Geography 316:  Biogeography

The Biogeography of The Great White Shark, a.k.a. Tommy (Carcharodon  carcharias)

by Paul Glendening, student in Geography 316

 
        The great white shark, Carcharodon carcharias, is one of the most feared and least understood animals known to humankind.  Also known as the white shark, blue pointer, white pointer, uptail, white death and tommy, few other creatures have captured our imagination to such depths as this mysterious, ancient and awesome predator of the sea.  It is simultaneously loved, hated, feared, respected, and exploited by humans.  Regardless of the actual or perceived threat that it poses to us, however, the fact remains that we are much more dangerous to it than it is to us (Ellis, 1991).  The white shark inhabited the seas long before the first ancestors of humans were roaming the land.  They evolved in the ocean, we on the land.  When we enter their territory we are guests in an alien world and should act accordingly.  Marine biologist Eugenie Clark says of sharks in general, “These animals are not the vicious, terrible things that people have built them up to be.  We’re invaders in their ecological environment and if we behave, they’ll behave too.” (Noxon, 1996).  But, being children on the evolutionary scene, we are not behaving.
        Bad publicity and a greatly expanded market for their parts has led to widespread, indiscriminate slaughter of millions of sharks of all species: for shark-fin soup ($50 per bowl in Hong Kong), leather, jewelry of their teeth, aphrodisiacs, sport and perceived “revenge”.  Despite the Jaws reputation and the media’s unavoidable sensationalization of every white shark attack that occurs, the truth is that they are not out to get us.  In fact, we are far more dangerous to them.  Shark expert Leonard J.V. Compagno states in his 1984 book Sharks of the World that in California ten to twenty or more white sharks are killed each year, versus 0.13 humans killed each year by white sharks.  This number includes only sharks killed as by-catch in fisheries; sport fishing of the species is not included in the count (Ellis, 1991).
        So what do we do?  Do we continue killing this ancient and graceful creature out of fear, mistrust and ignorance?  Or do we make an attempt to understand and appreciate the white shark’s place in the oceans and the web of life and, in so doing, possibly come to understand something about ourselves?  I propose the latter.
 

 
 
 Taxonomy

Kingdom:  Animalia
Phylum:  Chordata
Class:  Chondrichthyes
Order:  Lamniformes
Genus:  Carcharodon
Species: Carcharodon  carcharias

 

Range & Habitat

        The range of the white shark has proven difficult for biologists to determine with precision. Carcharodon carcharias has been sighted in all of the world’s oceans from tropical to cool temperate waters, and from shallow coastal areas to the deep sea (Lineaweaver, 1984).  But there are a few areas where it is much more commonly sighted than anywhere - the temperate waters off of South Africa, south and west Australia, and both coasts of North America.  Limited sightings have been recorded in the Mediterranean and Caribbean Seas, off the Hawaiian Islands, near The Seychelles in the Indian Ocean and near Bikini Atoll in the Marshall Islands of the South Pacific.  It is believed that they generally inhabit the deeper waters of continental shelves, frequently venturing into the shallows to feed (Gold, 1989).

 

        There is evidence that the great white migrates with the seasons, not north and south like many species, but from deeper waters during winter to coastal waters for the summer when seals and sea lions, a favorite food, are most abundant (Dingerkus, 1985).  Based on the sparse data available, it appears that the range of the white shark also varies depending upon the age, size and gender of individuals.  Males and larger individuals are more commonly found in the tropical waters of the overall range, while the females and young tend to occupy the temperate and cooler regions.  One hypothesis on this separation speculates that the females bear their young in rookeries in the temperate waters, though the location of any such area is unknown (Cailliet, 1990).
        The overall range of C. carcharias suggests a continuous cosmopolitan distribution.  But the distinctively higher concentrations in a few select regions provide persuasive evidence of a disjunct stenotopic pattern.  Perhaps this seemingly contradictory evidence is a result of the white shark’s high mobility, which would allow individuals to travel across vast expanses of ocean from one favorable region to another, or to vacate favored regions in times of food scarcity in search of better hunting grounds.

 

Abundance

        Compared to many other shark species the great white is relatively uncommon, though there is no reliable count or even an estimate of their numbers (Ellis, 1991).  During the two-year period 1985-86 the National Marine Fisheries Cooperative Shark Tagging Program tagged a total of 11,863 sharks of forty different species off the east coast of the United States.  Of this total only one was a white shark.  Of 51,379 sharks tagged from 1963 to 1983, only 59 were whites.  The first tag return from a white shark was in 1984.  The shark had been tagged off Long Island, New York and was recaptured off the coast of South Carolina 2.5 years later (Gold, 1989).  Similarly, of every 100,000 sharks caught in the Florida shark fishery only twenty-seven are great whites.  This number could be slightly higher if some of the individuals hooked did not escape their would-be captors by breaking the chains and 3,800-lb. wire ropes used to haul them in (Lineaweaver, 1984).

