Scientists have strapped cameras onto free-swimming sharks, capturing a shark's-eye view of their underwater world.
The footage from 14 tiger sharks, six Galapagos sharks, five sandbar sharks, five bluntnose sixgill sharks and a prickly shark is the first to be taken of sharks, by sharks in their natural environment.
One clip from a camera attached to a male sandbar shark show the pursuit of a female; another shows its wearer's point of view as it meets up with dozens of other sharks in a mixed group — including sandbars, oceanic blacktips and scalloped hammerheads — and swimming together for most of the day.
The discoveries, presented last week at the 2014 Ocean Sciences Meeting in Honolulu, reveal the complex daily lives of these fierce, sharp-toothed swimmers in their natural environment.
"I was really amazed by all the images we got back," said lead author Carl Meyer, a marine biologist at the University of Hawaii. "It's mesmerizing to see that shark's-eye perspective of the shark interacting with the coral reef and the fishes."
Sharks are among the top predators in the world's oceans, and so where they go and what they eat can have huge effects that reverberate through their ecosystem's food web. Scientists have attached sensors to sharks before to get an idea of their movements, but until recently they haven't had the technology necessary to get good video footage — or to get their cameras back.
The system the researchers used involves a camera and sensors to measure depth, acceleration and the magnetic field around the animal. It includes a VHF transmitter that the researchers can use to track down the devices once they detach from their host sharks and float up to the ocean's surface.
The scientists managed to mount these devices on sharks' pectoral fins by relying on a strange phenomenon known as "tonic immobility": flip a shark over onto its back, and it suddenly goes into a calm, trance-like state.
After performing this maneuver with a subject shark, they would then gingerly flip it back right side up and hustle to get the camera on before the shark woke up. (If it started to struggle, they'd flip it back on its belly again.) Then they released the sharks, and let the cameras roll.
The footage revealed surprising aspects of shark behavior. For example, the scientists think the sharks traveled in their mixed-species groups for most of the day for protection against tiger sharks, which are much larger and often prey on their smaller peers (much as herding animals like gazelles stay together for protection against hungry lions). Turns out it's a shark-eat-shark world under the sea.
"We had no idea that these mixed-species aggregations existed," Meyer said, "even though it's just a few miles offshore."
Most of the sharks used in the study hang out along the coast, but the scientists say they're hoping to get more footage from sharks like the bluntnose sixgill, which inhabit much deeper, more mysterious waters.
Meyer is also working with another colleague on an instrument that would be implanted in sharks' bellies and measure their stomach contents. Hidden inside a tasty bit of fish, this electronic pill would measure the current between two electrodes and be able to tell researchers when the animal was eating and — based on how long it takes to digest in the stomach — how big of a meal it had.