Great White Shark ( Carcharodon carcharias )
Sharks (Selachii, also Selachimorpha) are fish from the class of cartilaginous fish . Over 500 species are known worldwide. The word shark comes from the Dutch haai . This in turn comes from the Icelandic word haki , which means hook and is based on the hook-shaped tail fin of the sharks. Colloquially, the sharks are often called sharks .
Most sharks eat fish and other larger marine animals; the two largest shark species of up to 14 m long and 12 t Walhai and basking and the megamouth , feed mainly on plankton . Although only about five people die each year as a result of shark attacks, the animals are generally considered to be cold-blooded killers and ogre-eaters. Many species of shark are threatened by excessive fishing.
Sharks represent a group of over 500 species, some of which differ considerably in size and appearance. The smallest known species of shark are the dwarf lantern shark ( Etmopterus perryi ) and the cylindrical lantern shark ( E. carteri ) with a body length of only 16 to 20 centimeters and a weight of around 150 grams. The largest species are those up to 14 meters long and the 12-ton whale shark ( Rhincodon typus ) and the basking shark ( Cetorhinus maximus ) up to 10 meters long , both of which feed almost exclusively on plankton. Among the carnivorous and hunting species, the great white shark ( Carcharodon carcharias ) is the largest species with a maximum length of up to 7 meters, while the extinct basking shark megalodon ( Otodus megalodon ) probably reached a total length of 14 to a maximum of 20 meters. About half of all shark species reach a body length of about one meter, in 20 percent of all species it is more than 2 meters.
All shark species have a more or less pronounced spindle-shaped body, which in some ground-dwelling groups - such as the carpet and angel sharks or the sawshark-like - can be strongly flattened dorsoventrally, similar to the rays. The body can be divided into a head, trunk and tail area, each characterized by specific organs and fins. The primary sensory organs are located in the head area : the eyes , the nostrils , the Lorenzini ampoules and the mouth, which is used to take in food. Most shark species have a snout region that is more or less pointed. In some species, such as nurse or bull head sharks , it is not trained. In hammerheads , the head is very much widened sideways, which creates a very large distance between the eyes and nostrils at the respective head ends.
Between the eye and the gill slits is the spray hole , which is used to take in breathing water and is particularly large in soil-dwelling species. The actual gill slits are located at the transition from the head to the trunk. The most primitive sharks, the gray shark-like (Hexanchiformes), to which the collar shark ( Chlamydoselachus anguineus ) and the comb -toothed shark ( Hexanchidae) belong, as well as the six- gill sawshark ( Pliotrema warenni ) have six or seven open gill slits on each side of the body . In all other species of shark there are only five gill slits left.
The trunk area begins with the paired pectoral fins , which usually start in the area or behind the last cleft gill. On the belly side these are followed by the paired pelvic fins , which in the males are connected to the paired clasps , the sharks' copulatory organs. On the back panel there are usually two consecutive and unpaired dorsal fins , which often (especially in the then named dogfish each have a dorsal fin spine is formed). In most shark species, the first dorsal fin is significantly larger than the second, which is mostly in the area of the tail stem and can also be missing in some species. On the ventral side of the caudal peduncle there is also very often an unpaired anal fin . Fast swimming shark species also have laterally formed keels on the tail stalk.
The caudal fin consists of an upper and a lower fin lobes (lobi), whereby the lower lobe is often significantly smaller than the upper one. This is particularly the case with species living on the ground such as the cat sharks , but also with many species of open water, such as hammerhead sharks or, above all, the thresher sharks . The latter have a significantly elongated upper tail globe that is used for hunting. In contrast, in fast- swimming species such as the mako shark or the great white shark , the tail is almost symmetrical.
Skull and Axial Skeleton
The entire skeleton of sharks consists of hyaline cartilage , which only in larger and older animals in some areas of the jaw and vertebrae calcification ( calcifications has). The skull consists of a brain skull ( neurocranium ), which contains the brain , and the facial skull ( viscerocranium ); Cover bones are not developed in recent cartilaginous fish.
