Sharks: Positive Buoyancy Discovered in Two Types of Sharks off Japan and Hawaii

It turns out that while most sharks are not buoyant, two types of deep-sea sharks, those called six-gill and prickly, have what is called positive buoyancy.

In fact, these two are regular bouncy, floaty balls, compared with other sharks. When the sharks swim downward, they work harder than when swimming up. They float upward more easily. So, on the off chance that you're ever far out at sea, deep-sea diving, and you are below a sixgill shark, you're golden. Or nearly so.

That's at least according to a study recently published in Science Daily, released by scientists at the University of Hawaii - Manoa and the University of Tokyo.

Using an accelerometer to record sharks' swimming performance, including speed, heading, tail beat frequency and body orientation, researchers were able to determine whether sharks were positively, negatively, or neutrally buoyant, according to a press release at Eurekalert.org.

"We didn't expect to find evidence of positive buoyancy, and ran two sets of experiments to confirm our initial observations of this phenomenon. This finding was a total surprise," said Carl Meyer, assistant researcher at UHM's Hawaii Institute of Marine Biology (HIMB) and co-author of the study, according to the release.                    

Both types of deep-sea sharks, the bluntnose sixgill, Hexanchus griseus, and the pricky shark, Echinorhinus cookei, showed similar vertical migration patterns during the day and night: They swam at depths of 656-984 feet at night, and deeper than 1,640 feet during the day, according to a release by the two universities, posted at Public Library of Science.

The deep-sea sharks all showed higher swimming efforts during descent than ascent, and were able to glide uphill for periods of several minutes. Scientists are still working on how this bounciness might benefit the sharks. It could be adaptive for stealthy hunting – upward gliding to surprise prey from underneath) or, says the release, it could facilitate upward migrations when sharks' muscle temperatures are cool and they feel sluggish after a long day in cold water.