JOHNSEN, S; Duke University: Mirrors or colors? Successful fashions for crypsis in the pelagic realm

Mirrored and colored surfaces are common strategies for crypsis in pelagic habitats. In this study we examine the relative merits of these two forms of crypticity under different lighting regimes. Underwater radiance distributions were calculated using inherent optical properties measured in coastal waters 80 km off the coast of New Hampshire, USA. These radiance distributions were then used to calculate optimally cryptic diffuse and specular reflectance spectra as a function of depth, solar elevation, viewing angle and azimuth. In addition, the visibility of organisms cryptic in one situation viewed in a different situation was calculated, using the Atlantic Cod, Gadus morhua, as the viewer. In contrast to benthic organisms, pelagic organisms cryptic under one set of viewing conditions were quite visible under a different set, particularly when viewed from a different azimuth. The crypsis afforded by mirrored surfaces was generally more robust than that due to colored surfaces. However, because mirrored surfaces could not be perfectly cryptic when viewed in the azimuth of the sun, neither strategy clearly outperformed the other. In general, crypsis by colored or mirrored surfaces was of limited success in near-surface water, which may help explain both the predominance of transparent species in near-surface pelagic habitats and the vertical migration of many colored and mirrored species. The results also show that three common foraging strategies – circling, crepuscular activity, and driving prey toward the surface – all increase the visibility of cryptically colored or mirrored prey.