The brain skull consists of a uniform cartilage capsule that can be divided into various structural areas. The front area is formed by the rostrum , which forms the shape of the shark's snout. It is followed by the paired nasal capsules , which are open ventrally and connected dorsally via the internasal plate. This is followed by the roof of the cerebral cavity, which lies between the large anterior fontanelle and the posterior parietal fossa , the floor or basal plate of the brain capsule between the internasal plate and the occiput, and the two eye sockets (orbits). To the rear, the brain skull is closed off by the two ear capsules with the joint surfaces for the hyomandibular and the occipital region with the foramen magnum and the transition to the spine . This connection is rigid and has no movable occipital joint for vertical movement of the head in relation to the trunk.
The two jaws ( mandibular arch ), the hyoid arch and the adjoining gill arches are assigned to the facial skull . The lip cartilage, the gill rays and the extrabranchialia also belong to this head part. The two tooth-bearing jaw parts - dorsally the palatoquadratum as the upper jaw and ventrally the mandibular as the lower jaw - lie below the skull and can be connected to it by individual extensions of the palatoquadratum in the area of the nose and the eye sockets. The individual jaw branches are movably connected to one another via a symphysis . Most sharks have an articulated connection between the jaws and the cranium, indirectly via the hyomandibular in the area of the ear capsules ( hyostyly ), which results in a high degree of mobility of the entire jaw; the individual jaw parts can be lowered and pushed back and forth independently of each other. In the collar shark and many fossil sharks, on the other hand, the upper jaw lies broadly on the skull and is connected to it twice via the palatoquadratum and the hyomandibular ( amphistyly ).
The spine forms the axial skeleton of the sharks. It consists of a varying number of hourglass-shaped vertebrae , the 60 individual vertebrae in Zwergdornhai ( Squaliolus laticaudus ) to 477 in the thresher sharks ( Alopias can range). The number of vertebrae in the trunk spine is between 44 in the dwarf buckthorn shark and around 150, the number of caudal vertebrae can range from 12 in the dwarf buckthorn shark to more than 300 in the greatly elongated caudal fin of the thresher sharks. The number of individual species also varies between different populations, such as the dogfish ( Acanthias vulgaris ), whose Atlantic population has 79 to 85 and the Pacific population only 68 to 76 trunk vertebrae. From around the pelvic girdle, sharks and individual rays show a doubling of the recognizable cartilage elements of the individual vertebral bodies ( diplospondyly ), which increases the mobility of the caudal peduncle and the caudal fin.
Sharks have teeth that grow back, with several rows of teeth growing behind the first row of teeth. In a shark's life that can be up to 30,000 teeth. If a tooth breaks off in the first row, a new tooth moves up, just as a new ball moves up in the ball drum in a revolver (hence the name " revolver bit "). Sharks often lose numerous teeth when attacking seals and fish.
Shark teeth are modified placoid scales that contain a central cavity with the tooth pulp , encased in dentin and a hard surface made of tooth enamel . The shape of the teeth depends on the diet of the species. Species that feed on mussels and crustaceans have flattened teeth for chopping, those that feed on fish have needle-like teeth for gripping. Sharks, which hunt larger prey, have triangular teeth with jagged edges for cutting. The teeth of plankton eaters , such as the basking shark , are severely reduced and inoperable. New teeth are constantly being formed in a corresponding recess in the mouth of the hair. Their roots are only fixed by the skin, so they are more flexible. In this skin, the continuously newly formed teeth advance one position at a time.
The root of the shark tooth consists of the basal lip, a V-shaped indentation between the root lobes. This is adjacent to the Bourrelet, a dental band that forms a triangular area of very thin enamel between the crown and the root. The tooth crown, which can have serrations, is located above the bourrelet.
The rows of teeth are counted along the line of the jaw while the series of teeth are counted from the front of the jaw inward. A single row of teeth includes one or more functional teeth on the front of the jaw and several replacement teeth behind. For example, a bull shark's jaws may contain 50 rows of teeth in a 7-series. The small teeth on the symphysis where the two halves of the jaw meet are usually counted separately from the main teeth on either side.
