LOOK into a swordfish's eye and you'll see a burning desire to go hunting. For these ocean predators heat their retinas, a trick that allows them to see prey better.

Most fish are cold-blooded, but swordfish can maintain the temperatures of their eyes and brains at between 19 and 28 °C, even in nearly freezing water. Now Kerstin Fritsches of the University of Queensland in Australia and colleagues have found out one reason why. The swordfish's retina is extremely sensitive to temperature, reacting five times faster to visual stimuli for every 10 °C increase in temperature (Current Biology, vol 15, p 64).

Swordfish typically hunt in dim light at depths of between 100 and 300 metres. Here their warm eyes can spot a squid 7 to 12 times faster than they would if they were not warmed, the team found. It is also likely that the fish's eyes react more quickly than those of the cold-blooded squid.

The comparatively great advantage of a warm retina may be why the fish evolved this unique strategy of warming only its eyes and brain, while similar fish with heat-sensitive eyes, such as tuna, warm more of their bodies.

From issue 2482 of New Scientist magazine, 15 January 2005