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Sharks may sink in fresh water

Sharks may be fearsome predators, but they have one weakness: most of the sharks are not able to tolerate fresh water. About 40% of bony fish, categories including goldfish to rainbow trout, can breathe in fresh water, but only 5% of elasmobranchs (a subclass of Chondrichthyes or cartilaginous fish, that includes the sharks and the rays and skates) can manage this skill. 

Fresh water creates a problem of dehydration in them, dulls their senses, and affect their reproduction chances. According to the study published online this January in The Journal of Experimental Biology, Sharks may sink in fresh water. 

Modern fish with their fancy air bladders can tolerate fresh water, but the ancient
elasmobranchs have only their oily livers as flotation devices. Scientists developed a model of the swimming mechanics of a bull shark as shown in picture above, one species that lives for a short period of time in rivers, as if testing the aerodynamics of a new airplane design.

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They computed that sharks must spend about 50% more energy on lift, because of a loss of buoyancy, once it leaves the ocean. The team observed that freshwater species might recompense by fattening their livers to the
physiological max, but the added drag of the
resulting tubby body would likely make them less sleekly efficient
hunters than they could be at sea. 
New measurements of two freshwater
elasmobranchs from Fitzroy River in Western Australia back up the
calculations. The five bull sharks and 17 largetooth sawfish, a
bottom-dwelling relative, were less buoyant than 27 previously studied
ocean species, despite extra-fatty livers. Fossils suggest freshwater
sharks were once more common, but more research is needed to determine
if flotation problems or other factors finally drove them into the sea.