Sex-dependent shifts in visual detection thresholds under turbid conditions in an African cichlid

Turbidity is increasing in freshwaters globally due to human activities and is known to affect visually-mediated behaviors in fish. As anthropogenic impacts continue to degrade aquatic environments, it is critical to determine how sensory systems are affected and what this might mean for population persistence. We investigated the effect of turbidity on visual detection thresholds in an African cichlid fish (Pseudocrenilabrus multicolor) that experiences environmental extremes across its East African range. We tested the visual abilities of adult wild-caught fish from two sites representing the extremes of turbidity and oxygen (a high turbidity, high dissolved oxygen river and a low turbidity, low dissolved oxygen swamp). Further, we reared offspring of wild-caught parents from each population in a full-factorial high/low oxygen, high/low turbidity design to tease apart the influence of each stressor on visual detection thresholds. We used an optomotor response test to determine detection thresholds under increasing levels of turbidity for both wild-caught and lab-reared fish. Detection thresholds were higher in the wild-caught river population compared to the swamp population, and there was a strong sex difference, such that wild-caught males had higher detection thresholds than females, regardless of population of origin. Our results suggest that there are sex-based differences in contrast detection abilities that could play a critical role in visual ecology for populations experiencing divergent turbidity regimes. In the rearing experiment, sex-based differences in detection thresholds were influenced by different aspects of the rearing treatment. Detection threshold varied significantly by oxygen rearing treatment for males and by the interactive effects of oxygen and turbidity for females. This research improves our understanding of the effect of elevated turbidity on African cichlid vision and contributes to growing knowledge of how animals respond to environmental change.

Ethics statement: 

All research was approved by The Ohio State University Institutional Animal Care and Use Committee (2014A00000055-R1). Permission was given by the Commissioner of Fisheries Resources Management and Development, Uganda, for export of fish, and from the Uganda National Council for Science and Technology for permission to conduct research.

全球淡水水体因人类活动导致浊度持续升高，而浊度已知会干扰鱼类的视觉介导行为。随着人为扰动持续恶化水生环境，明确感官系统所受影响及其对种群存续的潜在意义，已成为亟待解决的关键科学问题。本研究以分布于东非、栖息环境跨度极大的非洲慈鲷（多色拟丽鱼，Pseudocrenilabrus multicolor）为研究对象，探究浊度对其视觉检测阈值的影响。我们采集了来自两个极端浊度与溶氧环境位点的野生成年个体：其一为高浊度高溶解氧河流种群，其二为低浊度低溶解氧沼泽种群，并测试了这两个种群个体的视觉能力。进一步，我们采用完全交叉的高/低溶氧、高/低浊度饲养设计，将上述两个种群的野生亲本所产后代置于不同环境中饲养，以厘清两种胁迫因子各自对视觉检测阈值的独立影响。本研究借助视动反应测试（optomotor response test），分别测定野生捕获个体与实验室饲养个体在梯度浊度条件下的视觉检测阈值。实验结果显示：野生捕获的河流种群个体的检测阈值显著高于沼泽种群；同时存在显著的性别差异——无论种群来源如何，野生雄性个体的检测阈值均高于雌性个体。本研究结果表明，基于性别的对比度检测能力差异，可能在经历不同浊度环境的种群的视觉生态中发挥关键作用。在饲养实验中，检测阈值的性别差异受到饲养环境不同维度的调控：雄性个体的检测阈值随溶氧饲养条件显著变化，而雌性个体的检测阈值则受溶氧与浊度的交互作用显著影响。本研究增进了我们对高浊度如何影响非洲慈鲷视觉系统的认知，也为探索动物应对全球环境变化的响应机制提供了新的实证依据。

伦理声明：
本研究所有实验操作均获得俄亥俄州立大学机构动物护理与使用委员会（2014A00000055-R1）批准。鱼类出口许可由乌干达渔业资源管理与发展专员签发，研究开展许可由乌干达国家科学技术委员会核准。