In this section of the site you find articles about true sharks (see articles below) and «freshwater sharks». When you purchase so called freshwater sharks from your local fish store, they are usually not sharks. Several types of small and silvery shark-like fish species are marketed as freshwater sharks even though they only have a body form that resembles that of a shark. There are true shark species that lives in freshwater, particularly in Australia, but these true sharks are not recommended for hobby aquarists. They will grow very large and you will need a huge aquarium if you want to provide them with a good home. Several species are also endangered and should not be removed from their natural environment unless needed for conservatory reasons, e.g. for a public aquarium. A majority of the most well known true shark species, such as the Great White shark and Hammerhead shark, are marine species.
There are also a few true shark species that can live in brackish waters and the Bull Shark can even leave the ocean and migrate several miles up in freshwater rivers and into lakes. The Bull Shark will usually do well in captivity, but due to its size it is only kept in public aquariums. It can reach a length of 3.5 meters and require plenty of space to thrive. It is also included in the IUCN Red List of Threatened Species and considered near threatened by CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora).
As mentioned earlier, the so called freshwater shark species are actually a better choice for a freshwater aquarium than the true freshwater shark species. If you have a saltwater aquarium you can also keep some of the smallest saltwater shark species. A saltwater aquarium is not suitable as a first aquarium; it is advisable to begin with a freshwater aquarium and gain some experience before you venture into the beautiful but somewhat more complicated saltwater world.
It is common for the so called freshwater sharks to come from rivers and lakes in Asia. They might look like sharks, but their anatomy and habits are very different from those of a true shark. The so called sharks are typically not even predatory. Always try to find out the true name of the “Shark” that you are offered in the fish store, since you should choose a species that will not outgrow your aquarium.
The Bala shark is one example of a so called shark that is commonly sold to hobby aquarists even though it will reach a mature size of 36 centimeters (14 inches) which makes it too big for a majority of the hobby aquarists. The Bala shark should also ideally be kept in a school consisting of at least six Bala sharks, since Bala sharks always form schools in the wild. Keeping six fishes that are longer than 30 centimeters in the same aquarium is unfeasible for most aquarists. The Black shark is another so called shark that is typically offered to aquarists when it is 5 centimeters (2 inches) long, and many fish stores are quite reluctant to tell the purchaser that the quite little fish that they just bought can become 82 centimeters (32 inches) long.
In the rostrum (snout), the cartilage can be spongy and flexible to absorb the power of impacts.
The fin skeletons are elongated and supported with soft and unsegmented rays named ceratotrichia, filaments of elastic protein resembling the horny keratin in hair and feathers.
Respiration [ ]
Buoyancy [ ]
Osmoregulation [ ]
Teeth [ ]
The tails (caudal fins) of sharks vary considerably between species and are adapted to the lifestyle of the shark. The tail provides thrust and so speed and acceleration are dependent on tail shape. Different tail shapes have evolved in sharks adapted for different environments. Sharks possess a heterocercal caudal fin in which the dorsal portion is usually noticeably larger than the ventral portion. This is due to the fact that the shark’s vertebral column extends into that dorsal portion, allowing for a greater surface area for muscle attachment which would then be used for more efficient locomotion among the negatively buoyant cartilaginous fishes. This is in contrast to the bony fishes, class osteichthyes, which possess a homocercal caudal fin.
Some tail adaptations have purposes other than providing thrust. The cookiecutter shark has a tail with broad lower and upper lobes of similar shape which are luminescent and may help to lure prey towards the shark. The thresher feeds on fish and squid, which it is believed to herd, then stun with its powerful and elongated upper lobe.
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. In the past, sharks’ skin has been used as sandpaper.
