Impacts of caffeine on fathead minnow behaviour and physiology

Pollution from regularly used substances such as pharmaceuticals, cleaning agents, and even food and beverages is an increasing problem in the environment. Caffeine, a commonly ingested stimulant, is one such contaminant that has been detected in aquatic environments worldwide. Yet, little is known about how ecologically relevant concentrations of caffeine influence the morphology, behaviour, and physiology of exposed organisms. To address this knowledge gap, we exposed fathead minnow (Pimephales promelas) to three caffeine treatments: a freshwater control (nominal: 0 ng/L), a low (nominal: 1,000 ng/L) and high environmentally relevant dose (nominal: 10,000 ng/L), for 35 days. We tested the learning abilities, anxiety, metabolic rates, and morphological features of exposed vs. control fish. Caffeine exposure did not affect the ability of fish to learn but did influence anxiety levels. Over the course of repeated anxiety testing, unexposed control fish visited a black square more often while fish exposed to low levels of caffeine did not, potentially indicating that these fish remained in a more anxious state. While caffeine did not impact metabolism, fish growth, or body size, it was associated with lower liver investment-although this response was only observed in our low caffeine treatment. Overall, our results suggest that even relatively low concentrations of caffeine may impact the liver size and anxiety of exposed fish, but further research is needed to assess how extended exposure to caffeine impacts fitness. Given the increase in anthropogenic contaminants in aquatic environments, it is important that we continue to investigate their effects on the organisms exposed to them.

日常使用的各类物质（如药品、清洁剂乃至食品与饮料）所产生的污染，正日益成为环境中的严峻问题。咖啡因作为一种广泛摄入的兴奋剂，便是这类污染物之一，目前已在全球范围内的水生环境中被检出。然而，学界对生态相关浓度的咖啡因如何影响暴露生物体的形态、行为与生理机能，仍知之甚少。为填补这一认知空白，本研究将黑头呆鱼（*Pimephales promelas*）暴露于三种咖啡因处理环境中：淡水对照组（标称浓度：0 ng/L）、低剂量组（标称浓度：1000 ng/L）以及高剂量生态相关浓度组（标称浓度：10000 ng/L），暴露时长为35天。本研究对暴露组与对照组鱼类的学习能力、焦虑状态、代谢速率及形态特征进行了检测。结果显示，咖啡因暴露并未影响鱼类的学习能力，但确实改变了其焦虑水平：在重复开展的焦虑测试中，未暴露的对照组鱼类会更频繁地造访黑色区域，而低剂量咖啡因暴露组鱼类则无此行为，这或暗示该组鱼类始终处于更为焦虑的状态。尽管咖啡因未对鱼类的代谢、生长或体型产生影响，但它与更低的肝体比相关——不过这一效应仅在低剂量咖啡因处理组中被观测到。总体而言，本研究结果表明，即便浓度相对较低的咖啡因，也可能对暴露鱼类的肝脏大小与焦虑状态产生影响，但仍需开展进一步研究，以评估长期咖啡因暴露对生物体适合度的影响。鉴于水生环境中人为源污染物的占比持续上升，持续探究此类污染物对暴露生物体的影响具有重要意义。