Most sharks are not buoyant and they could sink if they stop swimming but findings of a new study have shown that two species of deep-sea sharks, the prickly shark and the bluntnose sixgill shark, are characterized by what is known as positive buoyancy.

Sharks that are positively buoyant have to swim harder when they go downward than when they go up.  When they ascend, these sharks can also glide for minutes a time without the need to use their tail.

Researchers from the University of Tokyo and University of Hawaii - Manoa used an accelerometer to record the swimming performance of the sharks as the creatures swam up and down in their habitat.

By looking at the animals' speed, tail beat frequency, heading and body orientation, the researchers determined if sharks were neutrally, positively or negatively buoyant.  

Study author Carl Meyer, from the UHM's Hawaii Institute of Marine Biology (HIMB) said that they did not expect to find sharks that are positively buoyant but the results showed otherwise.  They had to conduct two sets of experiment to confirm their initial observation and the findings turned out to be a surprise.

"During vertical movements, all deep-sea sharks showed higher swimming efforts during descent than ascent to maintain a given swimming speed, and were able to glide uphill for extended periods (several minutes), indicating that these deep-sea sharks are in fact positively buoyant in their natural habitats," Meyer and colleagues wrote in their study, which was published in the journal PLOS One on June 10.

The researchers also used a shark-mounted camera on a deep-sea shark in order to gain insight into these creatures' deep-sea habitat. They found that the sharks live in the deeper part of the sea during the day and the cold water during this time may cool their swimming muscles and cause them to become sluggish swimmers.

Positive buoyancy appears to help the sharks thrive in their habitat. Being positively buoyant may be these marine animals' physiological strategy that allows them to exploit the deep and cold habitats with limited food supply and the researchers said they are interested in understanding this trait.

"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?" Meyer asked.

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