Devil Rays Dive 6,000 Feet to Feast

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The Chilean devil ray has always been considered a shallow-water swimmer, but new research shows that the species frequently dives to depths of more than 6,000 feet (1,800 meters), likely in search of food.

Prior to this research, marine biologists thought Chilean devil rays (Mobula tarapacana) did not descend below 3,280 feet (1,000 m). However, new satellite tracking data now shows that these rays are among the deepest-diving marine animals. Researchers think the rays spend most of their time in shallow water to warm themselves, and then dive down to extreme depths in search of small crustaceans and fish to eat.

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"The fact that they were traveling so far horizontally was not necessarily surprising, but the diving behavior was very surprising," Simon Thorrold, a senior scientist at Woods Hole Oceanographic Institution in Woods Hole, Massachusetts, told Live Science. "What they're doing down there is the big unknown." [In Photos: The Wonders of the Deep Sea]

It's common for ocean predators to dive into the mesopelagic zone, a stretch of ocean water 656 to 3,280 feet (200 to 1,000 m) below the surface, to feast on squid and krill. But few predators make it deeper than the mesopelagic zone to the bathypelagic zone.

The bathypelagic zone is a huge food resource, home to an estimated 10 billion tons of prey fish, but few ocean predators can withstand the extreme pressure, cold temperatures and low oxygen levels.

In deep ocean zones, the water can be as cold as 37 degrees Fahrenheit (3 degrees Celsius). Deep-diving ocean predators must maintain a higher brain temperature than the surrounding water, so they are equipped with a special organ called the rete mirabile. The organ functions as a heat-exchange system that warms the animal's brain and helps it function better in the extreme cold. The organ also helps the animal see better when it's hunting in deep, dark waters.

Scientists were puzzled as to why Chilean devil rays, believed to be surface dwellers, had the organ. Researchers originally suggested that the rete mirabile helped to cool the brains of the rays living in the warm and shallow tropical waters.