“Warm Eyes Provide Superior Vision in Swordfishes”, 2005-01-11 (; backlinks):
Large and powerful ocean predators such as swordfishes, some tunas, and several shark species are unique among fishes in that they are capable of maintaining elevated body temperatures (endothermy) when hunting for prey in deep and cold water1, 2, 3.
In these animals, warming the central nervous system and the eyes is the one common feature of this energetically costly adaptation4. In the swordfish (Xiphias gladius), a highly specialized heating system located in an extra-ocular muscle specifically warms the eyes and brain up to 10℃–15℃ above ambient water temperatures2, 5.
Although the function of neural warming in fishes has been the subject of considerable speculation1, 6, 7, the biological importance of this unusual ability has until now remained unknown. We show here that warming the retina substantially 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 10× 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.