Data from: Pushing the limits of photoreception in twilight conditions: the rod-like cone retina of the deep-sea pearlsides

Most vertebrates have a duplex retina comprising two photoreceptor types, rods for dim-light (scotopic) vision and cones for bright-light (photopic) and color vision. However, deep-sea fishes are only active in dim-light conditions; hence, most species have lost their cones in favor of a simplex retina composed exclusively of rods. Although the pearlsides, Maurolicus spp., have such a pure rod retina, their behavior is at odds with this simplex visual system. Contrary to other deep-sea fishes, pearlsides are mostly active during dusk and dawn close to the surface, where light levels are intermediate (twilight or mesopic) and require the use of both rod and cone photoreceptors. This study elucidates this paradox by demonstrating that the pearlside retina does not have rod photoreceptors only; instead, it is composed almost exclusively of transmuted cone photoreceptors. These transmuted cells combine the morphological characteristics of a rod photoreceptor with a cone opsin and a cone phototransduction cascade to form a unique photoreceptor type, a rod-like cone, specifically tuned to the light conditions of the pearlsides’ habitat (blue-shifted light at mesopic intensities). Combining properties of both rods and cones into a single cell type, instead of using two photoreceptor types that do not function at their full potential under mesopic conditions, is likely to be the most efficient and economical solution to optimize visual performance. These results challenge the standing paradigm of the function and evolution of the vertebrate duplex retina and emphasize the need for a more comprehensive evaluation of visual systems in general.

大多数脊椎动物拥有复层视网膜（duplex retina），包含两类感光细胞（photoreceptor）：视杆细胞（rod）负责弱光暗视觉（scotopic vision），视锥细胞（cone）则负责强光明视觉（photopic vision）与色觉。然而深海鱼类仅在弱光环境下活动，因此多数物种丢失了视锥细胞，转而拥有仅由视杆细胞构成的单一型视网膜（simplex retina）。尽管珍珠鱼（Maurolicus spp.）拥有这类纯视杆细胞视网膜，但其行为模式却与该单一型视觉系统相悖。与其他深海鱼类不同，珍珠鱼多在黄昏与黎明时段活跃于近表层水域，此处光照强度处于中间水平（暮光或间视觉（mesopic vision）环境），需要同时动用视杆与视锥感光细胞。本研究阐明了这一悖论：珍珠鱼的视网膜并非仅含视杆感光细胞，而是几乎完全由转化型视锥感光细胞（transmuted cone photoreceptors）构成。这类转化型细胞兼具视杆感光细胞的形态特征，同时拥有视锥视蛋白（cone opsin）与视锥光转导级联反应（cone phototransduction cascade），由此形成一种独特的感光细胞类型——类视杆视锥细胞（rod-like cone），其光谱响应特性专门适配珍珠鱼栖息地的光照环境（间视觉强度下的蓝移光）。将视杆与视锥的特性整合至单一细胞类型，而非使用两类在间视觉环境下无法充分发挥功能的感光细胞，或许是优化视觉性能的最高效且经济的解决方案。本研究结果挑战了当前关于脊椎动物复层视网膜功能与演化的主流范式，同时也强调了全面评估各类视觉系统的必要性。