May Elasmobranch of the Month: Bull Shark
Key Features: Bull sharks have a very stout appearance due to their high body width to length ratio. They also exhibit countershading, which means that their dorsal (top) side is a darker grey while their ventral (bottom) side is white in color. This coloration helps them blend in with their surroundings from any angle; if you view a bull shark from above, it’s dark back blends into the seafloor bottom, while if viewed from below, a bull shark’s light-colored belly blends into the sunlit waters above. Their small eyes and blunt snouts also add to their “bullish” appearance. Male bull sharks grow to an average of 7ft while females grow to a larger average of 11ft.
Where it’s found: Bull sharks are found in coastal waters all over the world, as well as in numerous river systems and a few freshwater lakes! In the United States, bull sharks are found off the East Coast and in the Gulf of Mexico. Bull sharks are the only species of shark that can readily be found in freshwater, and they use these habitats as safe pupping and nursery grounds.
Diet: These sharks favor a wide variety of animals in their diet, but prefer bony fish and smaller sharks. Sea turtles, dolphins, crabs, shrimp, sea birds, squid and stingrays have all been found inside a bull shark’s stomach! Bull sharks are usually found slowly cruising close to the sea floor but can capture smaller, more agile prey with sudden bursts of speed (up to 11 mph).
Status: IUCN Near Threatened (which means the bull shark is not considered endangered or vulnerable at this time). Threats: Since Bull sharks are found around coastlines, they are more at risk from pollution and habitat degradation. They are also regularly captured as bycatch in commercial fisheries around the world, usually on longlines, where their fins are sold to Asia for shark fin soup. Recreationally, bull sharks are also considered popular game fish and are targeted around the world during fishing tournaments.
Fun fact: Bull sharks are unusual because they can adapt readily to fresh water because they can adapt their process of osmogregulation- which is the ability of an organism to maintain a constant concentration of water in its body even when its outside environment should cause it to lose or gain water. The kidneys of bull sharks can be gradually adjusted to suit the salinity of the water they are in. When moved gradually into freshwater (perhaps by migrating from the ocean, to an estuary, and then upriver), bull sharks’ kidneys remove less salt and more urea from the bloodstream through urination, essentially reversing the normal marine shark method of osmoregulation. This adaptation allows bull sharks to live entirely in estuaries or freshwater!
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