Tags reveal Chilean devil rays are among ocean's deepest divers
Mainly thought to be surface dwellers, Chilean devil rays (Mobula tarapacana) are most often seen gliding through shallow, warm waters. But a new study by KAUST scientists and international colleagues reveals that these large and majestic creatures are actually among the deepest-diving and fastest-diving ocean animals.
"Very little is known about devil rays," said Michael Berumen, a biologist at KAUST and senior author of the paper, which was published July 1, 2014, in the journal Nature Communications. "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."
The researchers utilized pop-up satellite archival transmitting tags (PSATs) to record the movement patterns of 15 Chilean devil rays in the central North Atlantic Ocean during 2011 and 2012. The tags, which stay on the animals for up to 9 months, also measure water temperature, depth, and light levels of the waters. Once the PSATs detach or "pop off" from the tagged animal, they float to the surface and transmit data via the ARGOs satellite system back to computers on shore.
"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," said Simon Thorrold, a first author to the paper and a biologist from the Woods Hole Oceanographic Institution (WHOI) in the United States.
TRACKING THE RAYS IN THE OCEAN
Devil rays, which can grow as large as four meters (13 feet) across, are ocean nomads travelling large areas of the ocean. Dive data from the tags showed that individuals are also routinely descending at speeds up to 6 meters per second (13.4 miles per hour) to depths of almost 2,000 meters (1.24 miles) in water temperatures less than 4 degrees Celsius (39.2 degrees Fahrenheit).
The deep dives generally followed two distinct patterns. The most common involved descent to the maximum depth followed by a slower, stepwise return to the surface with a total dive time of 60 to 90 minutes. The tagged rays generally only made one such dive during a 24-hour period. In the second dive pattern, individuals descended and then remained at depths of up to 1,000 meters (3,280 feet) for as long as 11 hours.
During the day, the rays would spend time up at the surface—presumably heating up—immediately before and then again after a deep dive. This may explain how these animals deal with the cold temperatures of the deep ocean.
Additionally, a previous study in the 1970s found that several species of devil rays possess a physiological adaptation associated with thermoregulatory capabilities—well-developed retia mirabilia (networks of blood vessels) around the cranial cavity.
"They are basically heat exchange systems that allow animals to become endothermic, or in some sense warm blooded," explained Berumen. "We see them in other deep-diving elasmobranchs, such as mako and white sharks."
Though it was hypothesized at the time that the rays must be using this adaptation to cool down rather than warm up, the Berumen and his colleagues show that the adaptation could facilitate dives into deep, cold waters, thus solving a decades-old puzzle.
"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.
QUESTIONS ABOUT RAYS CONTINUE
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. Earlier this year, a KAUST-led team of scientists described this fish abundance in a report also published in Nature Communications.
"There's an enormous amount of biomass in the deep ocean that we're only starting to understand the significance of," said Camrin Braun, a coauthor and a graduate from KAUST's Marine Science program. "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." Braun is currently a PhD student in the MIT/WHOI Joint Program in Oceanography.
"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," added Pedro Afonso, a coauthor of the paper and 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).
Devil rays are coming under increasing pressures from the 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.
Little is known about the life span of devil rays or at what age they reproduce. Like other large rays, devil rays are thought to have just one pup born per litter about every two years.
"With such low reproductive rates, any increase in natural mortality rates will have a big impact on the species," Berumen said. "We don't know enough about devil rays to even know if we should be worried about their status. There are lines of evidence to suggest we ought to be worried, or at least that we should be trying to learn more about the basic biology and ecology of these rays. Movement ecology studies can be challenging, but the results are crucial to inform conservation efforts. And the more species we tag, the more we learn about their remarkable behaviors. Without such knowledge, it's hard to know even where to begin to protect a species."
Researchers from Massachusetts Marine Fisheries in the United States also contributed to the study. Funding for this research came from the National Science Foundation, The Harrison Foundation, Rodney and Elizabeth Berens, KAUST, the Portuguese Foundation for Science and Technology/ Ministry of Education and Science, the LARSyS Strategic Project, and WHOI.