Meeting Abstract

P1.78  Friday, Jan. 4  The polaro-cryptic mirror: a biological adaptation for open-ocean camouflage BRADY, P/C*; TRAVIS, K/A; MAGINNIS, T; CUMMINGS, M/E; University of Texas at Austin; University of Texas at Austin; University of Portland; University of Texas at Austin pbrady@physics.utexas.edu

With no object to hide behind in 3D-space, the open ocean represents a challenging environment for camouflage. Camouflaging to solar illumination poses particular problems due to the dynamic polarization aspect of the light field. Near the water surface, the degree of polarization can be up to ~70%, and the complex polarization distribution changes throughout the day. Polarization vision research predicts the importance of polarized light in predator-prey interactions. Presently understood underwater crypsis strategies (e.g. vertically held mirror-like surfaces) are effective against axially-symmetric spectral irradiance fields present in high solar elevation conditions (noon), yet ineffective against asymmetric polarized light fields present at lower solar elevation conditions (sunrise, sunset) in the first 15 meters of the open ocean. Here, we evaluated polarization camouflage strategies by measuring the Mueller matrix (a mathematical description of a surfaces polarization reflection property) of an open-ocean mirror-like fish, the Lookdown (Selene vomer). We calculated the range of Mueller matrix values that would maximize crypsis by approximating a fish as a vertically held plate and summing the polarization contrast values for all other predator-prey viewing angles. Our results show that the Lookdown’s Mueller matrix values occupy the minimization basin of the calculated polarization-contrast space, and suggest an evolutionary adaptation for polaro-crypsis. Lookdown reflectance properties exhibit significant gains in polaro-crypsis (up to 80%) from other reflective crypsis strategies by incorporating angle-specific depolarization and transformation of incident polarization.