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The bull shark (Carcharhinus leucas), also known as the Zambezi shark or, unofficially, as Zambi in Africa and Nicaragua shark in Nicaragua, is a requiem shark commonly found worldwide in warm, shallow waters along coasts and in rivers. The bull shark is known for its aggressive nature, predilection for warm shallow water, and presence in brackish and freshwater systems including estuaries and rivers.
Bull sharks can thrive in both saltwater and freshwater and can travel far up rivers. They have even been known to travel as far up the Mississippi River as Illinois, although there have been few recorded freshwater attacks. They are probably responsible for the majority of near-shore shark attacks, including many attacks attributed to other species.
The name "bull shark" comes from the shark's stocky shape, broad, flat snout, and aggressive, unpredictable behavior. In India, the bull shark may be confused with the Sundarbans or Ganges shark. In Africa, it is also commonly called the Zambezi River shark or just Zambi. Its wide range and diverse habitats result in many other local names, including Ganges River shark, Fitzroy Creek whaler, van Rooyen's shark, Lake Nicaragua shark, river shark, freshwater whaler, estuary whaler, Swan River whaler, cub shark, and shovelnose shark.
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The bull shark is a special case in evolutionary history. Some of its closest living relatives do not have the capabilities of osmoregulation. The bull shark's genus, Carcharhinus, is shared by the sand bar shark that is not capable of osmoregulation. Although, bull sharks share the same family, Carcharhinidae, with the little known river sharks as well as the same subclass, Elasmobranchii. This allows for some evolutionary predictions of where the bull shark first originated. Is it a river shark turned to the sea? Or a sea shark turned towards the river? The bull shark just has very much in common with river sharks, glyphis, and its own genus of Carcharhinus, but the evolutionary question still stands, where exactly did this shark come from and when? Much more evolutionary evidence is needed to draw a sound conclusion of the bull sharks phylogeny.
Anatomy and appearance
Bull sharks are large and stout, with females being larger than males. The bull shark can be up to 81 cm (2.66 ft) in length at birth. Adult female bull sharks average 2.4 m (7.9 ft) long and typically weigh 130 kg (290 lb), whereas the slightly smaller adult male averages 2.25 m (7.4 ft) and 95 kg (209 lb). While a maximum size of 3.5 m (11 ft) is commonly reported, there is a questionable record of a female specimen of exactly 4 m (13 ft). The maximum recorded weight of a bull shark was 315 kg (694 lb) but may be larger. Bull sharks are wider and heavier than other requiem sharks of comparable length, and are grey on top and white below. The second dorsal fin is smaller than the first. The bull shark's caudal fin is longer and lower than that of the larger sharks, and it has a small snout, and lacks an interdorsal ridge.
Bull sharks have a bite force of up to 600 kilograms-force (1,300 lbf), weight for weight the highest among all investigated cartilaginous fishes.
Distribution and habitat
The bull shark is commonly found worldwide in coastal areas of warm oceans, in rivers and lakes, and occasionally salt and freshwater streams if they are deep enough. It is found to a depth of 150 metres (490 ft), but does not usually swim deeper than 30 metres (98 ft). In the Atlantic, it is found from Massachusetts to southern Brazil, and from Morocco to Angola. In the Indian Ocean, it is found from South Africa to Kenya, India, and Vietnam to Australia.
Populations of bull sharks are also found in several major rivers, with more than 500 bull sharks thought to be living in the Brisbane River. One was reportedly seen swimming the flooded streets of Brisbane, Queensland, Australia, during the Queensland floods of late 2010/early 2011. Several were sighted in one of the main streets of Goodna, Queensland, shortly after the peak of the January 2011, floods. A large bull shark was caught in the canals of Scarborough, just north of Brisbane within Moreton Bay. There are greater numbers still in the canals of the Gold Coast, also in Queensland. In the Pacific Ocean, it can be found from Baja California to Ecuador. The shark has traveled 4,000 kilometres (2,500 mi) up the Amazon River to Iquitos in Peru and north Bolivia. It also lives in fresh water Lake Nicaragua, in the Ganges and Brahmaputra rivers of West Bengal and Assam in eastern India and adjoining Bangladesh. It can live in water with a high salt content as in St. Lucia Estuary in South Africa. The bull shark is generally prolific in the warm coastal waters and estuarine systems of the Mozambique Channel and southward, including Kwa-Zulu Natal and Mozambique. The species has a distinct preference for warm currents.
After Hurricane Katrina, many bull sharks were sighted in Lake Pontchartrain. Bull sharks have occasionally gone up the Mississippi River as far upstream as Alton, Illinois. They have also been found in the Potomac River in Maryland. A golf course lake in Queensland, Australia is the home to several bull sharks. They are believed to have become trapped following a flood in the 1990s. The golf course has capitalized on the novelty and now hosts a monthly tournament called the "Shark Lake Challenge."
