Deep-Sea Sharks are Positively Buoyant, Surprising Scientists

It turns out the deep-sea sharks may be positively buoyant, a fact which surprised scientists who believed that these animals were neutrally buoyant, which would make it easier for them to swim.

Conventional wisdom suggests that sharks are negatively, or sometimes neutrally, buoyant. Sharks have cartilaginous skeletons, which are lower in density than bone, and they generate buoyancy with their large, oil-filled liver. Despite these adaptations, though, most sharks are negatively buoyant and will sink if they stop swimming. Instead, they generate life by swimming forward.

"We didn't expect to find evidence of positive buoyancy , and ran two sets of experiments to confirm our initial observations of this phenomenon," said Carl Meyer, co-author of the new study, in a news release. "This finding was a total surprise."

In this case, the researchers fitted sharks with an accelerometer data logger to measure the animals' swimming performance as they swam up and down in their deep-sea habitat. This device can provide information on the shark's swim speed, heading, tail beat frequency, and body orientation From this data, the researchers found that two species of deep-sea sharks, the six-gill and prickly sharks, are both positively buoyant.

The researchers also deployed the first ever shark-mounted camera on a deep-sea sharp to get a better look at the shark's environment. This revealed that the shark spends most of its time higher up in the water column rather than near the seafloor.

"We want to better understand why these sharks are positively buoyant," said Meyer. "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 shallow water areas?"

The findings are published in the journal PLOS One.

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