Bull sharks are large, powerful, and highly aggressive predators. Among the sharks which have gained the most notoriety in the eyes of people, it is the bull shark which is considered the most dangerous. Though it may not grow as large as the infamous great white shark, make no mistake; the bull shark is a fearsome hunter in its own right. Preferring the same warm, shallow beaches which attract countless people every year and capable of surviving in freshwater environments for extended periods of time, the chances of running into a bull shark are much higher than those of encountering most other sharks. Being on the receiving end of an attack by one of these powerful animals is a terrifying experience, as an adult bull shark possesses more than enough strength to overpower any human. Yet it may be that this fearsome predator is the true victim. Along with many other shark species, the bull shark has been demonized and misunderstood by people throughout history. This has resulted in the needless slaughter of countless sharks since the earliest recorded shark attacks. This heinous persecution has drastically lowered the population numbers of many shark species, and the fear of these animals continues to this day.  It is only when we study these creatures and understand the reasons behind their behavior that we can find ways of properly coexisting with them. This is especially important with bull sharks, as they are among the most aggressive of sharks. Yet not even the bull shark is a mindless killer, and so it is of the utmost importance that people understand the purpose of its design and the reasons behind its actions.

The bull shark (Carcharhinus leucas) is a fairly large animal. Males of the species can grow to lengths of 7 feet and reach weights of almost 200 lbs while the much larger females can reach 11.4 feet in length and weigh up to 500 lbs.[1] Bull sharks possess short, broad snouts and rather small eyes. Their robust bodies consist of a barrel-shaped design and they possess two dorsal fins, the first of which are almost 3.2 times the height of the second dorsal fins. These sharks sport a gray upper body while their ventral sides are white in color. Their fins often end in noticeably dark tips, particularly in juvenile bull sharks.[2] Bull sharks possess a fearsome set of teeth. Their upper jaw teeth are broad, triangular, and heavily serrated while their lower jaw teeth are narrow and triangular with fine serrations. The sharks’ anterior teeth are erect and almost symmetrical while their posterior teeth become more oblique in shape.[3] This dentition ensures that whatever is on the receiving end of a bull shark’s bite is not going to fare too well.

Bull sharks can be found in tropical and subtropical seas throughout the world. The sharks have been caught in considerably deep water, though they prefer to reside in depths between 30 meters and waist deep. This preference for shallow water is partly why these sharks encounter humans and attack them more often than other sharks. Bull sharks also commonly inhabit murky waters, as these environments are favorable for hunting. Bull sharks also possess the interesting ability to survive in freshwater habitats for extended periods of time, a trait which few other sharks possess.[4] As a result, the species can be observed in lake and river habitats as well.

Though bull sharks are effective predators in marine environments, traveling into freshwater habitats has allowed them to take advantage of opportunities not available to most sharks. Yet how does a shark accomplish such an unusual feat? The secret lies in their capability to adapt the process of osmoregulation. Osmoregulation is an organism’s capacity to maintain a constant concentration of water in its body even when its outside environment would cause it to loose or gain water. Although all fish possess this ability, bull sharks can actually adapt their osmoregulatory processes to survive in a wide range of water salinity levels, from the salt water of the ocean to the freshwater of a common lake. The external environment and the internal environment of any organism are separated by a membrane across which substances can move. This movement takes place across both the cell membranes (on a cellular level) and across the skin (on the organism’s level). When the concentrations of water to solutes (any substances which have been dissolved in the water) is unequal between the external and internal environments, water will usually move from the area of its highest concentration to the area of its lowest concentration until the concentrations are all equal. This can lead some organisms to build up too much or too little water in their bodies in order to maintain life. In a marine environment, animals must prevent dehydration since their environment has a high concentration of solutes, while freshwater animals must be able to conserve their salts. This is where osmoregulation comes in.[5]

In the case of sharks, the normal osmoregulation mechanism in a marine environment is a high concentration of urea and other biological solvents in their blood and the removal of excess salt from the bloodstream through urine. The high salt concentrations within sharks’ bloodstreams allow them to absorb water from their marine environment, while any excess salt is simply removed from their bodies via urination. This process is mainly controlled by the kidneys. However, in most sharks this trait cannot be altered in any way. Should a shark enter a freshwater habitat and remain there, it will eventually absorb too much water relative to its bodily solvents and lose too much salt. After some time, the animal will die. Bull sharks, however, are different from most sharks. Their kidneys (and, to a lesser extent, the kidneys of a few other shark species) can be adjusted to suit the salinity of their current environment. As they move further into freshwater, bull sharks’ kidneys remove less salt and more urea from the bloodstream via urination, which is basically a reversal of a normal marine shark’s osmoregulation method. This incredible design means that bull sharks are free to enter a wide variety of aquatic environments as they wish.[6]

