Through the looking glass: Are silvery fish safe from viewers with polarization vision?

Many coral reef and open-ocean fish have mirrored sides built from stacks of guanine crystals. Because the underwater light field is approximately symmetric around the vertical axis, vertical mirrors are an excellent form of camouflage. Mirrors of this orientation reflect light from the same angle of elevation (though from the opposite azimuth), so the reflected image matches what would be seen if the animal were transparent. However, most surfaces affect the polarization of light that is reflected from it. Smooth surfaces strongly affect on the polarization of the reflected light, and rough surfaces depolarize the light. Either effect can be seen by animals with polarization vision, thus breaking the camouflage. We examined how the silvery surfaces of fish affect the polarization of light in two ways. First, using a custom-built in situ polarization imaging system, we photographed various species of fish with mirrored sides on the Great Barrier Reef. Second, using transfer matrix theory, we modeled how the polarization of light reflected from stacks of guanine platelets depended on the number of plates and the distribution of platelet thicknesses and angles. The in situ imaging showed that many fish species were less conspicuous to animals with polarization vision than would be predicted. The optical modeling showed that, while stacks of 10 or 20 platelets strongly affected the polarization of light for most angles of incidence, stacks of 50 or greater platelets with a moderate amount of randomness in both platelet thicknesses and angles reflected nearly 100% of both polarization components. Thus, these structures acted as polarization-preserving reflectors that would provide camouflage to both animals with normal vision and polarization vision. To our knowledge, this is the first example of a polarization–preserving reflector in nature