Shark

Large white shark; source : [1]
Sharks can be characterized as large, marine predators with a cartilaginous skeleton, multiple (usually five) gill slits along the sides or bottom of the head, dermal denticles covering the body, and rows of replaceable teeth in the mouth. There are exceptions to the "large", "marine" and "predatory" portions of the characterization. Sharks include everything from a hand sized deep sea species, to the whale shark, the largest 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 can move up into freshwater lakes and a few shark attacks have occurred in 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.

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".

Table of contents
1 Classification
2 Reproduction
3 Shark attacks
4 Shark senses
5 Shark fishery
6 Sharks in mythology
7 Related articles
8 External links

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. 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 7 inches in length) and suggested a fish that could grow 120 feet long. The jaw was realized to be inaccurate, and estimates revised downwards to around 50 feet.

Reproduction

Sharks can be easily sexed. 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 6" in size on a fish that reaches 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 - 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.

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 yards 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 7 1/2 foot 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 maco, silky shark, great hammerhead shark, grey reef shark and a few more, this list does not inclue the e.g. nurse shark that often bites humans after beeing disturbed, it would never attack on its own.

It is often said that sharks do not like the taste of humans. This belief has come about because in most cases, once a shark has made its first strike it then leaves the victim alone. 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.

Shark senses

Sharks have two senses that many animals do not have:

Shark fishery

Sharks are fished commercially and recreationally. Some are fished simply for the sport of landing a good fighting fish (mako sharks for instance), others for food (blacktip, mako and others), and some species for other products. In the past, sharkskin (covered in effect with tiny teeth - dermal denticles) was used for the purposes that sandpaper currently is. 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:

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.

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