Devil rays — long thought to be ocean surface dwellers — can actually make rapid dives that are among the deepest recorded for any marine animal.
The fish venture into deeper waters, almost 2,000 m below where temperatures can drop to less than 4°C, to forage for food in an area abundant with biomass.
A team of researchers, led by Simon Thorrold from the Woods Hole Oceanographic Institution and including Michael Berumen from the King Abdullah University of Science and Technology (KAUST) in Saudi Arabia, tracked 15 Chilean devil rays (Mobula tarapacana) off the northwest coast of Africa for nine months using satellite tagging, and published their findings in Nature Communications1.
"Very little is know about devil rays," says Berumen. "We suspected that they travelled long distances horizontally, but we had no idea that they were diving so deep or so fast. That was truly a surprise."
Several species of devil rays have a specialized organ called retia mirabilia around the cranial cavity that allows the warming of the brain, but its function was unclear in an animal thought to inhabit shallow waters.
The researchers, however, found that before and after these deep dives, the rays would spend a long time near the water surface during daytime, presumably to soak up heat and warm up for the next dive.
The warm brain is responsible for the brain activity and visual acuity that give the rays an advantage in cold waters.
“We lack even basic information on our study species, and it is impossible to devise effective management strategies without knowing where exactly they are likely to be vulnerable to fishing operations,” adds Thorrold. Listed as endangered, this understanding of the rays’ behaviour and movement may help in their conservation efforts.
This story was updated on 6 July, 2014, to add a quote from Michael Berumen.
Thorrold, S. R. et al. Extreme diving behaviour in devil rays links surface waters and the deep ocean. Nature Commun. (2014) doi:10.1038/ncomms5274.