Few animals make the move from seawater to freshwater habitat ; a slew of physiological and ecological barriers limits this . And for sharks , it ’s because they ’ll bury , according to a newstudypublished in theJournal of Experimental Biologylast workweek .
gristly Pisces the Fishes like sharks flourish in marine ecosystem across the planet , yet few have evolved to occupy lakes and rivers . Of the more than 1,000 specie of Elasmobranchii ( sharks , beam of light , and skates ) , only 5 percent can live in freshwater , and most of them do so part - time only . By comparison , Science explains , about 40 percent of bony fish ( from goldfish to rainbow trout ) survive in freshwater . researcher suspect it ’s because of the metabolic costs of osmoregulation — get to hold the right pressure by assure piddle and salt concentrations in the dead body .
But what about the density differences between saltwater and freshwater ? After all , one of the most noticeable conflict ( for us at least ) between , say , the Great Salt Lake or the Dead Sea and a swimming kitty is how well we can float . While many bony fish have swim bladders to help control their perkiness , the principal rootage of buoyancy in elasmobranchs is a lipoid - deep liver .
So , Stanford ’s Adrian Gleissand colleagues designed a hydromechanical model based on the bull shark ( genus Carcharhinus leucas ) , one of the few elasmobranchs that live in fresh water for part of their life Hz . They find that being in freshwater would lead to a two- to three - fold gain in damaging buoyancy for sharks and rays — which carries the up-and-coming price of increased drag because they have to lift themselves .
Sharks could essay to compensate for this increase negative buoyancy , but then the lipide - racy liver would necessitate to be eight sentence bigger by volume to maintain the same net buoyant effect they ’d love in nautical waters .
To back up these calculations , the squad also draw new data point on body compactness from fresh water - take elasmobranchs captured from Fitzroy River in western Australia : five Taurus sharks and 17 largetooth sawfish ( Pristis pristis ) . These have similar liver sizes as the 113 saltwater - inhabit cousins the team try , but lower liver densities . In fact , these 22 displayed the greatest minus buoyancies of any selachian to engagement .
