Data from: Aquatic prey use countershading camouflage to match the visual background

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.动物的三维躯体形态，结合多数自然光照环境中典型的定向照明，常会产生自身阴影；这类阴影既可能提升视觉捕食者对猎物的侦测概率，反之亦然。对于陆生动物而言，反荫蔽（countershading）图案指从深色背部到浅色腹部的亮度梯度色素沉着模式，其本质上通过反转入射光线在躯体表面的分布，抵消上述自身阴影带来的影响。尽管反荫蔽在水生捕食者与猎物中广泛存在，但目前仍不清楚其究竟是通过消除自身阴影（self-shadow concealment [SSC]）实现伪装，还是在多种观测视角下提升动物辐亮度与背景辐亮度的匹配度（background matching [BM]），亦或是两者兼具。本研究使用可变色淡水鱼的黏土模型，探究不同光照环境下实现SSC的最优图案，并将其与相同条件下饲养的鱼类的皮肤反射光谱特征进行对比。研究发现，鱼类会根据光照环境的变化调整其反荫蔽图案，但观测到的反射光谱特征与最优SSC的模型预测结果并不匹配。由此可见，鱼类调整躯体色素沉着的目的是匹配观测背景，而非隐藏其腹部阴影。我们推测，空气与水中光照特性的差异所带来的不同选择压力，导致陆生与水生猎物的反荫蔽表型出现了趋同演化。