Body temperature [ ]
Lifespan [ ]
The maximum lifespan of a shark varies from species to species. Most sharks live for 20 to 30 years, while the spiny dogfish lives a record lifespan of more than 100 years. [14] Whale sharks (Rhincodon typus) have been hypothesized to also live over 100 years. [15]
Etymology [ ]
Evolution [ ]
A collection of fossilised shark teeth
Evidence for the existence of sharks extends back over 450–420 million years, into the Ordovician period, before land vertebrates existed and before many plants had colonised the continents. [18] All that has been recovered from the first sharks are some scales. The oldest shark teeth are from 400 million years ago. The first sharks looked very different from modern sharks. [19] The majority of the modern sharks can be traced back to around 100 million years ago. [20]
Megalodon with the great white shark and a human for scale
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. It is currently thought that the sharks form a polyphyletic group: in particular, some sharks are more closely related to rays than they are to some other sharks.
There are more than 440 described species of sharks split across eight orders of sharks, listed below in roughly their evolutionary relationship from more primitive to more modern species:
Reproduction [ ]
Claspers of male spotted wobbegong, Orectolobus maculatus
Sharks have a different reproductive strategy from most fish. Instead of producing huge numbers of eggs and fry (a strategy which can result in a survival rate of less than 0.01%), sharks normally produce around a dozen pups ( blue sharks have been recorded as producing 135 and some species produce as few as two). [24] These pups are either protected by egg cases or born live.
There are three ways in which shark pups are born:
Parthenogenesis (asexual reproduction) [ ]
Shark senses [ ]
Sense of smell [ ]
Sharks generally rely on their superior sense of smell to find prey, but at closer range they also use the lateral lines running along their sides to sense movement in the water, and also employ special sensory pores on their heads ( Ampullae of Lorenzini ) to detect electrical fields created by prey and the ambient electric fields of the ocean.
Sense of sight [ ]
Oceanic whitetip shark with a small school of pilot fish
Sense of hearing [ ]
Although it is hard to test the hearing of sharks, there are indications that suggest that they have a sharp sense of hearing and can possibly hear prey many miles away. [29] A small opening on each side of their heads (not to be confused with the spiracle ) leads directly into the inner ear through a thin channel. The lateral line shows a similar arrangement, as it is open to the environment via a series of openings called lateral line pores. This is a reminder of the common origin of these two vibration- and sound-detecting organs that are grouped together as the acoustico-lateralis system. In bony fish and tetrapods the external opening into the inner ear has been lost.
Electroreception [ ]
Electroreceptors (Ampullae of Lorenzini) and lateral line canals in the head of a shark.
The Ampullae of Lorenzini are the electroreceptor organs of the shark, and they vary in number from a couple of hundred to thousands in an individual. Sharks use the Ampullae of Lorenzini to detect the electromagnetic fields that all living things produce. [30] This helps sharks (mostly the hammer head) find its prey. The shark has the greatest electrical sensitivity known in all animals. This sense is used to find prey hidden in sand by detecting the electric fields inadvertently produced by all fish. It is this sense that sometimes confuses a shark into attacking a boat: when the metal interacts with salt water, the electrochemical potentials generated by the rusting metal are similar to the weak fields of prey, or in some cases, much stronger than the prey’s electrical fields: strong enough to attract sharks from miles away. Template:Fact The oceanic currents moving in the magnetic field of the Earth also generate electric fields that can be used by the sharks for orientation and may be used in navigation. [31]
Lateral line [ ]
Behavior [ ]
When approached too closely some sharks will perform a threat display to warn off the prospective predators. This usually consists of exaggerated swimming movements, and can vary in intensity according to the level of threat. [34]
Aversion to dolphins [ ]
Feeding [ ]
Shark intelligence [ ]
Despite the common myth that sharks are instinct-driven «eating machines», recent studies have indicated that many species possess powerful problem-solving skills, social complexity and curiosity. The brain-mass-to-body-mass ratios of sharks are similar to those of mammals and other higher vertebrate species. [35]
Sharks have even been known to engage in playful activities (a trait also observed in cetaceans and primates ). Porbeagle sharks have been seen repeatedly rolling in kelp and have even been observed chasing an individual trailing a piece behind them. [37]
Shark sleep [ ]
Shark attacks [ ]
Main article: Shark attack
In 2006 the International Shark Attack File (ISAF) undertook an investigation into 96 alleged shark attacks, confirming 62 of them as unprovoked attacks and 16 as provoked attacks. The average number of fatalities per year between 2001 and 2006 from unprovoked shark attacks is 4.3. [39]
Sharks in captivity [ ]
Habitat [ ]
Conservation [ ]
The value of shark fins for shark fin soup has led to an increase in shark catches. Usually only the fin is taken, and the rest of the shark discarded, usually back into the sea.