Shark skin and scaling
While the bony fish have elasmoid or ganoid scales , the sharks have so-called plakoid scales , which start as teeth in the revolver dentition and spread from the mouth over the entire body. The teeth shrink and are continued as skin teeth, which in the sharks, in contrast to the rays, form an almost complete body covering.
The shark scales are aligned so that the skin feels smooth when you stroke the shark's back from head to tail. Conversely, the skin feels rough. Furthermore, the individual scales have a groove structure that extends seamlessly over the entire shark body. The groove structure (s. G. Riblets ) reduces the surface resistance. The grooves create many small water eddies. These reduce the laterally directed forces of the turbulent flow and reduce the effect of friction. However, this effect only works when swimming fast; when swimming slowly, it is better to have smooth surfaces. In addition to the lotus effect , the shark skin effect is one of the best-known findings in bionics . The effect is used to reduce the frictional resistance and thus the fuel consumption for ships, submarines and aircraft.
A shark's eyes are ten times more sensitive to light than a human, and most shark species are likely color-blind. He can look in almost all directions through the eyes on the sides of the head. When attacked, sharks protect their eyes either with a nictitating membrane that slides in front of the eye like an eyelid , or by rolling their eyes backwards. The number of rods (black and white vision) predominates, which increases the ability to see in poor light conditions. A further adaptation to the twilight is achieved by the tapetum lucidum . This consists of a layer of tiny mirror-like crystals. Since it lies behind the retina, it reflects the light that has passed through the retina back onto the receptors.
The shark's olfactory organs are located at the front of the snout. The olfactory center can make up up to two thirds of the brain's mass. Sharks can perceive blood in a billion-fold dilution. The shark can sense its prey from a great distance (over 75 m), even if no other sensory stimulus provides information. With constant head movement ("pendulum") he follows this scent trail to the destination. So-called Schneider's folds channel the incoming water in such a way that the nasal pits are constantly washed by water. A nasal pit is divided into two sections by a membrane, one each for incoming and outgoing water. The inner surface is folded, which increases the receptive area. Even a blind shark can find its food with such precision. The taste buds are located in the palate area. When the shark bites, it decides whether prey is edible or whether it is spat out again. Taste buds are also distributed over the entire skin of the shark, except for the eyes and fins; the taste can therefore also be perceived by nudging and rubbing a prey. A common myth is that sharks are attracted to human blood. In fact, sharks seem to immediately recognize that human blood is not prey blood. Therefore, they either show no reaction at all or even swim away from human blood. This has been proven in a large number of tests. Some of them were published on Youtube.
The hearing is easier to develop than in land animals. The ears are located on both sides of the skull, only a small pore is visible from the outside. Sharks react in the range of low-frequency tones, i.e. below about 600 Hz (for example wriggling fish, singing whales, grunting seals, fighting fish) and are highly sensitive to pulsating vibrations in the 100 Hertz range, such as those produced by sick or wounded animals. Sharks can hear lower frequencies than humans. Sound waves from 10 to 800 Hz are already perceived. Hearing is also important for the sense of balance and direction.
The shark's lateral line runs from the head region to the tip of the tail and serves - as with all fish species - to detect the smallest vibrations and pressure differences in the water. It contains sensory cells embedded in jelly. These can register water movements in the immediate vicinity, i.e. pressure differences (caused by prey, opponents or obstacles) along the surface of the body. Sharks can also perceive electrical fields that other living beings generate through heartbeat, muscle movements or brain waves. The sharks' electrical sensors are the Lorenzini ampoules . Sharks are also sensitive to the Earth's magnetic field that they use to navigate. In this way, they can return to specific regions precisely after they have been absent.
Sharks can regulate their salt balance hormonally ( osmoregulation ). As in most mammals, your body cells contain a low concentration of salts . However, they are isoosmolar with the surrounding seawater and belong to the group of osmoconformers . To prevent the penetration of salts from the sea water, they store more urea , trimethylamine oxide (TMAO) and other substances in their cells. The TMAO serves to weaken the harmful effects of urea on the cells. The urea is recovered in the kidneys through active transport. To maintain their low salt concentration, they release salt into the sea water. To do this, they have chloride cells in the rectal glands , which function on the same principle as in the salt glands of sea birds.
