Scientists have long speculated about whether sharks sink when they stop swimming. But new research is causing this theory to slowly unravel.
Scientists from the United States and Japan studied two species of deep-sea sharks near Hawaii and found that they exhibited positive buoyancy, meaning they are less dense than the water surrounding them ... making them prone to float.
Most non-air breathing marine organisms are negatively or neutrally buoyant since they do not need to surface to breathe. Unlike fish who use gas-filled bladders to alter buoyancy, sharks use oil-filled livers. Deep-sea sharks, in particular, have very large oil-filled livers constituting about 20 percent of their body mass.
Scientists previously believed that the intense pressure of the deep sea would negate the buoyancy capabilities of the large livers and that sharks would need to continuously swim in order to stay afloat. Now, this former belief has been negated, and the scientists could not be more shocked.
Accelerometer-magnetometers were attached to two species of six-gill bottom dweller sharks. The data collected indicated that the sharks exerted more energy when descending rather than ascending. They were observed “gliding” upwards without actively swimming, which is indicative of positive buoyancy, the study, recently published in the scientific journal PLOS One, concluded.
While the results are preliminary, the researchers said that positive buoyancy in deep-sea sharks could shed light on other organisms living in the lowest layer of the ocean. Theories about why a bottom dweller is inclined to float? The scientists suggest that floating is an evolved adaptation so that the sharks can “sneak up and catch prey from underneath”, or it may be a way for the shark to rest their muscles after exerting energy while hunting at colder depths.