 

Food & Energy Use

        Carcharodon carcharias is legendary as one of the most efficient predators on Earth.  Few animals inspire as much awe, fear and admiration in humans than this ultimate hunter of the sea.  In his 1868 book A History of the Fishes of the British Islands, Jonathan Couch wrote:

                                                    The White Shark is to sailors the most formidable of all
                                                    the inhabitants of the ocean; for in none besides are the
                                                    powers of inflicting injury so equally combined with
                                                    eagerness to accomplish it. (Lineaweaver, 1984)

Despite its reputation as a man-eater, however, the white shark would prefer a wide variety of other dishes before settling for a paltry meal of skinny, bony human.  Which may explain why, if they do attack a human, they will usually release the victim after the first bite.  If a person is killed by a white shark the cause is most commonly blood loss or organ damage (Dingerkus, 1985).  It prefers more substantial fatty and oily meals such as seals, sea lions, porpoises, menhaden, tuna and other sharks - including hammerheads, a formidable fish in its own right.  Younger white sharks smaller than 3m feed primarily on fish and have more slender teeth than the adults, who prefer marine mammals.  They will also occasionally feed on sea birds, and carrion, such as dead whales (Gold, 1989).
        In addition to these more obvious types of meals some surprising things have been found in the bellies of captured great whites.  J.L.B. Smith, in his 1950 book The Sea Fishes of Southern Africa described the stomach of one eighteen-footer that contained, “…the foot of a native, a small goat, two pumpkins, a wicker-covered scent bottle, two large fishes quite fresh, a small shark and unidentifiable oddiments” (Lineaweaver, 1984).  Great whites often swallow their prey intact.  There are reliably documented accounts of one caught in Florida in 1939 having within it two whole sandbar sharks six and seven feet long, and of another in Australia containing a whole horse (McCormick 298).  There is no explanation of what a horse was doing in the ocean.
        Like most sharks, the great white has several methods for locating potential prey.  A strong sense of smell allows it to detect blood in the water at concentrations as small as one part per 10 to 100 million (Budker, 1971), and it is well equipped to detect the irregular percussive pulses generated by injured or swimming animals flailing the water (Noxon, 1996).  Once the prey is located, the white shark depends on keen vision to lock on for a surprise attack from behind and below in an accelerated rush that ranges from twenty-five to forty miles per hour.  As it approaches the prey the shark raises its snout and drops its lower jaw.  As the shark gets closer still the upper jaw protrudes forward and the lower jaw moves slightly up and forward, further exposing the massive teeth.  In the final moments of the attack the shark depends on ampullae of Lorenzini, unique sensing organs in its snout, to keep track of the electromagnetic field surrounding the prey.  The eyes of the shark roll backward in its head and are covered by a membrane that protects them from the potentially thrashing victim (Gold, 1989).  Finally, the jaws snap shut and hundreds of 2-to 3-inch razor-sharp teeth grab hold of the prey with a force greater than one ton per square inch, potentially removing, with the ease of a scalpel, a 20- to 30- pound chunk of skin, flesh, and bone (Dingerkus, 1985).

 

 
                                                                                          Photos: Carl Roessler, 1995

 

        White sharks will stuff themselves if the opportunity arises and they are hungry (consider the horse) but it is likely that it does not need to eat constantly or even frequently to survive.  Experiments to calculate its energy intake indicate that it is extremely efficient and can, when well fed, go without a meal for three months or more (Gold, 1989).  Studies of other shark species in captivity show that they can remain in good health on a weekly food intake totaling only 3 to 14 percent of their body weight.  Moreover, a larger, more mature shark with a lower metabolism may take in even less. Eugenie Clark suggests that these figures may be less than a shark would require in its natural habitat where it would probably expend more energy on swimming, but that the shark may also be enjoying more food in captivity than when it has to find its own in the wild (Lineaweaver, 1984).

 

Reproduction

        Very little is known about the mating habits or reproduction of Carcharodon carcharias.  Courtship behavior has never been observed in the wild or in captivity. The white shark is believed to be ovoviviparous - the female incubating the eggs internally until they are ready to hatch – as is the case with the other sharks of the order Lamniformes.  They are also presumed to be oophagous: the developing embryos feeding on unfertilized eggs within the mother and on each other until only the largest and most aggressive remain.  As for the size and number of the white shark pups, one often-questioned report from the Mediterranean described a female with nine embryos two feet long and 108 pounds each.  The reason for the questionability of the account is that free-swimming specimens have been caught at five feet in length and only fifty to sixty pounds in weight.  The only well-documented account of a pregnant female white shark occurred in 1986 when a 4.7m individual with seven near-term embryos, 100 cm to 110 cm (about two feet) long, was captured off Japan (Gold, 1989).

