Fast-swimming predatory swordfish heat up their eyeballs so they can see rapidly moving prey better, says new research published today.
Most other fish are poikilothermic so they have the same body temperature as the water they live in. Swordfish, like tunas and certain sharks, are unusual in the fish kingdom in that they are endothermic and manage to maintain their body temperature several degrees higher than that of the surrounding water.
In some species and habitats, the body temperatures of endothermic fish can be over 10C higher than the surrounding water.
Previous studies have shown that much of the heat produced is localised around the eye and brain, but until now it wasn't known exactly why this was the case.
Now a new study from Fritsches, Brill and Warrant in the journal Current Biology has found out the answer: they heat up an extraocular muscle and warm the eyes and brain up to 10C-15C above ambient water temperature to significantly improves temporal resolution, and hence the detection of rapid motion, in fast-swimming predatory fishes such as the swordfish.
"Depending on diving depth, temporal resolution can be more than ten times greater in these fishes than in fishes with eyes at the same temperature as the surrounding water. The enhanced temporal resolution allowed by heated eyes provides warm-blooded and highly visual oceanic predators, such as swordfishes, tunas, and sharks, with a crucial advantage over their agile, cold-blooded prey."
For more details see the paper: Kerstin A. Fritsches, Richard W. Brill, and Eric J. Warrant. (2005) - Warm Eyes Provide Superior Vision in Swordfishes. Current Biology, vol. 15.