37.3 Saturday, Jan. 5 Why do giant squid have giant eyes? JOHNSEN, S.*; NILSSON, D.-E.; WARRANT, E.J.; Duke Univ.; Lund Univ.; Lund Univ. email@example.com
Giant and colossal deep-sea squid (Architeuthis and Mesonychoteuthis) have the largest eyes in the animal kingdom, but there is no explanation for why they would need eyes that are nearly three times the diameter and 27 times the volume of those of any other extant animal. While these eyes may simply be scaled-up version of the eyes of smaller squid, studies from vertebrate species show there is a significant negative allometry for eye size, with eye diameter peaking at roughly 9 cm and pupil diameter peaking at 3 cm. Here we develop a theory for visual detection in pelagic habitats, and demonstrate that such giant eyes are unlikely to evolve for detecting mates or prey at long distance, but are instead uniquely suited for detecting very large predators, such as sperm whales, either as shadows against the dim ambient light or via bioluminescence stimulated by the motion of the animals. We also provide photographic documentation of an eyeball of about 27 cm and a pupil diameter of 9 cm in the giant squid Architeuthis, and predict that, below 600 m depth in clear oceanic waters, it would allow detection of sperm whales at distances exceeding 120 m. With this long range of vision, giant squid can monitor a surrounding sphere of more than 7 million m3 of water, and get an early warning of approaching sperm whales. Interestingly, the distance at which giant squid are predicted to detect sperm whales visually is comparable to the sonar range of the whales. Our results thus suggest that the enormous eyes of giant squid may have evolved in an arms race with the sonar of large toothed whales. The equally enormous eyes of certain ichthyosaurs are also discussed in the context of the detection of large deep-sea targets in low-light conditions.