ABOUT 40 PERCENT OF BONY FISHES live in freshwater but only five percent of sharks, rays and skates can do so. There a number of reasons cartilaginous fishes don’t do well in freshwater, including problems with dehydration and reproduction.
SINK OR SWIM. MOSTLY SINK A team of American and Australian scientists have determined that another reason is an inability to maintain buoyancy control in freshwater. In short, in freshwater, sharks and their kin would tend to sink like newbie divers.
NEEDED: LARGER LIVERS Sharks, rays and skates rely on livers rich with oily lipids to maintain buoyancy. Through hydro-mechanical modeling, the researchers determined that being in freshwater involves a two- to three-fold increase in negative buoyancy for cartilaginous fishes and that they would require an eight-fold increase in liver volume to maintain the same net buoyancy in freshwater as in marine waters.
The study was published in the Journal of Experimental Biology. You can read a report on their findings, Sharks sink in fresh water, in ScienceNow, an online publication of the American Association for the Advancement of Science.
SOURCES: Mechanical challenges in freshwater residency in sharks and rays, Adrian C. Gleiss, Jean Potvin, James J. Keleher, Jeff M. Whitty, David L. Morgan, Jeremy A. Goldbogen, Journal of Experimental Biology, doi: 10.1242/jeb.114868; Sharks sink in fresh water, Nicholas Weiler, AAAS ScienceNOW, 15 January 2015,