The bull shark is the best known of 43 species of elasmobranch in ten genera and four families to have been reported in fresh water. Other species that enter rivers include the stingrays (Dasyatidae, Potamotrygonidae and others) and sawfish (Pristidae). Some skates (Rajidae), smooth dogfishes (Triakidae), and sandbar sharks (Carcharhinus plumbeus) regularly enter estuaries. The bull shark is a fish that is diadromous, meaning they can swim between saltwater and freshwater with ease. These fish also fall under the category of euryhaline fish. Euryhaline refers to an organism that is able to adapt to a wide range of salinities. The bull shark is one of the only cartilaginous fishes that have been reported in freshwater systems. Many of the euryhaline species are bony fish such as salmon and tilapia and do not qualify as fish that are closely related to bull sharks due to their physiological makeup. Evolutionary assumptions can be made to help explain this sort of evolutionary disconnect; one being that the bull shark encountered a population bottleneck that occurred during the last ice age. This bottleneck may have separated the bull shark from the rest of the elasmobranchii subclass and favored the genes for an osmoregulatory system. Elasmobranchs' ability to enter fresh water is limited because their blood is normally at least as salty (in terms of osmotic strength) as seawater through the accumulation of urea and trimethylamine oxide, but bull sharks living in fresh water show a significantly reduced concentration of urea within their blood. Despite this, the solute composition (i.e. osmolarity) of a bull shark in freshwater is still much higher than that of the external environment. This results in a large influx of water across the gills due to osmosis and loss of sodium and chloride from the shark's body. However, bull sharks in freshwater possess several organs with which to maintain appropriate salt and water balance; these are the rectal gland, kidneys, liver and gills. All elasmobranchs have a rectal gland which functions in the excretion of excess salts accumulated as a consequence of living in seawater. Bull sharks in freshwater environments decrease the salt-excretory activity of the rectal gland, thereby conserving sodium and chloride. The kidneys produce large amounts of dilute urine, but also play an important role in the active reabsorption of solutes into the blood. The gills of bull sharks are likely to be involved in the uptake of sodium and chloride from the surrounding freshwater, whereas urea is produced in the liver as required with changes in environmental salinity. Recent work also shows that the differences in density of freshwater to that of marine waters result in significantly greater negative buoyancies in sharks occupying freshwater, resulting in increasing costs of living in freshwater. Bull Sharks caught in freshwater have subsequently been shown to have lower liver densities than sharks living in marine waters. This may reduce the added cost of greater negative buoyancy 
Initially, scientists thought the sharks in Lake Nicaragua belonged to an endemic species, the Lake Nicaragua shark (Carcharhinus nicaraguensis). In 1961, following specimens comparisons, taxonomists synonymized them. They can jump along the rapids of the San Juan River (which connects Lake Nicaragua and the Caribbean Sea), almost like salmon. Bull sharks tagged inside the lake have later been caught in the open ocean (and vice versa), with some taking as little as seven to eleven days to complete the journey.
Bull sharks are able to regulate themselves to live in either fresh water or salt water. It is possible for the bull shark to live in fresh water for its entire life, but it has been observed that this does not happen for certain reasons, mostly due to reproduction. Young bull sharks will leave the brackish water in which they are born and move out into the sea in order to breed with bull sharks of the other sex. While theoretically, it may be possible for bull sharks to live in purely freshwater, it was observed that the bull sharks that were being experimented on had died within four years. The stomach was opened and all that was found were 2 small fishes that were unidentifiable. The cause of death could have been starvation since the primary food source for bull sharks resides in salt water.
The bull shark is known to be able to live in both saltwater and freshwater. It was found in a research experiment that, the majority of the time, the bull sharks were found to be at the mouth of an estuary. The water salinity was tested every month to determine if the difference in salinity is what drove bull sharks into the river. It was found that the bull shark stayed at the mouth of the river independent of the salinity of the water. The driving factor for a bull shark to be in freshwater or saltwater is its age; as the bull shark ages the tolerance for very low or high salinity increases. It was found that the majority of the newborn or very young bull sharks were found in the freshwater area, whereas the much older bull sharks were found to be in the saltwater, as they had developed a much better tolerance for the salinity. This is one of the reasons why adult bull sharks will travel into the river (in order to reproduce). This is thought to be a physiological strategy to help with juvenile survival rates and a way to increase overall fitness of bull sharks. The newborns are not born with a high tolerance for high salinity, so they are born in freshwater and stay there until they are able to travel out.
The bull shark's diet consists mainly of bony fish and small sharks, including other bull sharks, but can also include turtles, birds, dolphins, terrestrial mammals, crustaceans, echinoderms, and stingrays. They hunt in murky waters because it is harder for the prey to see the shark coming. Bull sharks have been known to use the bump-and-bite technique to attack their prey. After the first initial contact the bull shark continues to bite and tackle its prey until they are unable to flee.
The bull shark is known to be a solitary hunter, although there are brief moments in which the bull sharks will team up with another bull shark in order to make it easier to hunt and to trick prey.
Sharks are known to be opportunistic feeders, and the bull shark is no exception to this, as it is part of the Carcharhinus family of sharks. Normally, sharks eat in short bursts, and when food is scarce, sharks digest for a much longer period of time in order to avoid starvation. As part of their survival mechanism, bull sharks will regurgitate the food in their stomachs in order to escape from a predator. This is a distraction tactic, if the predator moves to eat the regurgitated food the bull shark can use the opportunity to escape.
