Researchers examining digital tags attached to Chilean devil rays have recently found the creatures are one of the deepest diving in the ocean.
The findings were part of a paper published this month by researchers at the Woods Hole Oceanographic Institution (WHOI).
"So little is known about these rays," said Simon Thorrold, a biologist at WHOI and one of the authors of the paper published in Nature Communications. "We thought they probably travelled long distances horizontally, but we had no idea that they were diving so deep. That was truly a surprise."
To make their findings the researchers looked at data from pop-up satellite archival transmitting tags that tracked 15 Chilean devil rays in the central North Atlantic. The tags stayed with the animals for up to nine months. These high-tech tags also measured "water temperature, depth, and light levels of the waters," the University reported. Once the tags fall off they float to the surface and beam radar back to the researchers.
"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 said.
Data from these tags showed the rays descended at speeds of 13.4 miles per hour and reached depths of 2,000 meters. The dives generally followed two distinct patterns. The most common type of dive lasts between 60 and 90 minutes while the second kind can last as long as 11 hours.
In the 1970s researchers discovered several species of rays had well-developed blood vessels around the cranial cavity that could serve as a heat exchange system. At the time, scientists hypothesized the rays used this to cool down rather than heat up.
"Rays were always seen in very warm water up at the surface, so why would they need an adaptation for cold water? Once we looked at the dive data from the tags, of course it made perfect sense that the rays have these systems. Sometimes they're down diving for two or three hours in very cold water - two to three degrees Celsius (35.6 to 37.4 degrees Fahrenheit)," Thorrold said.
Researchers believe the rays are making the dives in an effort to find prey.
"There's an enormous amount of biomass in the deep ocean that we're only starting to understand the significance of," said co-author Camrin Braun, a graduate student in the MIT/WHOI Joint Program in Oceanography. "This paper suggests that devil rays are aware of and regularly exploit this resource, which demonstrates an unexpected new link between the surface and deep ocean."
Devil rays are under mounting pressure from fishing. Their gill rakers are often used in Chinese medicine and their cartilage is used as a filler in shark fin soup.
"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 at the Institute of Marine Research (IMAR) at the University of the Azores and the Laboratory of Robotics and Systems in Engineering and Science (LARSyS).
