July 2, 2014
Chilean Devil Rays Found To Be Among The Deepest Sea Divers
April Flowers for redOrbit.com - Your Universe Online
Researchers have always believed that Chilean devil rays (Mobula tarapacana) mostly lived near the ocean's surface because they are most often observed gliding through shallow, warm waters. According to a new study led by the Woods Hole Oceanographic Institute (WHOI), that might not be the case. Their findings, published in Nature Communications, show that these are actually among the deepest-diving ocean animals.
"So little is known about these rays," said Simon Thorrold, a biologist at WHOI. "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 record the movements of 15 Chilean devil rays as they moved through the central Atlantic Ocean, the team of international researchers used pop-up satellite archival transmitting tags during 2011 and 2012. The tags are attached to the animals with darts.
"We actually jump in the water and tag them ourselves," he said. "We take a polespear, we swim down and deploy the tag. The animal's not restrained in any way."
The tags remain attached to the animal for up to 9 months, and record a wide variety of measurements, including water temperature, depth, and light levels of the water. When a tag detaches from the animal, it rises to the surface and transmits its data via the ARGO satellite system to computers back 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," Thorrold added.
Devil rays are large animals, growing to be as large as 13 feet across. They are nomadic, traveling vast areas of the ocean. The team retrieved data showing that individual rays routinely descended through the depths at speeds of up to 13.4 miles per second. They reached depths of nearly 1.24 miles in waters less than 39.2 degrees Fahrenheit.
The dives made by the devil rays typically followed one of two patterns. The first, and most common, pattern found the animals descending to the maximum depth. This was followed by a slower, stepwise return to the surface. The total dive time was between 60 and 90 minutes and only happened once in a 24-hour period. The second, less common, pattern saw the devil rays descend and remain at a depth of up to 3,280 feet for as long as 11 hours at a time.
Individuals did vary from these patterns. In particular, one ray made six consecutive dives, one a day, below 4,600 feet. Thorrold believes this to be near the limit of the diving behavior of devil rays.
Very few fish are known to dive this deep. Among those that do, the record holding dive is currently held by the beaked whale at almost 2 miles.
During daylight hours, the rays spend time near the surface, especially before and immediately after a deep dive. The researchers suggest this is a means of heating up their bodies.
In 1976, researchers discovered that many species of rays have a network of well-developed blood vessels around the cranial cavity that essentially serve as heat exchange systems. According to the BBC, this network is called the rele mirabile. The scientists of the time assumed this physiological adaptation was a means of cooling down their bodies, rather than warming them 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.
The scientists still are not certain what drives the rays to dive so deeply, however, they say the behavior suggests that the animals are foraging on large numbers of deep-water fish.
"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 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."
"We're hoping that the rays are going to lead us to some really interesting biology, at depth, that we wouldn't necessarily have been able to find ourselves - but we can use the rays to lead us to the biologically interesting layers and zones within those depths," Thorrold said. "These rays, in effect, connect the surface, epipelagic layers in the ocean, with the deep - the twilight zone."
Commercial fishing, especially in the Indian and Pacific Oceans, is putting increasing pressure on the devil ray populations. Chinese medicine prizes the manta grill rakers, and the cartilage is used as 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," added 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).
So far, scientists know very little about the life span or breeding age of the devil rays. Researchers assume that they, like other large rays, have only one pup per litter every two years.
"With those kinds of low reproductive rates, any type of mortality is going to have a big impact on the species," Thorrold 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 biology and ecology of these rays."
Image 2 (below): Mainly thought to be surface dwellers, Chilean devil rays (Mobula tarapacana) are actually among the deepest-diving ocean animals. Credit: Jack Cook, Woods Hole Oceanographic Institution