During the day, the rays would spend time up at the surfacepresumably heating upimmediately before, and then again, after a deep dive. How else might these animals be dealing with the cold temperatures of the deep ocean?
A previous study in the 1970s found that several species of devil rays possess a physiological adaptation well-developed blood vessels around the cranial cavity that essentially serve as heat exchange systems. At the time, it was hypothesized that the rays must be using this adaptation to cool down rather than warm 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.
While it's not certain what the rays are doing at these depths, the dive profiles suggest that they're foraging on large numbers of fish that live in deeper waters.
"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 coming under increasing pressures from fishing, particularly in the Indian and Pacific Oceans. Manta gill rakers are targeted for Chinese medicine, and their cartilage is used as filler in shark fin soup.
"Ultimately, answering whether these animals depend on the deep layers of the ocean for
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Woods Hole Oceanographic Institution