Sharks are cartilaginous fish and therefore relatively lighter than bony fish . They have a large, oil-rich liver that gives them some buoyancy . However, they do not have a swim bladder , so deep-sea sharks have to keep moving in order not to sink.
distribution and habitat
The different shark species can be found in all oceans and all marine habitats. They are often spotted near the coast because of the abundance of food there. In addition to shallow coastal waters, they also live in the high seas and in the deep sea . Some species are very specialized in living on the sea floor ( Benthal ), while others, mostly large species, prefer the open water ( Pelagial ). There are also species, such as the river sharks of the genus Glyphis or the bull shark , which occur regularly or exclusively in the brackish and fresh water of river systems or lakes with a connection to the ocean.
Way of life
Unlike most fish species, sharks grow very slowly and sometimes only reach sexual maturity at the age of 30 . Some species then only give birth to a few young every two years, so they have an extremely slow reproductive rate . Mating rituals are swimming, biting, or group circles.
Egg-laying sharks lay their eggs, which are surrounded by a thick shell to protect them from predatory fish, in rocks or seaweed. The embryos that feed on the yolk sac develop inside the eggs . The young hatch after a few days or weeks and are then left on their own in nature. In some species, the eggs can also ripen in an artificial environment and are carried out in several marine aquariums around the world.
Many species of shark are viviparous. The young sharks are first fed in the uterus via the placenta or a secretion called uterine milk. In some viviparous species, such as great white sharks, the young also feed on other embryos ( uterine cannibalism ). The shark cubs are so developed at birth that they are immediately viable.
It has long been known that various shark species regularly come together in groups - so-called schools. A study of Port Jackson bullhead sharks has shown that the animals in these schools not only recognize each other, but also specifically seek the company of certain individuals. National Geographic then headlined: "Sharks have best friends, like us."
In addition to the human who kills most sharks, sharks have other enemies as well. Smaller shark species in particular are regularly hunted by larger fish, rays and larger sharks. Small sharks are also caught by sea birds and seals near the coast.
Larger sharks, on the other hand, are only captured by killer whales and other sharks.
Evolution and systematics
Evolution of the sharks
The first species of shark, such as the Cladoselache , appeared as early as the Devonian geological age around 400 to 350 million years ago . However, sharks may have existed since the Ordovician , as studies from 2012 suggest. Until 1986, other primeval sharks were discovered that lived in the following Carboniferous Age . Many of them had one or more long spines on their heads ( Xenacanthiformes ). The meaning of these spines has not yet been adequately clarified; it is believed that they may have been used for defense.
The Neoselachii appeared in the Lower Jurassic . The largest shark known today was the megalodon , which lived 15 to 1 million years ago. Since the skeleton of sharks consists largely of cartilage, the only fossils found as fossils are mostly teeth and parts of the dorsal fins, which makes it extremely difficult to research the primeval sharks in detail. However, there are also some finds of primeval sharks whose cartilaginous skeleton has been completely preserved under very good conditions.
The cartilaginous fish can be divided into two main groups, which differ greatly in the number of their species. These are the more original sea cats (Holocephali) with about 50 recent species and the Elasmobranchii , which contain the sharks and rays with over 1100 known species. Among the fossil cartilaginous fish, the Cladoselachiformes of the Devonian to Carboniferous are regarded as a sister group of all sharks and rays that exist today, which together with some other extinct groups are summarized as Neoselachii and Euselachii.
The Neoselachii were traditionally divided into sharks and rays according to their external appearance. In 1996, the Neoselachi of de Carvalho and Shirai were independently subdivided into two monophyletic taxa according to morphological characteristics , the Galeomorphii (Galea near Shirai), which mainly include large open water sharks and the Squalea , which includes many bottom-dwelling sharks, deep-sea sharks and the rays belong. The sharks are thus merely a paraphyletic form taxon.
