Deep-sea sharks show surprising buoyancy
Mark Royer/University of Hawaii

Deep-sea sharks show surprising buoyancy

Many sharks reign atop their local food chain, but scientists thought the sleek hunters still had to bother with an unbefitting task: fighting their natural tendency to sink. Unlike other fish, which inflate air bladders to adjust their buoyancy on the fly, sharks rely on a skeleton of cartilage and a liver filled with lighter-than-water oil to help beat gravity’s pull. Even with the weight savings, scientists believed that sharks were either negatively or neutrally buoyant—tending to sink or stay in the same spot when they stopped swimming. Now, researchers have found that at least two species of deep-sea sharks have a small amount of positive buoyancy, the team reports this month in PLOS ONE. Scientists attached monitors to five bluntnose sixgill sharks (Hexanchus griseus, pictured) and a prickly shark (Echinorhinus cookei) to measure the animals’ depth, swimming speed, and acceleration. When the sharks beat their tails, the instruments registered a signature jolt. Researchers found that these spikes in acceleration appeared more frequently when sharks descended, indicating that swimming down was harder than swimming up. The team also found that the sharks could climb for minutes on end with almost no tail beating, a gliding behavior researchers believe is best explained by positive buoyancy. This propensity to rise could be an adaptation that allows the sharks to sneak up on prey from below, the team writes, or merely a way to allow muscles to relax after a day spent hunting for meals in colder, deeper water.