Polaro-cryptic mirror of the lookdown as a biological model for open ocean camouflage

With no object to hide behind in 3D space, the open ocean represents a challenging environment for camouflage. Conventional strategies for reflective crypsis (e.g., standard mirror) are effective against axially symmetric radiance fields associated with high solar altitudes, yet ineffective against asymmetric polarized radiance fields associated with low solar inclinations. Here we identify a biological model for polaro-crypsis. We measured the surface-reflectance Mueller matrix of live open ocean fish (lookdown, Selene vomer) and seagrass-dwelling fish (pinfish, Lagodon rhomboides) using polarization-imaging and modeling polarization camouflage for the open ocean. Lookdowns occupy the minimization basin of our polarization-contrast space, while pinfish and standard mirror measurements exhibit higher contrast values than optimal. The lookdown reflective strategy achieves significant gains in polaro-crypsis (up to 80%) in comparison with nonpolarization sensitive strategies, such as a vertical mirror. Lookdowns achieve polaro-crypsis across solar altitudes by varying reflective properties (described by 16 Mueller matrix elements m(ij)) with incident illumination. Lookdowns preserve reflected polarization aligned with principle axes (dorsal-ventral and anterior-posterior, m(22) = 0.64), while randomizing incident polarization 45° from principle axes (m(33) = -0.05). These reflectance properties allow lookdowns to reflect the uniform degree and angle of polarization associated with high-noon conditions due to alignment of the principle axes and the sun, and reflect a more complex polarization pattern at asymmetrical light fields associated with lower solar elevations. Our results suggest that polaro-cryptic strategies vary by habitat, and require context-specific depolarization and angle alteration for effective concealment in the complex open ocean environment.