Bull sharks mate during late summer and early autumn, often in the brackish water of river mouths. After gestating for 12 months, a bull shark may give birth to four to ten live young. They are viviparous; they are born live and free-swimming. The young are about 70 cm (27.6 in) at birth and take 10 years to reach maturity. Coastal lagoons, river mouths, and other low-salinity estuaries are common nursery habitats.
175 cm to 235 cm seems to be the size of a fully matured female bull shark that produce viable eggs for fertilization. The courting routine between bull sharks has not been observed in detail as of yet. It is speculated that the male bites the females on the tail until they turn upside down and they male can copulate at that point. At some points, the harassment of the male can become violent. It is not uncommon to see scratches and other marks on a mature female from the mating ritual.
Bull sharks have an unusual migratory pattern in comparison to other sharks. They are found in rivers all over the world. They have the ability to go from seawater and freshwater. They give birth in the freshwater of rivers. The young bull sharks are free from predators while they grow up in the river before they go out to the sea in order to find mates.
The ability to be able to survive in both freshwater and saltwater also gives another benefit that has been driven by evolution. Because the majority of sharks are only able to survive in saltwater, the bull shark has evolved to have their offspring in the freshwater where other sharks cannot enter. The freshwater acts as a protective area where the young are able to grow and mature without the threat of larger sharks preying on the younger bull sharks. This is an explanation for the behavior that is observed from the bull sharks as to why there would be any reason for the adult bull shark to ever travel into a freshwater area despite being able to tolerate the high salinity of marine water.
Bull sharks are born alive in freshwater. The size range of a litter for a female bull shark is around 1 to 13 pups. The average time span for a female bull shark to be pregnant is around 10 to 11 months. The male bull shark is able to begin reproducing around the age of 15 years while the female cannot begin reproducing until the age of 18 years. Unlike most sharks though, the bull shark does not rear its young like other sharks, the young bull sharks are born into flat, protected areas. Freshwater presents a natural defense against most larger predators, and the flat land is an added defense as most large predators will not swim in shallow areas. This increases their chance of survival since the parents do not rear the young in the traditional manner. This is also the reason why there is a high mortality rate in young bull sharks. Since the parents do not rear and protect the young, any predator that is able to attack a young bull shark is easily able to kill and eat the young bull shark without much resistance.
Interactions with humans
Since bull sharks often dwell in very shallow waters, they may be more dangerous to humans than any other species of shark, and along with the tiger shark and great white shark, are among the three shark species most likely to attack humans.
One or several bull sharks may have been responsible for the Jersey Shore shark attacks of 1916, which were the inspiration for Peter Benchley's novel Jaws. The speculation of bull sharks possibly being responsible is based on some attacks occurring in brackish and freshwater.
The bull shark is responsible for attacks around the Sydney Harbour inlets. Most of these attacks were previously attributed to great white sharks. In India, bull sharks swim up the Ganges River and have attacked people. Many of these attacks have been attributed to the Ganges shark, Glyphis gangeticus, a critically endangered river shark species that is probably the only other shark in India able to survive in fresh water, although the sand tiger shark was also blamed during the 1960s and 1970s.
The bull shark prefers coastal water which is less than 100 feet in depth. This is mostly due to their feeding patterns since they prefer murky waters. This is also a problem since this gives the most interaction with humans. It is known that bull sharks inhabit areas off the coast of Florida, and there have been reports of bull sharks getting close enough to the coast to attack humans since the bull shark is an aggressively territorial animal.
Behavioral studies have confirmed that sharks can take visual cues in order to discriminate between different objects. The bull shark is able to discriminate between colors of mesh netting that is present underwater. It was found that bull sharks tended to avoid mesh netting of bright colors rather than colors that blended in with the water. Bright yellow mesh netting was found to be easily avoided when it was placed in the path of the bull shark. This was found to be the reason that sharks are attracted to bright yellow survival gear rather than ones that were painted black. This is very important because it gives an insight into how bull sharks are able to pick up certain visual keys underwater that might give them an advantage when seeking out certain prey.
In 2008, researchers tagged and recorded the movements of young bull sharks in the Caloosahatchee River estuary. Specifically, they were testing to find out what determined the movement of the young bull sharks. It was found that the young bull sharks synchronously moved downriver when the environmental conditions changed. This large movement of young bull sharks were found to be moving as a response rather than other external factors such as predators. An interesting find was that the movement was directly related to the bull shark conserving energy for itself. One way the bull shark is able to conserve energy is that when the tidal flow changes, the bull shark uses the tidal flow in order to conserve energy as it moves downriver. Another way for the bull shark to conserve energy is to decrease the amount of energy needed to osmoregulate the surrounding environment.
Bull sharks are apex predators and seldom have to fear being attacked by other animals. Humans are their biggest threat. Larger sharks, such as the tiger shark and great white shark, may attack them. There have been stories documented in the media of saltwater crocodiles preying on young bull sharks in the rivers and estuaries of Northern Australia.
- Outline of sharks
- List of sharks
- List of fatal, unprovoked shark attacks in the United States by decade
Notes and references
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