One of the advantages of this trait is that bull sharks are not restricted to marine prey. The combination of their wide range and ability to survive in freshwater environments results in a great list of prey items for these powerful hunters. Among their usual prey items are mullet, tarpon, catfishes, menhaden, gar, snook, jacks, mackerel, snappers, stingrays, and smaller sharks. Other food items reported in bull sharks include crabs, shrimp, sea birds, squid, sea turtles, and dolphins.[7] On rare occasions, some truly shocking food items are reported inside of these sharks, such as human and hippopotamus remains.[8] Adult bull sharks themselves do not appear to possess predators of their own, though juvenile bull sharks can fall prey to larger sharks and, in certain areas of their range, crocodiles.[9]

Bull sharks develop somewhat slowly when compared to many other animals. Both males and females reach sexual maturity at the ages of 8-10 years.[10] When two bull sharks are prepared to mate, the male approaches the female shark and begins to nip at her back. He then grasps one of her pectoral fins in his mouth before the two animals copulate. This interesting courtship behavior usually leaves the females with noticeable scars, as those sharp teeth can make it difficult for the shark to grasp something without damaging it. In the western north Atlantic off Florida and the Gulf of Mexico, the young (known as pups) are born in late spring or early summer. Off Nicaragua, female bull sharks are known to produce young year round. The sharks are viviparous, meaning they give birth to live young like most mammals do. In the case of bull sharks, the size of a single litter can vary, with a maximum of 13 pups per litter. These young sharks, which can measure 60-80 cm at birth,[11] can fend for themselves even at this fragile stage in their lives. Common breeding areas for these sharks are the brackish waters where rivers meet the open ocean.[12] These areas are less dangerous for the young sharks, as they are less likely to cross paths with other, larger sharks.

The bull shark’s capacity to enter freshwater habitats has allowed it to feed on a wider variety of prey than most other sharks can and produce young in less dangerous habitats. However, this freshwater activity, along with the fact that the bull shark is commonly found in warm, shallow waters, has caused the shark to come into contact with humans more often than any other shark species. It is no surprise, then, that the bull shark is implicated in a high number of attacks on humans. Though they may be aggressive, bull sharks behave similarly to many other sharks when it comes to searching for food. Since sharks lack hands or other appendages that they could be used to investigate an object, they must learn from their surroundings using their jaws.[13] A shark which is presented with something it does not immediately recognize will most likely bite the object of interest in order to learn more about it. If the shark recognizes it as food, it will proceed to consume it. If it does not, it will merely leave the object be. However, as stated previously, many sharks, including bull sharks, possess powerful jaws lined with sharp, serrated teeth. An attempt by a shark to investigate, say, a human will most likely lead to a serious injury for the person due to the shark’s great strength. Furthermore, swimmers or divers which approach sharks are at a constant risk of being attacked.[14] Sharks, especially aggressive species such as bull sharks, are unpredictable (as are all animals) and may suddenly attack a person if it feels threatened. There is also the possibility that a shark could simply confuse a swimmer for its usual prey due to low visibility in their environment.[15] The silhouette of a person lying on a surfboard may appear similar to the body shape of a seal to a shark swimming below the swimmer, for example. In the case of bull sharks, which prefer to hunt for prey in murky waters, the chances of attacking a human due to mistaken identity can be much greater. There are many reasons why sharks may attack human beings, but it appears that bull sharks attack people mainly because of the environment it prefers to hunt in. If both humans and large, powerful predators remain in the same environment for extended periods of time, then an attack is inevitable. We must simply understand that sharks are designed to behave in such ways that aid them in their survival, and to interfere in their usual behavior is to risk one’s own life.

It appears that even the bull shark, with its reputation as a mindless killer, is a misunderstood creature much like its larger, more famous relatives. However, that does not change the fact that these sharks are dangerous. Not only must one exercise caution when entering waters which may contain sharks, but a better understanding of what makes these creatures attack humans and how we can help lower the chances of these conflicts must be attained. Sharks themselves, are victims of attacks by people. Throughout history, humans have persecuted sharks with the idea that the animals are mindless, killing machines, or basically just evil. This demonization has caused much more damage to sharks than sharks have ever caused to humans. We must simply understand that these are animals which must be treated with the respect they deserve, and that respect extends to practicing caution when entering their waters, their territory, and their world.

» Ethology, Ichthyology, Marine Biology » Predator of the Shallows: The...
On August 10, 2011
By Cendan Luis
aggressive, breaking, featured, marine predator, notorious, pictures, sharks

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