Many other threats to sharks include habitat alteration, damage and loss from coastal developments, pollution and the impact of fisheries on the seabed and prey species.
A Canadian-made documentary, Sharkwater is raising awareness of the depletion of the world’s shark population.
Shark fishery [ ]
The number of sharks being caught has increased rapidly over the last 50 years.
An estimate states that, every year, 26 to 73 million (median value is at 38 million) sharks are killed by people in commercial and recreational fishing. [48] In the past, sharks were killed simply for the sport of landing a good fighting fish (such as the shortfin mako sharks). Shark skin is covered with dermal denticles, which are similar to tiny teeth, and was used for purposes similar to sandpaper. Other sharks are hunted for food (Atlantic thresher, shortfin mako and others), and some species for other products. [49]
Sharks generally reach sexual maturity slowly and produce very few offspring in comparison to other fish 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 [ ]
In other Pacific Ocean cultures, Dakuwanga was a shark god who was the eater of lost souls..
Sharks in cultural tradition [ ]
A popular myth is that sharks are immune to disease and cancer ; however, this remains to be proved. Sharks may get cancer. [54] [55] There are both diseases and parasites that affect sharks. The evidence that sharks are at least resistant to cancer and disease is mostly anecdotal and there have been few, if any, scientific or statistical studies that have shown sharks to have heightened immunity to disease. [56]
Тело тонкое, обычно темно-серое. На спине иногда встречаются темные пятна. Брюхо светлее тела. Рыло плоское и закругленное. Спинные плавники смещены далеко назад. Кончики плавников окрашены в белый цвет, из-за чего акула в некоторых странах называется «whitetip reef shark» (белокончиковая рифовая акула).
Рифовая акула. Фото и видео рифовой акулы
Держится открытой прозрачной воды. Днем отдыхает на дне, иногда собираясь в небольшие группы. В отличие от других акул рифовые акулы способны лежать на дне неподвижно, активно работая жабрами, процеживая через них воду. Другие акулы не способны так делать, и им надо постоянно двигаться, чтобы в жабры постоянно был приток свежей воды и соответственно кислорода.
Рифовая акула. Фото и видео рифовой акулы
Охота и поведение
С наступлением темноты рифовые акулы выходят на охоту. Рацион питания составляют другие рыбы, а так же осьминоги и крабы. Благодаря своему небольшому размеру акула способна проникать в узкие расселины коралловых рифов и охотится на тех животных, которых обычно другие акулы не могут достать (например, мурен). Даже если жертва забилась в очень узкую щель, рифовая акула может отломить целые куски кораллов, чтобы добраться до своего обеда.
Рифовая акула. Фото и видео рифовой акулы
Однако «обжорой» это животное назвать нельзя. Акула может обходиться без еды до 6 недель.
Совместно рифовые акулы не охотятся, но могут собираться в небольшие стаи, занимая одну территорию на рифе.
Рифовая акула и человек
Рифовая акула. Фото и видео рифовой акулы
Международный союз охраны природы (МСОП) оценил вид рифовых акул как «близкий к уязвимому», поскольку его численность заметно снизилась за последнее десятилетия, в основном из-за нерегулируемого вылова. Ограниченный ареал обитания и не высокая рождаемость заставляют уже сейчас задуматься о сохранения этого вида. В заповедных зонах сокращение популяции рифовых акул удалось замедлить, но в рыболовных зонах продолжается сокращение на 6-8% каждый год.
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shark, any of numerous species of cartilaginous fishes of predatory habit that constitute the order Selachii (class Chondrichthyes).
Sharks, together with rays and skates, make up the subclass Elasmobranchii of the Chondrichthyes. Sharks differ from other elasmobranchs, however, and resemble ordinary fishes, in the fusiform shape of their body and in the location of their gill clefts on each side of the head. Though there are exceptions, sharks typically have a tough skin that is dull gray in colour and is roughened by toothlike scales. They also usually have a muscular, asymmetrical, upturned tail; pointed fins; and a pointed snout extending forward and over a crescentic mouth set with sharp triangular teeth. Sharks have no swim bladder and must swim perpetually to keep from sinking to the bottom.
