Shark
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See Classification and Shark taxonomy |
Sharks are a group (superorder Selachimorpha) of fish, with a full cartilaginous skeleton, a streamlined body plan with between 5 and 7 gill 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.
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Characteristics
Sharks have keen olfactory senses with abilities to smell one part blood in one million parts seawater. They are attracted even more by the chemicals found in the gut of many species, and often linger near and in sewage outfalls. Some species have even external barbels (Nurse Shark) that aid even more in sensing prey. Shark eyes are similar to the eyes of other vertebrates, including similar lenses, corneas and retinas, though their eyesight is well adapted to their marine environment. Some sharks have stronger nocturnal adaptations, allowing them to see in dark conditions. Some sharks 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, the shark also uses the lateral lines running along the side of the shark to sense electrical pulses 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 prey, while the top are used for cutting into it. (Gilbertson, 7.3)
There are exceptions to the "large", "marine" (as in 'ocean-going') and "predatory" portions of the characterization. Sharks include everything from the 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 (and in lake Nicaragua). 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.
In many cases, a shark will suffocate if it stops active swimming. Sharks get their oxygen by extracting it from seawater as that passes over their gills. Because of their size and the nature of their metabolism, their demand for oxygen is higher than that of most fish and they cannot rely on ambient water current to provide an adequate supply of oxygenated water. If a shark stops swimming, the necessary water circulation for respiration becomes too low and the animal suffocates. Some sharks, like the blacktip reef shark and nurse shark, can pump water over their gills as they rest. Because sharks have no swim bladders, they sink when they stop swimming; a resting shark sinks to the sea bed.
A shark if inverted enters a natural state of paralysis. This state is called 'tonic' and the shark usually becomes dull and unresponsive for a while. Researchers use this condition for handling sharks safely. The condition is termed tonic immobility.
A shark is immune to all known diseases.
Etymology
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 have been derived from the Mayan word for shark, xoc, pronounced "shock" or "shawk".
The term for a group of sharks is a shiver.
Classification
- See shark taxonomy for the complete classification.
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. 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 elongated, toothed snout that they use for slashing the fishes that they subsequently 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. The bite marks can also come from the courtship of the sharks. The male may come and bite the edges of the female to show his interest. In some species, females have evolved thicker skin to withstand the sharks bite marks during mating.
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.
PortJacksonShark'sEgg20050417c.JPG
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. When they wash up empty on beaches, the egg cases are sometimes called mermaid's purses. Oviparous sharks include the horn shark, catsharks, Port Jackson Sharks, 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. Dogfishes also have the longest known gestation period of any shark, 22 months. 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. Most ovoviviparous sharks generally give birth in sheltered areas, including bays, river mouths, and shallow reefs. They choose such areas mainly because of the protection from predators (mainly other sharks) and the abundance of food.
A slightly modified form of ovoviviparity, oophagy is where the young hatch inside of the uterus within the first three months of gestation and consume the eggs, which the mother makes during the gestational period. Utilized by the lamnoid, great whites, crocodile sharks, makos, sand tigers, threshers, and false catsharks. Even more complex is embryophagy, where the first of the embryo?s to hatch kills its brothers and sisters before consuming the eggs provided for nourishment from its mother.
Shark senses
Sharks have two senses that many animals do not have:
- Electroreception: The Ampullae of Lorenzini are 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 in commercial and recreational fishing. In the past they were fished simply for the sport of landing a good fighting fish (mako sharks for instance). 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.
However, sharks are most often killed for shark fin soup, in which many sharks are hunted for their fins, which are cut off with a hot metal blade before the live animal is tossed back into the water. There have been cases where hundreds of de-finned animals were swept up on local beaches without any way to convey themselves back into the sea. Conservationists have campaigned for changes in the law to make finning illegal in the U.S.
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.
Organizations like the Shark Trust are trying to limit shark fishing.
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