Float like... a shark? In an unexpected discovery, researchers who tagged deep-sea sharks with data loggers reveal that at least two species are surprisingly buoyant, according to a PLoS ONE study published last week.
Unlike many other fish, which inflate their gas-filled swim bladders to increase their buoyancy, sharks use their livers, which are filled with oil and as such are less dense than water. When they stop swimming, they tend to either sink (negatively buoyant) or stay in the same spot (neutrally buoyant). Researchers weren’t expecting any noticeable amount of positive buoyancy. In fact, this is why more sharks haven’t invaded freshwater habitats. Earlier this year, experiments with a mechanical bull shark revealed how freshwater leads to a two- to three-fold increase in negative buoyancy.
Deep-sea sharks have especially large oil-filled livers, and they’re thought to be neutrally buoyant in their cold, high-pressure environments. To see if this is really the case, a trio of researchers led by Itsumi Nakamura from the University of Tokyo, Kashiwanoha, captured five bluntnose sixgill sharks (Hexanchus griseus) and one prickly shark (Echinorhinus cookei) in Kane’ohe Bay off Oahu, Hawaii, using fishing lines baited with fish scraps.
They attached accelerometer–magnetometer data loggers to the sharks to measure their swimming performance – from depth and temperature to swimming speed and acceleration – for 36 days. Two of the bluntnose sixgill sharks also got cameras. At night, both sharks swam at depths of 200 to 300 meters (where the ambient water is about 15°C), and during the day, they swam down to deeper than 500 meters (and colder than 7°C).
To maintain swimming speed during these epic vertical movements, the sharks seemed to exert more effort during their descent than during their ascent and they were able to glide uphill for several minutes. These observations suggest that the deep-sea sharks are, in fact, positively buoyant. In their natural habitats, swimming downward is sometimes more of a challenge than swimming up.
This positive buoyancy may be an adaptation for stealthy hunting, helping sharks to surprise prey from beneath. Alternatively, it may help with upward migrations during the evenings after spending the day in deep, cold water; this is when muscle temperatures are at their coolest and the shark’s swimming is at its most sluggish.