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.

动物的三维形态结合多数自然光照环境中典型的定向光照，常会产生自身阴影（self-shadows），此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，反荫蔽图案（countershading patterning）指的是从背部深色到腹部浅色的亮度梯度色素沉着模式，其本质是通过逆转入射光线在身体表面的分布来抵消上述效应。尽管反荫蔽在水生捕食者与猎物类群中广泛分布，但目前仍不清楚其究竟是通过消除自身阴影遮蔽（self-shadow concealment [SSC]）实现伪装，还是通过在多视角下让动物的辐射亮度与背景辐射亮度相匹配（背景匹配 [background matching [BM]]）来达成目的，亦或是两者协同作用的结果。本研究以可变色淡水鱼的黏土模型为对象，探究了不同光照环境下实现SSC的最优图案，并将该结果与相同条件下饲养的鱼类皮肤反射光谱特征进行了对比。实验结果显示，鱼类会根据光照环境的变化调整其反荫蔽图案，但观测到的皮肤反射光谱特征与最优SSC的模型预测结果并不相符。由此可见，鱼类调整身体色素沉着的目的是匹配观测视角下的背景，而非遮蔽腹部阴影。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类群趋同演化出了相似的反荫蔽表型。