85-4 Wednesday, Jan. 6 11:00 Another beautiful hypothesis slain by an ugly fact: Polarization vision does not increase the sighting distance of silvery fish JOHNSEN, S.*; GAGNON, Y.L.; MARSHALL, N.J.; CRONIN, T.W.; Duke Univ.; Univ. of Queensland; Univ. of Queensland; Univ. of Maryland, Baltimore firstname.lastname@example.org http://sites.biology.duke.edu/johnsenlab/
Although the function of polarization vision in terrestrial and benthic species is generally well-established, its purpose in pelagic species (squid and certain fish and crustaceans) is poorly understood. A long-held hypothesis is that polarization vision in the water column is used to break the mirror camouflage of silvery fish, since biological mirrors can change the polarization of reflected light. While the addition of polarization information likely increases the conspicuousness of silvery fish at close range, direct in situ evidence that silvery fish, or indeed any pelagic animals, are visible at longer distances using polarization vision than they are when using radiance vision is lacking. Here we combine in situ polarization imagery with novel models of visual detection that allow us to directly compare the sighting distances of underwater animals when viewed by polarization-sensitive and polarization-insensitive visual systems. We find that, in general, the sighting distances of the lateral surfaces of these animals is shorter for polarization vision than for radiance vision. The primary factors limiting the polarization sighting distances are the relatively low degrees of polarization of underwater light and the low polarization sensitivity of the photoreceptors themselves. The lateral surfaces of silvery fish may be particularly hard to detect via polarization vision in horizontal viewing because the incident light that is reflected is generally similar to the background in both radiance and polarization.