Out of all the species walking, flying, slithering or swimming, there aren't many who have been around as long, survived as well, or come in so many shapes and kinds as the shark. The earliest evidences of sharks are isolated spines, teeth and scales that appeared about 430 million years ago in the Silurian Period, known as the "Age of Fishes". Sharks have a sleek, streamlined design which helps them swim without using up a lot of energy.They certainly need to conserve their energy because they never really sleep and most of them never stop swimming.

Until the 16th century, sharks were known to mariners as "sea dogs". The etymology of the word "shark" is uncertain. One theory is that it derives from the Yucatec Maya word xok, pronounced 'shok'. Evidence for this etymology comes from the OED, which notes the name "shark" first came into use after Sir John Hawkins' sailors exhibited one in London in 1569 and used the word "sharke" to refer to the large sharks of the Caribbean Sea.

An alternate etymology states that the original sense of the word was that of "predator, one who preys on others" from the German Schorck, a variant of Schurke "villain, scoundrel" (cf. card shark, loan shark, etc.), which was later applied to the fish due to its predatory behaviour.

Sharks are a group of fishes characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimorpha (or Selachii), and are the sister group to the rays. However, the term "shark" has also been used for extinct members of the suborder Elasmobranchii outside the Selachimorpha, such as Cladoselache and Xenacanthus. Under this broader definition, the earliest known sharks date from more than 420 million years ago.

Since that time, sharks have diversified into over 400 species. They range in size from the small dwarf lanternshark (Etmopterus perryi), a deep sea species of only 17 centimetres (6.7 in) in length, to the whale shark (Rhincodon typus), the largest fish in the world, which reaches approximately 12 metres (39 ft). Despite its size, the whale shark feeds only on plankton, squid, and small fish by filter feeding. Sharks are found in all seas and are common down to depths of 2,000 metres (6,600 ft). They generally do not live in freshwater although there are a few known exceptions, such as the bull shark and the river shark that can survive in both seawater and freshwater. They breathe through five to seven gill slits. Sharks have a covering of dermal denticles that protects their skin from damage and parasites in addition to improving their fluid dynamics. They also have several sets of replaceable teeth.

Well-known species such as the great white shark, tiger shark, blue shark, mako shark, and the hammerhead shark are apex predators—organisms at the top of their underwater food chain. Their predatory skill fascinates and frightens humans, even though their survival is threatened by human-related activities.

Some sharks are fierce predators, and would be happy to eat you if they encountered you. Almost any shark six feet or longer is a potential danger, but three species have been identified repeatedly in attacks: the Great White Shark, the Tiger Shark and the Bull Shark . All three live world wide, reach large sizes and eat large prey such as marine mammals or sea turtles. But most sharks never grow longer than five feet and never even see anyone with legs and arms anyway. People kill thousands more sharks every year than sharks kill people.

Sharks take about as long to mature as we do. Some of them become adults in their teens. A mother shark carries her babies inside her body while they develop, sometimes for more than a year. Even so, some sharks are born inside an egg which they have to crack open. They spend early portions of their lives in nursery grounds. Some of the advantages sharks have over people is that they keep growing new teeth, they don't have breakable bones, and they are not prone to get cancer. Sometimes sharks are referred to as swimming computers because of the six senses which they possess: vision, hearing, vibration, smell, taste and electro-perception.

The great white shark, Carcharodon carcharias, also known as the great white, white pointer, white shark, or white death, is a large lamniform shark found in coastal surface waters in all major oceans. It is known for its size, with the largest individuals known to have approached or exceeded 6 metres (20 ft) in length, and 2,268 kilograms (5,000 lb) in weight. This shark reaches maturity at around 15 years of age and can have a life span of over 30 years.

The great white shark is arguably the world's largest known extant macropredatory fish and is one of the primary predators of marine mammals. It is also known to prey upon a variety of other marine animals including fish and seabirds. It is the only known surviving species of its genus, Carcharodon, and is ranked first in a list of number of recorded attacks on humans. The IUCN treats the great white shark as vulnerable, while it is included in Appendix II of CITES.

The best selling novel Jaws by Peter Benchley and the subsequent blockbuster film by Steven Spielberg depicted the great white shark as a "ferocious man eater". In reality, humans are not the preferred prey of the great white shark.

TAXONOMY

Sharks belong to the superorder Selachimorpha in the subclass Elasmobranchii in the class Chondrichthyes. The Elasmobranchii also include rays and skates; the Chondrichthyes also include Chimaeras. It is currently thought that the sharks form a polyphyletic group: some sharks are more closely related to rays than they are to some other sharks.

The superorder Selachimorpha is divided into Galea (or Galeomorphii), and Squalea. The Galeans are the Heterodontiformes, Orectolobiformes, Lamniformes, and Carcharhiniformes. Lamnoids and Carcharhinoids are usually placed in one clade, but recent studies show the Lamnoids and Orectoloboids are a clade. Some scientists now think that Heterodontoids may be Squalean. The Squalea is divided into Hexanchoidei and Squalomorpha. The Hexanchoidei includes the Hexanchiformes and Chlamydoselachiformes. The Squalomorpha contains the Squaliformes and the Hypnosqualea. The Hypnosqualea may be invalid. It includes the Squatiniformes, and the Pristorajea, which may also be invalid, but includes the Pristiophoriformes and the Batoidea.

