Meeting Abstract

68.2  Monday, Jan. 6 08:15  Through the looking glass: The structure of the oceanic light field and its implications for mirror-based camouflage JOHNSEN, S*; SAWICKA, E; REYNOLDS, R; STRAMSKI, D; Duke Univ.; Scripps Inst. Oceanography; Scripps Inst. Oceanography; Scripps Inst. Oceanography sjohnsen@duke.edu

Many pelagic species, in particular teleosts, have silvered lateral surfaces. While these can make an animal obvious under certain lighting conditions, mirrored sides are thought to primarily serve as a form of camouflage. The underlying argument for this is that the underwater light field is cylindrically symmetrical; thus a vertical mirror reflects a region of the water column that matches the region directly behind the mirror. Previous research on the biological mirrors themselves has shown that they are exquisitely adapted for camouflage if the light field is indeed symmetric, but the the symmetry of the light field itself has not been assessed. We first modeled the underwater light field using measured optical properties and radiative transfer software. This showed that the light field was surprisingly asymmetrical for most of the day for most latitudes. In addition, it was found that the maximum asymmetry at depth occurred not when the sun was near the horizon, but when it was 45 degrees above it. We validated these modeled results using a custom-built light meter on a cruise of the RV Kilo Moana off the coast of Hawaii in 2012. In addition to confirming the model, the measured light field showed that the asymmetry was even higher at longer wavelengths, suggesting that predators may gain an advantage by viewing prey in this spectral range. The modeled and measured asymmetry of the light field implies that mirror camouflage is not as successful as originally thought and suggests that there may be further refinements (such as the ability to dynamically tune the reflectance of the skin) that have not been previously considered.