There are more than 400 living species of sharks, taxonomically grouped into 14–30 families, according to different authorities. Several larger species can be dangerous to humans. Numerous sharks are fished commercially. However, overfishing in the late 20th and early 21st centuries substantially reduced the populations of some shark species.
Description and habits
Shark species are nondescript in colour, varying from gray to cream, brown, yellow, slate, or blue and often patterned with spots, bands, marblings, or protuberances. The oddest-looking sharks are the hammerheads (Sphyrna), whose heads resemble double-headed hammers and have an eye on each stalk, and the wobbegongs (family Orectolobidae), whose skin flaps and protective coloration closely resemble the seafloor. The vernacular of shark names indicate colours in living species, such as the blue (Prionace glauca), the white (Carcharodon carcharias; also known as the great white shark), and the lemon (Negaprion brevirostris) shark.
The whale shark (Rhincodon typus) and the basking shark (Cetorhinus maximus), both of which may weigh several tons, are harmless giants that subsist on plankton strained from the sea through modified gill rakers. Whale sharks may grow up to 18 metres (59 feet) in length, whereas basking sharks may reach 14 metres (46 feet) fully grown. All other sharks prey on smaller sharks, fish, squid, octopuses, shellfish, other invertebrates, and, in some species, trash. The largest among the more predatory species is the voracious 6-metre (20-foot) white shark, which attacks seals, dolphins, sea turtles, large fish, and occasionally people. The more sluggish Greenland shark (Somniosus microcephalus) of cold deep waters feeds on seals, large fish, and even swimming reindeer; they may also scavenge whale carcasses.
Normally, sharks feed on fish, often attacking in schools. Open-ocean species such as the mackerel (Lamna), mako (Isurus), and thresher (Alopias) sharks frequently feed near the surface and are much sought after with rod and reel for sport. Beautifully streamlined and powerful swimmers, those open-ocean sharks are adept at feeding on fast tuna, marlin, and the like. Bottom-feeding species of sharks are stout, blunt-headed forms that tend to have more-sluggish habits. The shellfish eaters among them have coarse, pavementlike, crushing teeth.
Fertilization in sharks is internal. The male introduces sperm into the female by using special copulatory organs (claspers) derived from the pelvic fins. The young in many species hatch from eggs within the female and are born alive. Other species may lay eggs or nurture their young in the uterus with a placental attachment to the mother, like humans. Some species may even consume their siblings before they are born.
The origin of sharks is obscure, but their geologic record goes back at least to the Devonian Period (419.2 million to 358.9 million years ago). Fossil sharklike fish appeared in the Middle Devonian Epoch and became the dominant vertebrates of the Carboniferous Period (358.9 million to 298.9 million years ago). Modern sharks appeared in the Early Jurassic Epoch (201.3 million to 174.1 million years ago) and by the Cretaceous Period (145 million to 66 million years ago) had expanded into the present-day families. Overall, evolution has modified shark morphology very little except to improve their feeding and swimming mechanisms. Shark teeth are highly diagnostic of species, both fossil and modern.
Sharks’ geographic ranges are not well known. Their extensive movements are related to reproductive or feeding activities or to seasonal environmental changes. Tagging returns from large sharks on the east coast of the United States indicate regular movements between New Jersey and Florida, and blue sharks have been recovered after crossing the southern Atlantic Ocean. A tagged spiny dogfish (Squalus acanthias) was recovered after traveling about 1,600 km (1,000 miles) in 129 days. Advances in tracking technologies include the use of satellite tags that continuously beam a signal to orbiting satellites whenever the shark surfaces and in that manner reveal north-south movements, transoceanic movements, and roaming and migratory patterns that are beginning to be understood for many species.
Some members of the Carcharhinus genus—most notably the bull shark (C. leucas)—enter fresh waters. Riverine sharks are small to medium-sized and are exceptionally voracious and bold.
