Shark

Sharks are a group of fish with a full cartilaginous skeleton, a streamlined body plan with 5, up to 7gill slits along the sides (most often) or side of the head (the first modified slit is behind the eye and called a spiracle), dermal denticles covering the body to protect from parasites, and rows of replaceable teeth in the mouth.

Sharks have keen olfactory senses with abilities to smell one part blood in one million parts seawater. Some species have even external barbels (Nurse Shark) that aid even more in sensing prey. Shark eyes have similar parts of their eyes that other vertebrates have like a cornea, lens, and retina. So their eyesight is well adapted to their marine environment. There are some sharks with green eyes that even adapted them so well they are able to see in dark. They have a nictitating membrane to protect the eye during predation. The sharks rely more on their superior sense of smell to find prey, however. Once the shark is in the general area of the prey, then the shark begins to use a combination of its smell coupled with the use of the lateral lines running along the side of the shark. The lateral lines are used to sense electrical pulses that are sent out by wounded or dying fish. Their teeth are not attached to the jaw, but embedded in their flesh. The lower teeth are primarily used for holding the prey, while the top are used for cutting into it.(Gilbertson, 7.3)

There are exceptions to the "large", "marine" and "predatory" portions of the characterization. Sharks include everything from a hand sized pygmy shark a deep sea species, to the whale shark, the largest fish (although sharks are not closely related to bony fish) which is believed to grow to a maximum length of 18m (59 feet) and which, like the great whales, feeds only on plankton. The bull shark is a unique species in that it can swim in both salt water ocean and fresh water rivers. A few of the larger species, the Mako and White shark, are mildly homeothermic, able to maintain their body temperature at a level above the ocean's temperature.

Large white shark; source: www.photolib.noaa.gov

Table of contents

6 Surviving shark attacks

7 Shark senses

8 Shark fishery

9 Sharks in mythology

10 Related articles

11 External links

The name

Until the late 16th century sharks were usually referred to in the English language as sea-dogs. The name "Shark" first came into use around the late 1560s to refer to the large sharks of the Caribbean Sea, and later to all sharks in general. The name may derive from the Maya language word for shark, xoc, pronounced "shock" or "shawk".

References

Gilbertson, Lance (1999). Zoology Laboratory Manual. New York, McGraw-Hill Companies, Inc.

Classification

Sharks are a member of Class Chondrichthyes which includes the rays, skates, and Chimaeras. There are 368 recognized species of sharks.

The first sharks appeared in the oceans 400 to 350 million years ago. Most of the species we know today are as old as the Jurassic period. There are eight orders of sharks, listed below in roughly their evolutionary relationship from more primitive to more modern species:

Hexanchiformes: Examples from this group include the cow sharks, frilled shark and even a shark that looks on first inspection to be a marine snake.
Squaliformes: Examples from this group include the bramble sharks, dogfishes and roughsharks.
Pristiophoriformes: These are the sawsharks, with an elongate, toothed snout that they use for slashing the fishes that they then eat.
Squatiniformes: Angel sharks.
Heterodontiformes: They are commonly 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 largest of all fishes, the whale shark.
Carcharhiniformes: They are commonly referred to as the groundsharks, and some of the species include the blue, tiger, bull, 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.
Lamniformes: They are commonly referred to as the mackerel sharks. They include the goblin shark, basking shark, megamouth, the threshers, mako shark and great white shark. They are distinguished by their large jaws and ovoviviparous reproduction.

The Lamniformes contains the extinct Megalodon (Carcharodon megalodon), which like all extinct sharks is only known from its teeth (the only bone found in these cartilaginous fishes, and therefore the only fossils produced). A reproduction of the jaw was based on some of the largest teeth (up to almost 17 cm (7 inches) in length) and suggested a fish that could grow 15 m (50 feet) long. The jaw was realized to be inaccurate, and estimates revised downwards to around 6 m (20 feet).

Reproduction

The sex of a shark can be easily determined. The males all have their pelvic fins modified into a pair of claspers. The name is somewhat misleading as they are not used to hold on to the female, but are the shark's version of the mammalian penis. (As a side note, Class Chondrichthyes has the distinction of having the animal with the largest intromittent organ - an organ used for transmitting sperm - in relation to body length. This animal is the clearnose skate (Raja eglanteria) which has claspers of 15 cm (6 in) in size on a fish that reaches 1 m (3 feet) in length.)

Mating has rarely been observed in sharks. The smaller catsharks often mate with the male curling around the female. In the less flexible species the two sharks swim parallel to each other while the male inserts the clasper into the female's oviduct. Many females in the larger species have bite marks that appear to be a result of a male grasping her to maintain position.

Sharks have a much different reproductive strategy than most fishes. Instead of producing huge numbers of eggs and larvae (99.9% of which never reach sexual maturity in fishes that use this strategy) sharks normally produce around a dozen pups, some species up to 70-80 and some as few as 2-3. These pups are either protected by egg cases or born live. No known sharks provide parental protection for their young, but females have a hormone that is released into their blood during the pupping season that apparently keeps them from feeding.

