In the frigid gloom of the ocean, sharp vision can mean the difference between eating and starving.
In the struggle for survival, swordfish and some other large marine predators have evolved huge instruments of sight: eyes the size of tennis balls, even grapefruit. Once it spots its prey, this gladiator of the sea can charge into a school of fish with its muscular body, flicking its head from side to side and slashing the smaller creatures to pieces.
Thanks in part to its oversized eyes, swordfish can spot tiny changes in brightness caused by barely detectable movements of fish, squid and crustaceans hiding in a world where everything is as dim as starlight and food can be hard to come by.
"The open ocean has been described as essentially a desert," said Kerstin Fritsches, a researcher at the University of Queensland in Australia.
To get enough to eat, Fritsches said, swordfish dive to 2,300 feet, where the temperature is barely above freezing.
Most of the world's 25,000 species of bony fish are coldblooded, but swordfish and 21 other species of bony fish have developed various ways of keeping warm.
Bigeye tuna and yellowfin tuna warm their entire bodies using a heat -exchange system: Blood rushing to the gills to capture oxygen gets cooled to the temperature of the water, then absorbs heat from nearby vessels carrying blood warmed by the fish's swimming muscles.
But swordfish, as well as other tuna and shark species, have developed a more specialized adaptation. "It's not clear why, but they don't seem to need whole-body heating," Fritsches said in a telephone interview.
Instead, she said, the swordfish's eyes are warmed by a muscle behind them that no longer plays a role in eye movement.
A layer of fat covering the eye and brain helps hold that heat in, keeping the eyes 10 to 15 degrees Celsius warmer than the surrounding water. By comparison, a bigeye tuna's eye is only 3 to 7 degrees warmer than the water.
Scientists had known for more than 20 years that swordfish have this built-in heater but did not understand why. Now Fritsches believes she has the answer. Her study, published last month in the journal Current Biology, indicates that swordfish can see 10 times better than fish whose eyes are the same temperature as the water.
The warmer the eye, she explained, the faster it can move and the better the swordfish can detect rapidly moving prey.
Fritsches and her team figured this out by comparing the rate at which fishes' retinas responded to a flickering light as the temperature was lowered. They found that the retinas of yellowfin tuna, which feed close to the surface, and bigeye tuna, which, like the swordfish, dive deep for food, were less sensitive to temperature. A drop of 10 degrees resulted in almost a 40 percent reduction in a tuna's ability to see fast-flickering light, but the swordfish's seeing ability was reduced more than 80 percent in the same temperature range.
With warmed eyes, however, swordfish at more than 900 feet deep can detect a fast-flickering light seven times faster than they could with cold eyes.
