Behavioural red-light sensitivity in fish according to the optomotor response

Various procedures have been adopted to investigate spectral sensitivity of animals, e.g., absorption spectra of visual pigments, electroretinography, optokinetic response, optomotor response (OMR), and phototaxis. These works have led to various conclusions about animal vision. However, visual sensitivity should be evaluated consistently for a reliable comparison. In this study, we retrieved behavioural data of several fish species that used a unified procedure for the OMR test and compared the sensitivities to near-infrared light. Besides cavefish that lack eyes, some species were not appropriate for the OMR test because they either stayed still or changed swimming direction frequently. Eight fish species out of 13 species tested were OMR positive. Detailed analyses using goldfish, zebrafish, guppy, medaka, stickleback, and cichlid revealed that all the fish were sensitive to light at λ ≥ 750 nm, where the threshold wavelengths varied from 750 nm to 880 nm. Fish opsin repertoire enabled the perception of red light. In contrast, the copy number of long-wave-sensitive (LWS) genes did not necessarily improve red-light sensitivity. The duplication of LWS and other cone opsin genes that have occurred extensively during fish evolution might not aid increasing spectral sensitivity, but provide some other ophthalmic function, such as enhanced spectral discrimination.

学界已采用多种实验手段探究动物的光谱敏感性，例如视觉色素吸收光谱、视网膜电图（electroretinography）、视动反应、视动反射（optomotor response, OMR）以及趋光性。此类研究已产出诸多关于动物视觉的相关结论。然而，若要开展可靠的跨物种比较，需对视觉敏感性采用统一标准进行评估。本研究检索了13种受试鱼类的行为学数据，这些鱼类均采用统一的视动反射测试流程，并以此比较它们对近红外光的敏感性。其中，除了无眼的穴居鱼类外，另有部分物种因始终保持静止或频繁改变游泳方向，不适用于视动反射测试。13个受试物种中有8种呈现视动反射阳性结果。针对金鱼、斑马鱼、孔雀鱼、青鳉、棘鱼和慈鲷的详细分析显示，所有受试鱼类均对波长λ≥750 nm的光线敏感，其敏感性阈值波长范围为750 nm至880 nm。鱼类视蛋白组（opsin repertoire）赋予其红光感知能力。与之相反，长波敏感（long-wave-sensitive, LWS）基因的拷贝数未必能提升红光敏感性。鱼类演化过程中广泛发生的长波敏感基因及其他视锥视蛋白基因复制事件，或许并非用于提升光谱敏感性，而是赋予其他视觉功能，例如增强光谱辨别能力。