they put their reproductive energy into producing a few larger young that are likely to survive to maturity and reproduce.  This is called ‘k-selection’ .

Male & Female Sharks

There is a visible difference between male and female sharks, although both male and female sharks have a hole near the pelvic fins called the cloaca.

Male sharks have modified pelvic fins called claspers.  They also have two muscular sacs (siphon sacs) in their abdominal wall, which they fill with seawater.  Internally males have testes that produce sperm and secrete hormones that control the development of male characteristics. 

Females tend to be larger than males, externally the only difference is the absence of claspers.  Females have a pair of ovaries where eggs are produced; in most species only the right ovary makes eggs. The ovaries, like testes in the male, produce hormones that control the development of female characteristics.

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Courtship & finding a mate

Male and female sharks tend to live in different areas of the ocean the only time they come together is to mate.  Courtship between sharks is something that is rarely seen by humans.  Courtship is likely to involve all the shark’s senses. Chemicals released into the water by the females stimulate the male sharks.  A shark’s courtship can look like fighting, and female sharks often end up with teeth marks or ‘love bites’.  Male Whitetip Reef Sharks (Triaenodon obesus) bite the female on her fins or back during mating.  This doesn’t hurt the female shark because their skin is thicker and can be up to three times thicker than a male sharks' skin.

Male Catsharks have been seen twisting their body around female sharks before mating.  Larger sharks, like the Whaler(Carcharhinus brachyurus), Lemon (Negaprion brevirostris) and Nurse (Ginglymostoma cirratum) Sharks, have been seen with the male swimming parallel and head-to-head with the female shark.

Fertilization

During fertilisation one clasper is inserted into the female cloaca.  The claspers have hooks and spines on them, helping them to stay firmly in the female.  Sperm passes from the sperm sacs and goes down the clasper.  Water from the muscular sacs is washed down the clasper to help the sperm move into the female.  The eggs are then fertilised one at a time in the shell gland of the female shark.

How do baby sharks start life? 

Some sharks, like Dogfish and Hornsharks, lay eggcases (oviparity) that have large yolk sacs in them. The yolk nourishes the developing shark as oxygen passes in through the eggcase and body wastes seep out.  When the baby shark (pup) hatches it is a fully formed copy of its parents.  The female shark can spend a long time laying her eggcases as they must be in a securely fixed in a safe place as it can take between six to ten months for the pups to hatch.  Port Jackson Sharks lay large eggs shaped like a screw, which are adapted to wedge firmly between rocks.

(Example of a shark developing in an eggcase, Marc Dando)

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Most shark species, including Mako, White, Thresher, Crocodile and Sandtiger, do not lay eggs (2).Instead the fertilized eggs develop inside the mother (aplacental viviparity or ovoviviparity).  The embryos develop within an eggcase but this is not hard like the ones that are laid instead it has a thin membrane-like covering.  The embryos develop in the same way as oviparous sharks (above) but within the female there is a good supply of oxygen and little danger from predators meaning a greater chance of survival for the pups.  Once the food supply from the yolk is finished the pup will hatch out of the eggcase and be born.

In some species the pups are not born immediately after hatching out of the eggcases.  Instead they stay in a part of the oviduct called the uterus where they are provided with more food.  In some species, such as the Porbeagle, the pups eat unfertilised eggs (oophagy).  Sandtiger Sharks (Carcharius taurus) do not just eat unfertilised eggs but eat their un-hatched siblings as well.  The first pup to hatch will eat all the other developing pups this is called intrauterine cannibalism.

The final, and most advanced, strategy is to form an umbilical cord with the mother (viviparity).  The pups can then absorb nutrients and oxygen from the mother and excrete their waste products.  This process is similar to that employed by mammals.  Bull Sharks (Carcharhinus leucas) and Hammerhead Sharks are species that reproduce in this way.

Birth

When the young shark hatches, or is born, they are miniature versions of the adults and generally have to fend for themselves.  To help their young survive, some sharks give birth or lay their eggs in nursery areas.  The water in nursery areas is usually warm, shallow and has lots of fish for the young shark to feed on.  There are no adult sharks or other major predators in these areas, so the young are able to grow in comparative safety to a bigger size before facing the real underwater world by themselves.

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Why are shark numbers decreasing?

Sharks only reproduce a few young.  The number of offspring varies from two for the Bigeye Thresher (Alopias vulpinus) to 135 for the Blue Shark (Prionace glauca).  The length of pregnancy in sharks is relatively long, averaging between 9-12 months.  The longest pregnancy is 22 months for the Piked Dogfish (Squalus acanthias) (4).  In addition, not all shark species reproduce every year; many have a resting stage of 1-2 years before reproducing again.  Young sharks are slow to grow, and reach maturity late.  For example, Lemon Sharks take 15 years to mature, and Piked Dogfish take 20 years.  Many sharks are killed before maturity by other marine predators or man.  And of the ones that survive, again these only produce a few young that may take another 15-20 years to mature.

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Glossary

Bycatch: in the fishing industry, the part of the catch made up of non-target species.

Cetaceans: a group comprising all whale species; therefore including dolphins and porpoises.

Crustaceans: diverse group of arthropods (a phylum of animals with jointed limbs and a hard chitinous exoskeleton) characterised by the possession of two pairs of antennae, one pair of mandibles (parts of the mouthparts used for handling and processing food) and two pairs of maxillae (appendages used in eating, which are located behind the mandibles). Includes crabs, lobsters, shrimps, slaters, woodlice and barnacles.

Dorsal fin: in fish, the unpaired fin found on the back of the body.

Gestation: the state of being pregnant; the period from conception to birth.

Molluscs: a diverse group of invertebrates, mainly marine, that have one or all of the following; a horny, toothed ribbon in the mouth (the radula), a shell covering the upper surface of the body, and a mantle or mantle cavity with a type of gill. Includes snails, slugs, shellfish, octopuses and squid.

Ovovivipary: method of reproduction whereby the egg shell is weakly formed and young hatch inside the mother; they are nourished by their yolk sac and then born.

Pectoral fins: in fish, the pair of fins that are found one on each side of the body just behind the gills. They are generally used for balancing and braking.

Pelagic: inhabiting the open oceans.

Page 25: The Great White Shark - Essay

Fonti:

www.sharktrust.org The Shark Trust is the UK charity for shark conservation. The Trust works to advance the worldwide conservation of sharks through science, education, influence and action

http://it.wikipedia.org/wiki/Carcharodon_carcharias

http://www.arkive.org/great-white-shark/carcharodon-carcharias/#src=portletV3api

Traduzione di Valentina Zaro