In the meantime, however, there are several molecular biological studies that confirm a basic dichotomy between sharks and rays. The morphological correspondences of the squalomorphic sharks with the rays then developed convergent . Since the rays, just like the modern sharks, can be traced back to the fossil record since the early Jurassic , an ancestry of the rays at the end of a long evolutionary line of the Squalea is not supported by paleontological data.
One version of each of the two different concepts of the internal systematics of Neoselachii is presented in the following tables.
Relationship with people
Endangerment of humans from sharks
In the temperate zone, the possibility that a shark could attack and kill a human unprovoked was considered unlikely until the beginning of the 20th century. It was known that sharks occasionally kill people in tropical waters. However, it was thought to be abnormal events that occurred as fishing accidents. This attitude only changed with the shark attacks on the New Jersey coast in 1916 . Five people were attacked by sharks between July 1 and July 16. Four of the victims were killed. The events formed the basis for the 1974 novel Jaws by Peter Benchley , which was filmed in 1975 by Steven Spielberg under the title Jaws . Both the events of 1916 and their literary and cinematic processing strongly influenced public opinion regarding sharks. Attacks by and accidents with sharks on humans are nowadays carried out by various organizations such as: B. the International Shark Attack File or the Global Shark Attack File is recorded, reconstructed and analyzed. Data from shark accidents are e.g. B. available on the Internet through the Shark Accident Victim Network .
All sharks that can be dangerous to humans belong to the superior order of the real sharks (Galeomorphii). The danger of shark attacks is often exaggerated; it is 47 times more likely to be killed by lightning than by a shark. An average of 100 shark attacks are recorded in a year; about five to ten of them are fatal. Many of these attacks are the result of:
- Human Harassment: There are instances when individuals, mostly teenagers, want to impress their peers by trying to grab the fin of a slow nurse shark . These sharks usually go away, but there have been cases where they turned around and attacked the affected person. Bottom-dwelling Port Jackson bullhead sharks are known for bite injuries when bathers step on them.
- Confusion: The shark considers the target to be prey, for example a seal. A great white shark ( Carcharodon carcharias ) can do the most damage in such an attack.
- Curiosity shark attacks on Surfer ( surfer ) be attributed, inter alia, that the surfboard causes noise, which animate the Hai to a "sample bite". Often, however, great white sharks simply observe swimmers and divers without attacking, and attacks often consist of lightly grasping and holding on as opposed to the powerful killing bites used against prey. The behavior towards people was therefore also interpreted as an investigation out of curiosity or agonistic behavior .
Although many people almost automatically think of a great white shark when it comes to more serious shark attacks, in reality the bull shark ( Carcharhinus leucas ) is also responsible for many attacks. One of the reasons for this is that these sharks often swim up the rivers and linger there for weeks (Amazon, Lake Nicaragua, Zambezi).
In addition to the great white shark and bull shark, white tip deep sea sharks ( Carcharhinus longimanus ) and tiger sharks ( Galeocerdo cuvieri ) have been shown to have killed humans. Most of the attacks occurred in the Pacific .
Another ten species have already bitten humans, but with no fatal outcome. These species include the mako shark ( Isurus spec. ), The silk shark ( Carcharhinus falciformis ), the lemon shark ( Negaprion brevirostris ) and the hammerhead shark ( Carcharhinus falciformis ).
It is often said that sharks do not like human flesh. This claim arises from the fact that sharks often only bite once and then disappear again. This behavior can also be explained differently. When a shark attacks a sea lion or seal , the eyes represent the most vulnerable parts of a shark's body that an attacked animal can still reach. To protect itself from injuries that can arise from the sharp claws of the fighting animal, the shark disappears for a short time. He waits until his victim has lost enough blood to attack again in the weakened state. People who have been bitten are often rescued during this waiting period: After the first shark attack, other water sports enthusiasts or fishermen take the bitten person out of the water. So the shark cannot attack again. However, even assuming this theory, it is believed that humans are not preferred prey for sharks.
