The linearly polarized light field in clear, tropical marine waters: Spatial and temporal variation of light intensity, degree of polarization and e-vector angle

Sensitivity to polarized light is widespread among marine animals, includin g crustaceans, cephalopods and some fishes. They use this ability to orient and find prey, and possibly for a number of other visual tasks. Unlike the ultraviolet-sensitive polarization receptors of most insects, the polariza tion receptors of marine invertebrates tend to be maximally sensitive near 500 nm, suggesting that polarized light in water differs from that in air. The underwater field of partially linearly polarized light has been studied for nearly 50 years, but data are still limited and sparse. We measured th e submarine polarized light field from 350 to 600 nm throughout the day on a coral reef in the Florida Keys at a depth of 15 m using the underwater la boratory Aquarius as a research platform. Our results show that the angle o f polarization as viewed along any given line of sight at this depth is a r elatively simple function of solar position and that the degree of polariza tion is greatest 60-90 degrees from the sun. Both e-vector angle and degree of polarization vary only slightly with wavelength, although light is some times less polarized in the ultraviolet. Since light is most intense at med ium wavelengths and polarization is nearly maximal at these wavelengths, in vertebrate polarization photoreceptors are spectrally well placed. Also, th e relative spectral constancy of the angle and degree of polarization suppo rts fish polarization sensitivity, which relies on spectrally diverse photo receptor sets.