Sharks may be fearsome predators, but they have a simple weakness: Most can’t tolerate fresh water. About 40% of bony fish, from goldfish to rainbow trout, live in fresh water, but only 5% of elasmobranchs (the sharks, rays, and skates) can manage this feat. Fresh water dehydrates them, dulls their senses, and compromises their reproduction. It also makes them sink, according to a study published online this month in The Journal of Experimental Biology. Unlike more modern fish, with their fancy air bladders, the ancient elasmobranchs have only their oily livers as flotation devices. Researchers modeled the swimming mechanics of a bull shark (pictured), one species that lives part time in rivers, as if testing the aerodynamics of a new airplane design. They computed that because of a loss of buoyancy, the shark must spend about 50% more energy on lift once it leaves the ocean. Freshwater species might compensate by fattening their livers to the physiological max, the team calculated, 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.