Cryptic coloration and mirrored sides as camouflage strategies in near‐surface pelagic habitats: Implications for foraging and predator avoidance

Abstract

Mirrored and colored surfaces are common adaptations for crypsis in pelagic habitats. Although highly successful when optimized for a particular situation, either may become less successful if it is then viewed in a different situation. In this study we examine the relative robustness of these two strategies by determining how visible an organism becomes when viewed under optical conditions different from those under which the camouflage is optimal. Underwater radiance distributions were calculated using inherent optical properties measured in coastal waters 80 km off the coast of New Hampshire. 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. Then the visibilities of organisms cryptic in one situation viewed in a different situation were calculated, using the Atlantic Cod, Gadus morhua, as the viewer. In contrast to benthic organisms, pelagic organisms cryptic under one set of optical 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 resulting from colored surfaces. However, because mirrored surfaces could never 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 not robust in nearsurface 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.

A highly distributed Bragg stack with unique geometry provides effective camouflage for Loliginid squid eyes

“…The squid silver structure, with its slightly curved Bragg stacks, primarily appears to exhibit specular reflectance with some diffuse scattering. Placed in the context of the modelled radiance from the work of Johnsen & Sosik [18], the reflectances of the squid silver tissue that we measured is a good average of the solutions to camouflage in the top 50 m of open water ( figure 11). The 84 per cent average reflectance across all wavelengths produced by the tissue of the squid eye appears to represent a good compromise strategy between the optimal reflectance at the surface and at 50 m depth, which overlaps with the depth distribution of loliginids [19].…”

Plasticity in coloration as an antipredator strategy among zooplankton

“…This includes transparency and crypsis (Johnsen 2001), as well as behavioral adaptations, like hiding in dark, deep water during daytime (Gliwicz 1986).…”

“…Optical conditions change with depth and time, turbidity, and viewing angle. Crypsis will therefore be depth and time dependent (Johnsen 2002). In well-illuminated epipelagic layers, transparency is the only form of camouflage that is successful from all viewpoints (Johnsen 2001), and epipelagic zooplankton species are generally transparent (Herring and Roe 1988).…”

“…Crypsis will therefore be depth and time dependent (Johnsen 2002). In well-illuminated epipelagic layers, transparency is the only form of camouflage that is successful from all viewpoints (Johnsen 2001), and epipelagic zooplankton species are generally transparent (Herring and Roe 1988). Mesopelagic species, on the other hand, tend to be half-red (red coloration of guts and opaque tissue), while bathypelagic species are uniformly red, purple, or black (Herring and Roe 1988).…”

“…Approximately 90% of all the animals in the mesopelagic zone (200-1,000 m) are bioluminescent, primarily emitting blue light (400-479 nm) (Herring 1983). In this light regime, red, purple, or black coloration may be more cryptic than transparency, because even a perfectly transparent object causes surface reflection (Johnsen 2001).…”