Open water camouflage via 'leaky' light guides in the midwater squid Galiteuthis

Galiteuthis, a midwater squid, has photophores on the ventral surfaces of its eyes. These photophores emit bioluminescence to counter-illuminate the shadows cast by the eyes in downwelling sunlight, thereby hiding the eyes from upward-looking predators. The photophores consist of laminated fibre-like cells with semi-coaxial protein-dense layers around axial cytoplasm. These cells have been suggested to function as light guides: bioluminescence is an isotropic process used to hide in an anisotropic light environment, so any emission must be reshaped to be effective. We found a wide variation in cross-sectional geometries of photophore cells; some were more efficient at light guiding than others. We used a set of optical models to place these photophores in the context of the radiance where Galiteuthis lives and discovered a possible adaptive reason for this variation. In Galiteuthis's horizontal and vertical range, ocean radiance is also quite variable. For complete camouflage, photophores must reproduce this variation in radiance using an isotropic source. Our models show that variation in the geometry of the photophore light guides reproduces the predicted variation in ocean radiance experienced by this species. By selectively activating geometrically distinct populations of photophore cells, the animal may reproduce the angular distribution of light at all positions in its habitat.

Galiteuthis（帆乌贼属）是一类中层水域鱿鱼，其眼部腹侧表面分布有发光器（photophores）。此类发光器可发射生物发光，以抵消下行太阳光在其眼部投射的阴影，从而将眼部从向上窥伺的捕食者视野中隐藏。该发光器由层状纤维状细胞构成，轴细胞质周围包裹有半同轴的蛋白质致密层。这类细胞被认为可作为光导（light guides）发挥作用：生物发光属于各向同性过程，而鱿鱼所处的光环境为各向异性，因此必须对发射光进行塑形才能实现有效的伪装效果。研究发现，发光器细胞的横截面几何形态存在显著差异，部分细胞的光导效率优于其他细胞。我们通过一系列光学模型，将此类发光器置于该鱿鱼栖息环境的辐亮度背景中进行分析，揭示了这种几何形态差异可能存在的适应性动因。在Galiteuthis的水平与垂直栖息范围内，海洋辐亮度同样存在显著波动。为实现完全的伪装，发光器需借助各向同性的发光源复现这种辐亮度变化。我们的模型结果表明，发光器光导的几何形态差异，恰好能够复现该物种所经历的海洋辐亮度预测变化。通过选择性激活几何形态各异的发光器细胞群，该鱿鱼可在其栖息环境的所有位置复现光线的角分布。