More than 440 species of sharks split across eight orders, listed below in roughly their evolutionary relationship from ancient to modern:-

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  Hexanchiformes: Examples from this group include the cow sharks and frilled shark, which somewhat resembles a marine snake.

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• Squaliformes: This group includes the bramble sharks, dogfish and roughsharks, and prickly shark.

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  Pristiophoriformes: These are the sawsharks, with an elongated, toothed snout that they use for slashing their prey.

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• Squatiniformes: Also known as angel sharks, they are flattened sharks with a strong resemblance to stingrays and skates.

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• Heterodontiformes: They are generally referred to as the bullhead or horn sharks.
  Orectolobiformes: They are commonly referred to as the carpet sharks, including zebra sharks, nurse sharks, wobbegongs and the whale shark.

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• Carcharhiniformes: Commonly known as groundsharks, the species include the blue, tiger, bull, grey reef, blacktip reef, Caribbean reef, blacktail reef, whitetip reef and oceanic whitetip sharks (collectively called the requiem sharks) along with the houndsharks, catsharks and hammerhead sharks. They are distinguished by an elongated snout and a nictitating membrane which protects the eyes during an attack.

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• Lamniformes: They are commonly known as the mackerel sharks. They include the goblin shark, basking shark, megamouth shark, the thresher sharks, shortfin and longfin mako sharks, and great white shark. They are distinguished by their large jaws and ovoviviparous reproduction. The Lamniformes include the extinct megalodon, Carcharodon megalodon.

ANATOMY

Teeth

Shark teeth are embedded in the gums rather than directly affixed to the jaw, and are constantly replaced throughout life. Multiple rows of replacement teeth grow in a groove on the inside of the jaw and steadily move forward in comparison to a conveyor belt; some sharks lose 30,000 or more teeth in their lifetime. The rate of tooth replacement varies from once every 8 to 10 days to several months. In most species, teeth are replaced one at a time as opposed to the simultaneous replacement of an entire row, which is observed in the cookiecutter shark.

Tooth shape depends on the shark's diet: those that feed on mollusks and crustaceans have dense and flattened teeth used for crushing, those that feed on fish have needle-like teeth for gripping, and those that feed on larger prey such as mammals have pointed lower teeth for gripping and triangular upper teeth with serrated edges for cutting. The teeth of plankton-feeders such as the basking shark are small and non-functional.

Skeleton

Shark skeletons are very different from those of bony fish and terrestrial vertebrates. Sharks and other cartilaginous fish (skates and rays) have skeletons made of cartilage and connective tissue. Cartilage is flexible and durable, yet is about half the normal density of bone. This reduces the skeleton’s weight, saving energy. Because sharks do not have rib cages, they can easily be crushed under their own weight on land.

Jaw

Jaws of sharks, like those of rays and skates, are not attached to the cranium. The jaw's surface (in comparison to the shark's vertebrae and gill arches) needs extra support due to its heavy exposure to physical stress and its need for strength. It has a layer of tiny hexagonal plates called "tesserae", which are crystal blocks of calcium salts arranged as a mosaic. This gives these areas much of the same strength found in the bony tissue found in other animals.

Generally sharks have only one layer of tesserae, but the jaws of large specimens, such as the bull shark, tiger shark, and the great white shark, have two to three layers or more, depending on body size. The jaws of a large great white shark may have up to five layers. In the rostrum (snout), the cartilage can be spongy and flexible to absorb the power of impacts.

Fins

Fin skeletons are elongated and supported with soft and un-segmented rays named ceratotrichia, filaments of elastic protein resembling the horny keratin in hair and feathers. Most sharks have eight fins. Sharks can only drift away from objects directly in front of them because their fins do not allow them to move in the tail-first direction.

Dermal denticles

Unlike bony fish, sharks have a complex dermal corset made of flexible collagenous fibers and arranged as a helical network surrounding their body. This works as an outer skeleton, providing attachment for their swimming muscles and thus saving energy. Their dermal teeth give them hydrodynamic advantages as they reduce turbulence when swimming.

Tails

Tails provide thrust, making speed and acceleration dependent on tail shape. Caudal fin shapes vary considerably between shark species, due to their evolution in separate environments. Sharks possess a heterocercal caudal fin in which the dorsal portion is usually noticeably larger than the ventral portion. This is because the shark's vertebral column extends into that dorsal portion, providing a greater surface area for muscle attachment. This allows more efficient locomotion among these negatively buoyant cartilaginous fish. By contrast, most bony fish possess a homocercal caudal fin.

Tiger sharks have a large upper lobe, which allows for slow cruising and sudden bursts of speed. The tiger shark must be able to twist and turn in the water easily when hunting to support its varied diet, whereas the porbeagle shark, which hunts schooling fish such as mackerel and herring, has a large lower lobe to help it keep pace with its fast-swimming prey. Other tail adaptations help sharks catch prey more directly, such as the thresher shark's usage of its powerful, elongated upper lobe to stun fish and squid

SHARK LINKS:

New England Aquarium  http://www.neaq.org/

This is the well-designed Web site of the New England Aquarium, cosponsor with WGBH-TV of "Island of the Sharks," on which this Online Adventure Web site is based.

