Chilean devil rays-known primarily as ocean surface dwellers-regularly dive to extreme depths according to research published in Nature Communications this week.
A number of apex, deep-diving, predators, such as the great white shark, are known to maintain brain temperatures above that of the surrounding water. It has long been proposed that brain activity and visual acuity, allowed by higher brain temperature, conveys significant selective advantages when foraging in cold, deep water. A specialised organ called the retia mirabilia is thought to allow some rays to warm their brains through heat exchange but its purpose in a species, thought to primarily inhabit shallow, warm waters, was unclear.
Simon Thorrold and colleagues documented the movements of 15 Chilean devil rays off the northwest coast of Africa, over a period of several months, using satellite tags. The measurements revealed frequent dives to depths of up to 2 km-among the deepest dives recorded for marine animals-where the temperature can reach as low as 3°C.
The recorded dive profiles suggest that the rays are foraging at those depths where an abundant biomass is present. Both before and after deep-water expeditions, the rays tended to spend a significant amount of time near the surface during daytime, presumably to warm-up.
In addition to providing an explanation for the presence of an elaborate heat-exchange system in the ray, the observed diving behaviour provides an important ecological link between surface waters and the deep ocean. As the group to which the devil ray belongs has also recently been listed as an endangered species, an understanding of their behaviour may become crucial to future conservation efforts.