Devil rays dive deep beneath the ocean's surface

Ocean life is most abundant in its surface waters. That's where scientists assumed devil rays remained. But a new study has revealed the unusual and impressive travel patterns of devil rays, featuring dives that see the creatures venturing more than mile beneath the ocean surface. They're some of deepest and fastest ocean dives ever before observed, with the creatures swimming nearly straight down at 13 miles per hour.
Over the course of several months, marine biologists at the Woods Hole Oceanographic Institution in Massachusetts -- along with a team of cooperating international scientists -- tracked 15 Chilean devil rays (Mobula tarapacana) over the course of several months.
The winged fish are most often observed gliding through clear blue warm waters of the shallows. But the new study -- published this week in the journal Nature Communications -- and the ray's newly discovered deep diving behaviors help explain why the species feature a sponge-like mesh of arteries that surrounds the brain with warm blood.
"It was a mystery as to why they had this system, which is a way of keeping brain activity high, even in a cold environment," said Simon Thorrold, a biologist at WHOI and one of the study's authors.
"So little is known about these rays," added Thorrold. "We thought they probably travelled long distances horizontally, but we had no idea that they were diving so deep."
"That was truly a surprise," said Thorrold -- a surprise that explains the need to keep the brain extra warm.
Thorrold and his fellow researchers were able to keep tabs on the school of rays by affixing tags to them. The tags were programmed to remain on the animals for nine months while measuring water temperature, depth, and the water's light levels. At nine months, the tags popped off, floated to the surface and beamed data back to the eagerly awaiting researchers via the ARGO satellite system.
"Data from the tags gives us a three-dimensional view of the movements of these animals, and a window into how they're living in their ocean habitat -- where they go, when, and why," Thorrold explained.
By further studying the rays, scientists hope they can better understand the links between the different layers of the ocean and the ecosystems unique to each.
"These rays, in effect, connect the surface, epipelagic layers in the ocean, with the deep -- the twilight zone," said Thorrold.
And this connection could offer new clues to the health of the oceans as a whole.
"Ultimately, answering whether these animals depend on the deep layers of the ocean for their feeding and survival could have major implications for their management and that of oceanic habitats," said co-author Pedro Afonso, a researcher from the University of the Azores.