Few animals make the move from saltwater to freshwater habitats; a slew of physiological and ecological barriers limits this. And for sharks, it’s because they’ll sink, according to a new study published in the Journal of Experimental Biology last week.
Cartilaginous fish like sharks thrive in marine ecosystems across the planet, yet few have evolved to occupy lakes and rivers. Of the more than 1,000 species of Elasmobranchii (sharks, rays, 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) live in freshwater. Researchers suspect it’s because of the metabolic costs of osmoregulation—having to maintain the right pressure by controlling water and salt concentrations in the body.
But what about the density differences between seawater and freshwater? After all, one of the most noticeable differences (for us at least) between, say, the Great Salt Lake or the Dead Sea and a swimming pool is how well we can float. While many bony fish have swim bladders to help control their buoyancy, the primary source of buoyancy in elasmobranchs is a lipid-rich liver.
So, Stanford’s Adrian Gleiss and colleagues designed a hydromechanical model based on the bull shark (Carcharhinus leucas), one of the few elasmobranchs that live in freshwater for part of their life cycle. They found that being in freshwater would lead to a two- to three-fold increase in negative buoyancy for sharks and rays—which carries the energetic cost of increased drag because they have to lift themselves.
Sharks could try to compensate for this increased negative buoyancy, but then the lipid-rich liver would need to be eight times bigger by volume in order to maintain the same net buoyant effect they’d enjoy in marine waters.
To back up these calculations, the team also describe new data on body density from freshwater-occupying elasmobranchs captured from Fitzroy River in western Australia: five bull sharks and 17 largetooth sawfish (Pristis pristis). These have similar liver sizes as the 113 saltwater-dwelling cousins the team sampled, but lower liver densities. In fact, these 22 displayed the greatest negative buoyancies of any elasmobranch to date.