Size, Growth & Age

        Adult white sharks average fifteen feet in length, but they can grow to over twenty feet (Dingerkus, 1985).  The largest white shark whose length was accurately measured and scientifically confirmed was captured in Cuba in 1948; it was twenty-one feet long and weighed 7,300 pounds (Ellis, 1991).  The liver alone accounted for 1,005 pounds of the animal’s total weight (Budker, 1971).  Larger white sharks, quite possibly accurately measured but not scientifically confirmed, have been reported.  In 1987 a twenty-two footer was captured off Kangaroo Island in Australia, and a fisherman hauled in a twenty-three footer near the island of Malta in the Mediterranean in 1988.  Fishermen and sailors have reported sighting individual sharks estimated at twenty-five feet in length.
        Limited studies suggest that the growth rate of white sharks varies with geography.  Those in the Atlantic appear to grow at a rate of approximately 20 cm per year, compared with 25 to 30 cm per year for younger, and 22 cm per year for older individuals in the eastern Pacific.  A 16-foot white shark from the Atlantic is estimated to be twenty years old.  But an individual of the same size in the eastern Pacific is estimated to be only thirteen or fourteen years old.  Studies suggest that the white shark has a life span in the range of twenty-five years (Gold, 1989).

 

Evolution

        Sharks first appeared on Earth approximately 400 million years ago during the Devonian Period – well before dinosaurs, birds or mammals.  The oldest of the ancient sharks are known only from fossilized teeth embedded in rock of the Late Middle Devonian.  One shark, known as Cladoselache, is known from teeth, vertebrae, and rare impressions of skin, kidneys and muscles discovered in shales along the shore of Lake Erie.  Cladoselache was about 6 feet long and similar in appearance to modern sharks, with one major difference being a mouth on the front of the snout rather than beneath.  This ancient shark apparently disappeared by the end of the Carboniferous, 290 million years ago, and was replaced by more advanced species (Gold, 1989).
        From Cladoselache the early sharks produced at least twelve groups of fossil sharks.  This is quite a variety considering that all modern sharks are placed in the one group of Neoselachii, meaning “new sharks”.  These twelve groups have greater differences from one to another than do even the seemingly most unrelated of modern sharks.
The theme of the evolution of sharks since Cladoselache has been one of conservation.  Since they first developed their streamlined, torpedo-like form, sharks have evolved more on the inside than on the outside.  Once the fast-moving, efficient shape of the underwater predator was developed only refinements on the internal workings were necessary.  Changes in the teeth and skeleton have been especially dramatic (Dingerkus, 1985).
        The closest known ancestor to today’s white shark seems to be just a much larger version of the same animal.  Carcharodon megalodon swam the seas some 10 to 30 million years ago and is so closely related to Carcharodon carcharias that it is commonly called “the extinct great white shark”.  It is known only from fossil teeth – commonly found in Florida, Georgia and the Carolinas - that appear nearly identical to those of the modern white shark except in size.  Whereas the modern white shark’s teeth are about three inches long in the largest individuals, Megalodon had razor sharp teeth ten inches long.  This tooth size suggests a monstrous shark up to sixty feet long (Dingerkus, 1985).

        In the final analysis the white shark is a very complex and mysterious animal.  We are slowly but surely learning more about them and their cartilaginous cousins, but there is much more about them that we do not, and may never, understand.  Sharks large and small play a vital role in the oceanic food chain.  For this reason alone it is imperative that we scale back our assault on them.  If we do not we may find ourselves asking, once again, who is the most vicious, soulless, indiscriminate killer on Earth.
 
 

 
 
 

Bibliography

Budker, Paul.  The Life of Sharks.  New York: Columbia University Press, 1971.

Cailliet, Gregor and Ava Ferguson.  Sharks and Rays of the Pacific Coast.  Monterey,
        California: Monterey Bay Aquarium Foundation, 1990.

Dingerkus, Guido.  The Shark Watchers’ Guide.  New York: Julian Messner, 1985.

Ellis, R. and McCosker, John E. “What Fate Awaits the Great White Shark?”.  Audubon
        vol. 93 Sep/Oct 1991:  93:100-8.

Gold, Joy P. and Victor G. Springer.  Sharks in Question; The Smithsonian Answer
        Book.  Washington, D.C.: Smithsonian Institution Press, 1989.

Lineaweaver, Thomas H., III and Richard H. Backus.  The Natural History of Sharks.
        New York: Nick Lyons Books/Schocken Books, 1984.

Noxon, Nicholas.  National Geographic Video: The Sharks.  WQED/Pittsburgh,
        1996.
 
 

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