Aquatic prey use countershading camouflage to match the visual background

Animals often have darker coloration on their top surface than on their underside ("countershading"). In terrestrial prey, this patterning eliminates the body's shadows to facilitate camouflage. In a freshwater fish that can change color, countershading provides optimal camouflage for different visual backgrounds and viewing angles rather than to reduce shadowing. Countershading camouflage may operate differently in terrestrial and aquatic habitats. An animal's 3D form, combined with the directional lighting that is typical of many natural light environments, often results in the production of self-shadows, which may increase prey detectability to visual predators or vice versa. In terrestrial animals, countershading patterning, a luminance gradient from dark dorsal to pale ventral pigmentation, acts to counterbalance this effect by essentially reversing the distribution of light incident across the body surface. Although widespread among aquatic predators and prey, it is unclear whether countershading facilitates camouflage through elimination of self-shadows (self-shadow concealment [SSC]), enhances the match between an animal's radiance and that of the background for multiple viewing angles (background matching [BM]), or a combination of both. We used clay models of a color-changing freshwater fish to determine the optimal patterning for SSC in different light environments, and we compared this to the skin reflectance profile of fish held under the same conditions. Fish adjusted their countershading pattern in response to changes in the light environment, but the observed reflectance profiles did not match the modeling predictions for optimal SSC. Thus fish adjusted their body pigmentation to match the viewing background rather than to conceal their ventral shadows. We suggest that different selection pressures resulting from the dissimilar characteristics of light in air and water have resulted in convergence of similar countershading phenotypes in terrestrial and aquatic prey.