IUCN Shark Specialist Group  http://www.flmnh.ufl.edu/fish/organizations/ssg/ssg.htm

Learn how the Shark Specialist Group is working to help conserve threatened species of sharks worldwide.

Mote Marine Laboratory: The Center for Shark Research  http://www.mote.org/~rhueter/shark_research.phtml

This nonprofit institution's site offers substantial info on shark diversity, tagging, and attacks as well as descriptions of their research programs on shark vision, feeding, and more.

The Pelagic Shark Research Foundation  http://www.pelagic.org/

This foundation works to develop projects that contribute to a better understanding of sharks. Check out the shark video clips, find out about the sharks of Monterey Bay, and read about shark evolution and anatomy.

 

 

 

 

 

 

 

Shark Awareness Day July 15  http://www.123greetings.com/events/shark_awareness_day/

 

Time to spare a thought for these endangered and rare species. Reach out to all your friends and loved ones to spread awareness through our ecards.

Send Free Online Greeting Cards to your friends & loved ones. ALL cards are absolutely FREE !!!

 

 

Shark Myths  http://www.marinelab.sarasota.fl.us/~rhueter/sharks/myths.phtml

This site, also created by Mote Marine Laboratory, debunks common shark myths with the help of witty cartoons.

Costa Rica!  http://www.costarica.com/

The official Web site for the country, this rich site offers detailed information on everything from adventure travel to business opportunities. Click here for the Embassy of Costa Rica as well.

The Undersea Hunter  http://www.underseahunter.com/

This thorough site gives you everything you'd ever want to know about the Undersea Hunter, the live-aboard dive boat on which this expedition is based at Cocos Island.

The Cocos Island Research Center  http://www.istmo.com/cocos/index.html
In this quirkily fun site, delve into the island's unique history, including its recent designation as a UNESCO World Heritage site, and explore scads of useful links. A description of Cocos Island as a tourist destination. Offers a map as well as a link to an article on scuba diving at Cocos Island.

Secrets of the Ocean Realm  http://www.pbs.org/oceanrealm

This Web site, which accompanied the acclaimed PBS series of the same name, features an interview with Howard and Michele Hall on the challenges of underwater filmmaking.

Cocos Island | Sharkmasters | World of Sharks | Dispatches
E-mail | Resources | Site Map | Sharks Home

The Shark Research Institute Web Site

The Shark Research Institute (SRI), a multi-disciplinary non-profit 501(c)(3) scientific research organization, was created to sponsor and conduct research on sharks and promote the conservation of sharks. Founded in 1991 at Princeton, New Jersey, USA, SRI has field offices in Canada, the Galapagos Islands, Honduras, Mexico, South Africa and the Seychelles. A new data collecting site has been established in Australia.

SRI works with the scientific community, individuals and organizations concerned about the health of our marine ecosystem, and marine resource users: subsistence fishermen, sport divers, and the dive tourism industry. SRI works to correct misperceptions about sharks and stop the slaughter of 100 million sharks annually. A primary goal is creating value for sharks as sustainable natural resources for the dive tourism industry, particularly in developing countries. By so doing, a steady revenue stream is also generated for local fishers that might otherwise slaughter the sharks for immediate gain. Current programs involve visual and satellite tracking, behavioral and DNA studies of sharks, environmental advocacy, publications and public education.

Island of the Sharks Site Map

Scientific Classification
Habitat and Distribution
Physical Characteristics
Senses
Behavior
Diet and Eating Habits
Reproduction

Anatomy and Physiology
Hydrodynamics
Longevity and Causes of Death
Appendix: Classification
Bibliography
Books for Young Readers

Adventure to Cocos Island

Dispatches
View the Undersea World
Legends and Lore
Explore the Island

Dispatches

"This is Cocos, This is Cool" (Oct. 19)
Taken by Surprise (Oct. 17)
Of Booby And Beebe (Oct. 15)
Courtship of the Marbled Rays (Oct. 13)
The Search for Lake Cocos (Oct. 11)
The Magnificent Seven (Oct. 9)
Swimming with Whitetip Reef Sharks (Oct. 7)
The Director's Cut (Oct. 5)
Assault on Cocos (Oct. 3)
Hammerheads Sighted (Oct. 1)
The PIG and the Process (Sept. 29)
Nature Reigns at Cocos (Sept. 27)
Get Used To It (Sept. 25)
Hammerheads or Bust (September 23)

Sharkmasters

Dr. Hammerhead
Exploding Myths
Howard & Michele's Excellent Adventure
Ask the Expert

Questions and responses posted: October 16, 1998
October 8, 1998
October 4, 1998
October 2, 1998
September 25, 1998
September 23, 1998

World of Sharks

Who's Who of Sharks
Close Encounters
Clickable Shark
The Hunt (Hot Science)
Shark Bytes
Shark Attack! Teacher's Guide

Resources

Links
Books

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