Another theory is based on the fact that these so-called test bites often only result in a small flesh wound. According to new findings in shark research, sharks are intelligent animals with complex social behavior. This is particularly clear with the gray reef shark ( Carcharhinus amblyrhynchos ), whose warning behavior - swimming with lowered pectoral fins - has often been ignored by humans, which has resulted in accidents. It is conceivable that other sharks show such warning behavior and that large competitors (sharks, small whales , humans, etc.) are attacked with bites in order to drive them away. This would explain the fact that some shark attacks often only result in small flesh wounds.
The increasing number of shark attacks off Recife since 1992 have shown that anthropogenic environmental changes can change the hunting behavior of sharks.
In 2012, the prominent bodyboard surfer from South Africa, David Lilienfeld , was killed by a shark attack. His case attracted worldwide attention and shows the endangerment of surfers by sharks.
In 2015, a surfing competition was canceled due to a shark attack. Mick Fanning had fought against the swimming shark and was unharmed.
Shark endangerment from humans
According to the FAO, around 700,000 to 800,000 t of cartilaginous fish are caught every year, a large part of which is by-catch by large industrial fleets. This amount corresponds to 70 to 100 million individual animals, about 60% of which are sharks and 40% rays. According to Compagno et al. In 2005, however, these numbers are clearly too low, as a large number of states report too low catch numbers; he assumes at least twice the amount.
The economic use of the shark has ultimately meant that over 70 species are already threatened with extinction . Most of these species are already designated as no longer viable. In the Mediterranean , more than half of the shark species are threatened with extinction.
Hunting and fishing methods
Shark hunting was and is still a popular sport . The prepared shark bite or the whole head as a trophy are popular . Sharks are mostly hunted for their fins, which are used to make Chinese shark fin soup ; this has the shark finning (including shark finning ) developed a particularly cruel type of hunting: the shark fins are removed alive and the animal is then transported back into the sea where it is destroyed. Many animals perish on the longline before they reach sexual maturity. Shark cartilage is used as a dietary supplement for chronic joint diseases.
The shark as a food fish
The meat of the shark is often used in various cuisines around the world. In Asia, for example, the shark fin is often processed: to make shark fin soup or, in China , as a whole. But sharks are also used as food in other countries: The Inuit in Greenland dry the meat of the Greenland shark or ferment it into a regional delicacy called “tipnuk” . The Icelandic dish Hákarl also consists of fermented Greenland Shark.
Since the shark is at the top of the food chain , methylmercury ( MeHg ) builds up in the shark meat . Therefore, even the consumption of relatively small amounts of shark meat is sufficient to exceed harmless amounts of MeHg, so that toxic exposure levels can easily be reached.
Sharks that are special and valuable in terms of cuisine are sold under other names. The industry markets the fish cautiously, but it is often consumed unconsciously. The table below shows some examples of this naming.
|Rocksalmon||Dogfish from Ireland|
|Hemonets||Dog shark from France|
|Sea eel||Dogfish in jelly|
A number of organizations have made it their business to protect sharks. The #stopfinningEU initiative has launched a European citizens' initiative with which it wants to achieve that the Fins Naturally Attached Regulation is extended to the export, import and transit of sharks and rays. This regulation stipulates that the whole shark must be landed - so cutting off the fins and then throwing the body into the water is prohibited at sea.
The marine conservation organization Sea Shepherd , in cooperation with local authorities, has repeatedly succeeded in arresting illegal ships that hunted sharks. The owners of these ships were sometimes fined millions of euros. In addition, the lives of over a million sharks were saved as the arrested vessels could not fish while in port. Sea Shepherd also promotes the protection of sharks. This is done through free lectures in schools, the promotion of various petitions for the protection of sharks, advertising campaigns and through their merchandising products.
The Sharkproject funds research, raises public awareness, carries out projects in schools and publishes books on sharks.
Other organizations that strive to protect sharks are the German Marine Protection Foundation , Shark Savers Germany , Stop Finning Germany eV , Blue Shark Conservation , All For Blue , the Shark Allies , Shark Guardian, the Shark Citizens and Sharks Educational.
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