The eye size of the bluefin killifish (Lucania goodei) varies from springs to swamps

Variation in lighting environments creates different demands of visual systems for the successful detection and interpretation of visual signals in a given setting. Eye size is a critical property of the visual system as it is has strong effects on visual acuity and visual sensitivity. While many comparative studies have examined eye size across fishes that live in disparate lighting environments (i.e., caves versus surface habitats, mesopelagic versus pelagic depths, turbid versus clear water), fewer have investigated differences in eye size as a function of water clarity at the among population - within species level. Here, we compared relative eye size (eye size residuals on standard length) between wild-caught individuals from swamps and springs and across four drainages in Florida. We also performed a laboratory experiment where we reared animals in clear or tea-stained water, which mimic spring and swamp conditions to determine whether phenotypic plasticity as a function of lighting conditions influences relative eye size. Field caught animals greatly varied in relative eye size among populations, but there was no clear relationship with lighting environment. Fish from southern populations (where swamps are common) had greater relative eye size than those from northern populations (where springs are common). However, in North Florida, bluefin killifish from swamps had smaller eyes than those from springs. This is consistent with the results of our laboratory rearing experiment, which indicated that animals raised in tea-stained water had slightly smaller eyes than those raised in clear water conditions. These two disparate patterns suggest that the determinants of eye size are currently unknown.

光照环境的异质性会对视觉系统提出差异化需求，以保障个体在特定生境中成功检测并解读视觉信号。眼大小是视觉系统的核心特征之一，因其对视觉敏锐度与视觉敏感度均具有显著影响。尽管已有诸多比较研究针对栖息于不同光照环境的鱼类开展了眼大小相关分析（例如洞穴生境与地表生境、中层深海与远洋水深、浑浊水体与清水水体的对比），但针对同一物种内不同种群间尺度下，眼大小随水体透明度变化的差异研究仍相对较少。本研究对比了佛罗里达州4个流域中，采自沼泽与泉水生境的野生个体的相对眼大小（即眼大小相对于标准体长的残差）。此外，本研究还开展了室内饲养实验：将实验对象饲养于清水或茶褐色水体中（分别模拟泉水与沼泽生境的水质），以探究光照条件驱动的表型可塑性是否会对相对眼大小产生影响。野外捕获个体的相对眼大小在不同种群间存在显著差异，但并未与光照环境呈现明确的关联。南部种群（沼泽生境较为常见）的个体相对眼大小显著大于北部种群（泉水生境较为常见）的个体。但在佛罗里达州北部，采自沼泽生境的蓝鳍鳉（bluefin killifish）的眼大小反而小于采自泉水生境的个体。这一结果与本研究室内饲养实验的结论一致：饲养于茶褐色水体中的个体，其眼大小略小于饲养于清水环境中的个体。这两种相悖的观测结果表明，目前我们仍未明确鱼类眼大小的决定性影响因素。