Zoologists have answered the intriguing question of why swordfish keep their eyes warm while the rest of the body remains resolutely cold-blooded: it’s all the better to see their prey with.

Heat-assisted eyes work more than ten times faster than those cooled to the coldest deep-sea temperatures of around 3 ºC, report Kerstin Fritsches of the University of Queensland in Brisbane, Australia, and her colleagues. This increased ‘temporal resolution’ helps swordfish to catch dinner in the inky depths.

Researchers already knew that swordfish (Xiphias gladius) can selectively warm their eyes and brains. The fish have a specially adapted heating organ in the muscle next to their tennis-ball-sized eyes, which can raise temperatures in the surrounding tissue some 10-15 ºC above that of the water in which the fish is swimming.

But heating takes a lot of energy, and until now experts were confused as to why the swordfish goes to the trouble. Heat is lost around 3,000 times more quickly to water than to air, and of the 25,000 or so species of bony fish, only 22 – including swordfish, marlin, tuna and some sharks – have been found to possess any kind of heating mechanism.

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