Data from: Three-dimensional midwater camouflage from a novel two-component photonic structure in hatchetfish skin

The largest habitat by volume on Earth is the oceanic midwater, which is also one of the least understood in terms of animal ecology. The organisms here exhibit a spectacular array of optical adaptations for living in a visual void that have only barely begun to be described. We describe a complex pattern of broadband scattering from the skin of Argyropelecus sp., a hatchetfish found in the mesopelagic zone of the world's oceans. Hatchetfish skin superficially resembles the unpolished side of aluminum foil, but on closer inspection contains a complex composite array of sub-wavelength-scale dielectric structures. The superficial layer of this array contains dielectric stacks that are rectangular in cross-section, while the deeper layer contains dielectric bundles that are elliptical in cross-section; the cells in both layers have their longest dimension running parallel to the dorsal-ventral axis of the fish. Using the finite-difference time-domain approach and photographic radiometry, we explored the structural origins of this scattering behavior and its environmental consequences. When the fish’s flank is illuminated from an arbitrary incident angle, a portion of the scattered light exits in an arc parallel to the fish's anterior-posterior axis. Simultaneously, some incident light is also scattered downward through the complex birefringent skin structure and exits from the ventral photophores. We show that this complex scattering pattern will provide camouflage simultaneously against the horizontal radially symmetric environmental light in this habitat, the predatory bioluminescent searchlights that are ubiquitous here. The structure also directs light incident on the flank of the fish into the downwelling, silhouette-hiding counterillumination of the ventral photophores.地球上体积最大的生境为海洋中层水体（oceanic midwater），同时也是动物生态学领域认知程度最低的生境之一。此处栖居的生物演化出了一系列极为精妙的光学适应策略，以在这片视觉荒漠中生存，而这类适应机制迄今仅得到了极为初步的研究与描述。我们对银斧鱼属（Argyropelecus sp.）——一种广泛分布于全球海洋中层带（mesopelagic zone）的斧鱼——的皮肤展开研究，揭示了其皮肤所呈现的复杂宽带散射（broadband scattering）模式。斧鱼的皮肤外观近似未抛光的铝箔，但在高倍镜下可观察到，其由复杂的亚波长尺度介电结构复合阵列构成。该阵列的表层包含横截面呈矩形的介电堆叠结构，深层则包含横截面呈椭圆形的介电束结构；两层细胞的最长轴均平行于鱼类的背腹轴。本研究借助时域有限差分法（finite-difference time-domain method）与摄影辐射光度法（photographic radiometry），探究了该散射行为的结构起源及其环境效应。当从任意入射角照射鱼类的体侧时，部分散射光会沿平行于鱼类前后轴的弧形路径出射。与此同时，部分入射光也会通过该复杂的双折射皮肤结构向下散射，并从腹侧发光器射出。我们证实，这种复杂的散射模式可同时实现两项伪装功能：一是对抗该生境中水平方向的径向对称环境光，二是抵御此处无处不在的掠食性生物发光探照灯。该结构还可将照射在鱼体侧面的入射光引导至向下的光场中，配合腹侧发光器实现的反照明（counterillumination），以隐藏自身的剪影轮廓。