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Hot-blooded killers. Large ocean predators such as these southern bluefin tuna may warm their eyes to help see prey in dim ocean waters.

Hot Eyes for Cold Fish

Some cold water predators such as swordfish spend a lot of energy keeping their eyes warm, but the purpose of this ocular heating has puzzled scientists for years. Now a new study shows that warm eyes endow these hunters with superior vision, helping them capture quick-moving prey.

To warm the blood traveling to the eyes and brain, swordfish have converted a muscle once used to move their eyes into an organ dedicated to generating heat. The mechanism can keep their eyes more than 10 degrees warmer than ambient water temperature.

To determine why these creatures devote so much energy to this process, neuroscientist Kerstin Fritsches of the University of Queensland and colleagues examined the retinas of freshly caught swordfish. They placed each retina in a water bath, connected it to electrodes, and measured its electrical response to rapid flashes of light. The researchers report in the 10 January issue of Current Biology that eyes warmed to their natural heated state in swordfish (around 20 degrees Celsius) registered flickers of light more than 10 times faster than eyes kept at deepwater ocean temperatures (about 10 degrees Celsius). Experiments on bigeye and yellowfin tuna retinas yielded similar results, though swordfish eyes proved the most sensitive to temperature change.

Such a rapid visual response is like a fast shutter speed in a camera—it helps spot fast-moving objects, especially in the dim light of the deep ocean, says Fritsches. Warm eyes, then, should give swordfish, which regularly dive to 500 meters, a critical edge over prey. "And because none of the squid or other fish have a way to keep their eyes warm, they won't see the swordfish coming," she says.

Marine biologist Kim Holland of the University of Hawaii at Manoa agrees with this interpretation. "Animals don't go through the processes of developing and maintaining these complicated systems unless it really makes a difference to them," he says. Holland also praises the authors' methodological approach: "This is the next best thing to being able to swim around with the fish and see what it's up to."

Related sites
Vision in Billfish
Kerstin Fritsches' homepage