A recent study found two species of deep-sea sharks are positively buoyant, meaning they must work hard to swim downwards and can easily glide upwards.
Most sharks are negatively buoyant, and will sink if they stop swimming, but this new research shows the same is not true of six-gill and prickly sharks, the University of Hawaii at Manoa reported.
"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 Hawai'i Institute of Marine Biology (HIMB) and co-author of the study.
To make their findings, the researchers fitted sharks with an accelerometer data logger to measure their swimming performance as they moved upwards or downwards in the water. The data recorded by the device included "swim speed, heading, tail beat frequency and body orientation." This information allowed the researchers to determine whether the sharks were positively or negatively buoyant.
"When I first downloaded the camera, I thought it had failed because all I saw were thousands of completely black frames," Meyer said. "Suddenly a string of images appeared with a brightly-lit, alien-looking reef and strange deep-sea invertebrates. I was elated and realized that the black frames resulted from the shark swimming around too high in the water column for the camera strobe to illuminate the seabed."
The researchers suggest that since the sharks live in deeper water by day than night, the cold water could make them "sluggish" swimmers. This means positive buoyancy could be a physiological strategy that allows the sharks to live in cold habitats with limited food resources.
"We want to better understand why these sharks are positively buoyant," Meyer said. "Does this trait perhaps give them a 'stealth' advantage during hunting, allowing them to glide motionless upward to capture prey above them in the water? Or does it help them with nightly migrations to shallower areas?"
The findings were published in a recent edition of the journal PLOS One.
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