There are three ways in which shark pups are born:

Oviparity - Some sharks lay eggs. In most of these species, the developing embryo is protected by an egg case with the consistency of leather. Some of these cases are corkscrewed into crevices for protection. Oviparous sharks include the horn shark and the swell shark.
Viviparity - These sharks actually maintain a placental link to the developing young, more analogous to mammals than other fishes. The young are born alive and fully functional. Hammerheads, the requiem sharks (like the bull and tiger sharks), the basking shark and the smooth dogfishes fall into this category. The blue shark produces the most young of sharks that have had the number of pups recorded, the maximum reported being 82.
Ovoviviparity - Most sharks utilize this method. The young are nourished by the yolk of their egg and by fluids secreted by glands in the walls of the oviduct. The eggs hatch within the oviduct, and the young continue to be nourished by the remnants of the yolk and the oviduct's fluids. As in viviparity, the young are born alive and fully functional. Sometimes they are functional even before being born, as some species practice oophagy, where the first to hatch eat the remaining eggs in the oviduct. Sand tigers, makos, threshers, porbeagles and possibly great whites have oophagous young. The survival strategy for the species that do this is that the young are able to grow to an even larger size before being born. The whale shark is now considered to be in this category after having been classified as oviparous for a long time. Whale shark eggs found are now thought to have been aborted.

Shark attacks

The danger of a shark attack has been sensationalized by the media. A person is more likely to be struck by lightning than attacked by a shark. [1] There are, on average, 100 shark attacks per year, with 5 to 15 of them being fatal. Many attacks are the result of the following factors:

Harassment by humans - Cases have occurred when individuals, teenagers in particular, sometimes show off to their peers by grabbing the tails of slow moving, generally placid nurse sharks. Usually the sharks will simply move away from the annoyance, but there have been cases of them turning and attacking the perpetrator.
Mistaken identity (see "Hit and Run" below) - The shark that can cause the most damage in an attack is the great white. While it has attacked swimmers, this usually occurs in murky waters. Most often, the attacks are made on bodysurfers. From below, the silhouette of a surfer on a board looks very much like the shark's preferred prey - a seal.

There are 3 generally recognised shark attack types, Hit and run, Bump and Bite and Sneak.

Hit and run attacks are the most common. They typically occur in the surf zone. The victim seldom sees the shark and it inflicts one bite or slash. Thought to be mistaken identity due to poor visibility. Wounds are usually lacerations to the leg and are seldom life threatening.
Bump and Bite attacks result in more deaths. The attacks typically occur in deep water where the shark harasses or bumps the victim prior to the actual attack.
Sneak attacks occur without warning. Repeat attacks are common and the injuries severe, often resulting in death. It is believed that these are the result of feeding or antagonistic behavior rather than mistaken identity.

While the great white is the shark that most people immediately think of when shark attacks are mentioned, the bull shark may be responsible for the most attacks on humans. Part of the reason is that they often move up rivers for short distances. One of the most famous shark attack cases occurred in New Jersey along the coast and upriver. From July 1st to July 6th, 1916, five people were attacked by sharks, four of them fatally. The climax came on the 6th when an 11 year old boy named Lester Stillwell was attacked and pulled under. A 24 year old man named Stanley Fisher was one of those who dove into the water to try to rescue Lester. Fisher was bitten on the thigh and died in the hospital. A mere 400 m away, a group of boys were told of the attacks and were in the process of climbing out of the water. Twelve year old Joseph Dunn was bitten on the leg but recovered fully. A 2,25 m (7 1/2") great white was captured in the ensuing shark hunt and the stomach contained flesh and bones which were reported to be human, but a positive identification of this was never made. The attacks did stop after this, but many ichthyologists believe the shark that made the attacks was a bull shark.

In addition to the great white shark and bull shark, the only other sharks proven to have killed humans are the oceanic white tipped shark and the tiger shark, implicated in attacks most often in the tropical Pacific, including Hawaii.

There are about 10 other species that have attacked humans and bitten them but not been proven to have killed, this includes the mako, silky shark, great hammerhead shark, grey reef shark and a few more, this list does not include, for example, the nurse shark that often bites humans after being disturbed, it would never attack on its own.

It is often said that sharks do not like the taste of humans, perhaps because most sharks eat a diet of oily fish (which larger sharks occasionally supplement with blubbery sea mammals), making human flesh taste unappealingly "low calorie" by comparison. This belief has come about because in most cases, once a shark has made its first strike it then leaves the victim alone, a typical "Bump and Bite" attack. There is another possibility, at least in the case of sharks that normally prey on seals and sea lions. The most vulnerable portion of a shark that an attacked animal can reach is the eye. While a nictitating membrane can slide over the eye to protect it, the eye is still vulnerable to the sharp claws of the usual prey. Therefore, the shark attacks and waits for the prey to weaken from loss of blood before coming back to finish off the victim. Even with the above hypothesis, humans are obviously not the preferred prey of sharks, given the evidence.

