Chilean devil rays (Mobula tarapacana) have been found at almost twice the depths previously suspected, overturning our ideas on these strangely structured cartilaginous fishes.
Rays spend most of their time near the surface, basking in the warmth. Since this is also where it is easiest for us to find them, most of what we know about ray behavior has been in this environment. Nevertheless, it has previously been established that many ray species dive into the mesopelagic zone, 200-1000m beneath the surface of the ocean, to feed.
That is common behavior for large marine predators. However, few are able to go deeper, into the cold waters and enormous pressures of the bathypelagic zone. Yet when Dr Simon Thorrold of the Woods Hole Oceanographic Institution put satellite tracking tags on 15 Chilean devil rays off the coast of northern Africa some were found to reach depths of almost 2000m.
In Nature Communications Thorrold reported that the rays dove at speeds of 6ms-1 (compared to 1-2ms-1 for deep diving whales) before rising in a series of steps, foraging in between. Dives last up to three hours.
"So little is known about these rays," said Thorrold. "We thought they probably traveled long distances horizontally, but we had no idea that they were diving so deep. That was truly a surprise."
Dives predominantly took placed during daylight. This would make little difference in terms of the light available to feed by at such depths. Thorrold thinks diving into the frigid bathysphere is easier with a chance to warm oneself before and after.
Although the observations were unexpected, they do explain the mystery of the rays' retia mirabilia, a sort of heat exchange system around the brain. Deep diving species need to keep their brain warm as they plunge to depths where the temperature can be below 4°C. Retia mirabilia keep brain temperatures up at the expense of the rest of the body, raising the question of why apparently surface dwelling species such as the mobula genus of rays would have them.
“The presence of a heat-exchange system in these species was perplexing as mobulids were thought to be surface-dwelling filter feeders inhabiting warm temperate to tropical epipelagic waters,” the paper notes. “The authors concluded that the cranial rete cooled rather than warmed the brain as these rays were known to bask at the surface in tropical waters.” However, the dive observations reversed this thinking.
"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 co-author Pedro Afonso.
