Adult bull (Carcharhinus leucas) at Shark Reef Marine Preserve, Beqa Lagoon, Fiji
Bull sharks are aggressive, common, and usually live near high-population areas like tropical shorelines. They are not bothered by brackish and freshwater, and even venture far inland via rivers and tributaries. Because of these characteristics, many experts consider bull sharks to be the most dangerous sharks in the world. Historically, they are joined by their more famous cousins, great whites and tiger sharks, as the three species most likely to attack humans.
Bull sharks get their name from their short, blunt snout, as well as their pugnacious disposition and a tendency to head-butt their prey before attacking. They are medium-size sharks, with thick, stout bodies and long pectoral fins. They are gray on top and white below, and the fins have dark tips, particularly on young bull sharks.
They are found cruising the shallow, warm waters of all the world’s oceans. Fast, agile predators, they will eat almost anything they see, including fish, dolphins, and even other sharks. Humans are not, per se, on their menus. However, they frequent the turbid waters of estuaries and bays, and often attack people inadvertently or out of curiosity. Bull sharks currently are not threatened or endangered. However, they are fished widely for their meat, hides, and oils, and their numbers are likely shrinking. One study has found that their average lengths have declined significantly over the past few decades.
Freshwater Tolerance[]
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 few cartilaginous fishes that have been reported in freshwater systems. Many of the euryhaline fish are bony fish such as salmon and tilapia and are not closely related to bull sharks. 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.[27] 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.
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. 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 two small, unidentifiable fishes. The cause of death could have been starvation since the primary food source for Bull sharks resides in salt water.
In a research experiment the Bull sharks were found to be at the mouth of an estuary for the majority of the time. 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, however, 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. Reproduction is one of the reasons why adult Bull sharks will travel into the river—it is thought to be a physiological strategy to improve juvenile survival 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.
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.
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