Surviving shark attacks

If a shark is swimming near you, immediately move into shallow waters to limit its mobility. If a shark bites you, beat and claw its eyes, nose, and gills. If you damage these sensitive regions it will most likely release you and retreat. If available, surfboards or other flotation devices can be shoved into a shark’s mouth to confuse or distract it. If you are wounded, immediately get out of the water as the blood will attract more sharks quickly. If for some reason a shark has to be pulled ashore (perhaps to retrieve a severed limb) grasp it around the tail and pull it ashore backwards (most of the shark’s strength is derived from its tail motions, so grabbing it there makes it much weaker.) If you are forced to grapple with a shark it this manner, be wary of its skin, which has the texture of sandpaper and can tear flesh from your body as easily as its teeth.

The best way to survive a shark attack is to avoid one. Do not swim when you are bleeding or in areas that have high population densities of aggressive sharks.

Shark senses

Sharks have two senses that many animals do not have:

Ampullae of Lorenzini - These small pits in the head detect electricity. The shark has the greatest electricity sensitivity known in all animals. This sense is used to find prey hidden in sand in bottom feeding sharks, by detecting the nerve impulses. It is this sense that sometimes confuses a shark into attacking a boat, when the metal interacts with the salt water.
Lateral line - This system is found in most fishes, including sharks. It is used to detect motion or 'sound' in the water. The shark uses this to detect other organisms moving, especially wounded fish. The shark can 'hear' frequencies in the range of 25 to 50 Hz using this sense.

Shark fishery

Each year, 100 million sharks are killed by people because they are fished commercially and recreationally. In the past they were fished simply for the sport of landing a good fighting fish (mako sharks for instance), In the past, sharkskin (covered in effect with tiny teeth - dermal denticles) was used for the purposes that sandpaper currently is, others for food (Atlantic thresher, mako and others), and some species for other products. Though the number one killer is in the making of Shark Fin Soup millions of sharks are brutally murdered for their fins that are cut off with a hot metal blade then the live animal is tossed back into the water to perhaps die. Their have been cases where hundred of animals were swept up on local beaches live and kicking without anyway to pull themselves back into the sea to rot. Conservationists have been doing their best to pass laws to make finning illegal in the U.S., but the people just don't care. Sharks generally reach sexual maturity slowly and produce very few offspring in comparison to other fishes that are harvested. This has caused concern among biologists regarding the increase in effort applied to catching sharks over time, and many species are considered to be threatened.

Sharks in mythology

Sharks figure prominently in the Hawaiian mythology. There are stories of shark men who have shark jaws on their back. They could change form between shark and human at any time desired, and for any length. A common theme in the stories was that the shark men would warn beach goers that sharks were in the waters. The beach goers would laugh and ignore the warnings and go swimming, subsequently being eaten by the same shark man who warned them not to enter the water.

Hawaiian mythology also contained many shark gods. They believed that sharks were guardians of the sea, and called them Aumakua. A listing of them follows:

Kamohoali'i - The most well known and revered of the shark gods, he was the older and favored brother of Pele, and helped and journeyed with her to Hawaii. He was able to take on all human and fish forms. A summit cliff on the crater of Kilauea is considered to be one of his most sacred spots. At one point he had a he'iau (temple or shrine) dedicated to him on every piece of land that jutted into the ocean on the island of Moloka'i.
Ka'ahupahau - This goddess was born human, with her defining characteristic being her red hair. She was later transformed into shark form and was believed to protect the people who lived on O'ahu from sharks. She was also believed to live near Pearl Harbor.
Kaholia Kane - This was the shark god of the ali'i Kalaniopu'u and he was believed to live in a cave at Puhi, Kaua'i.
Kane'apua - Most commonly, he was the brother of Pele and Kamohoali'i. He was a trickster god who performed many heroic feats, including the calming of two legendary colliding hills that destroyed canoes trying to pass between.
Kawelomahamahai'a - Another human, he was transformed into a shark.
Keali'ikau 'o Ka'u - He was the cousin of Pele and son of Kua. He was called the protector of the Ka'u people. He had an affair with a human girl, who gave birth to a helpful green shark.
Kua - This was the main shark god of the people of Ka'u, and believed to be their ancestor.
Kuhaimoana - He was the brother of Pele and lived in the Ka'ula islet. He was said to be 30 fathoms long and was the husband of Ka'ahupahau.
Kauhuhu - He was a fierce king shark that lived in a cave in Kipahulu on the island of Maui. He sometimes moved to another cave on the windward side of island of Moloka'i.
Kane-i-kokala - A kind shark god that saved shipwrecked people by taking them to shore. The people who worshipped him feared to eat, touch or cross the smoke of the kokala, his sacred fish.

In other Pacific Ocean cultures, Dakuwanga was a shark god who was the eater of lost souls.

In ancient Greece, shark flesh was forbidden to be eaten at women's festivals.

In Greek mythology, Cerberus saved Delia from the stomach of a shark, fell in love with her and became her protector.