Rotifers negative phototaxis under UVA stress

Ultraviolet radiation (UVR) is an environmental stressor in several ecosystems and can affect organisms’ survival and reproduction, and community structure. Rotifers cope with UVR stress adopting preventive behavioral and metabolic mechanisms. However, the demonstration of an immediate behavioral response in rotifers is missing. We investigated the short-term response of rotifers to UVR, by combining video analysis and movement ecology methods, in three common species: Brachionus calyciflorus, Keratella cochlearis, and Keratella quadrata. We recorded the behavior of B. calyciflorus (both sexes), K. quadrata, and K. cochlearis (females) exposed to white light, and to intermittent cycles of UVR (30:30 s). Individual trajectories were extracted from videos with open-source software. We found that B. calyciflorus females exposed to UVR exhibited strong negative phototaxis with increased swimming speed, and a weak positive phototaxis in males. Keratella cochlearis and K. quadrata showed a weaker response. Our study reveals a species-specific behavioral response to UVR in rotifers. Furthermore, we highlight how sexual dimorphism in B. calyciflorus does not only occur in morphology and movement, but also in behavioral traits. Our results help to understand zooplankton community dynamics by providing a mechanistic explanation of UVR response in one major zooplankton taxonomic group.

紫外线辐射（Ultraviolet radiation, UVR）是多种生态系统中的环境胁迫因子，可影响生物的存活、繁殖以及群落结构。轮虫（Rotifers）可通过采取防御性行为与代谢机制来应对UVR胁迫，但目前尚无轮虫即时行为响应的相关实证报道。本研究结合视频分析与运动生态学方法，针对萼花臂尾轮虫（Brachionus calyciflorus）、龟甲轮虫（Keratella cochlearis）以及矩形龟甲轮虫（Keratella quadrata）这三种常见轮虫，探究了其对UVR的短期响应。我们记录了暴露于白光环境以及UVR间歇循环（30秒:30秒）环境下的萼花臂尾轮虫（雌雄两性）、矩形龟甲轮虫和龟甲轮虫（雌性）的行为活动，并通过开源软件从视频中提取个体运动轨迹。研究发现，暴露于UVR环境中的萼花臂尾轮虫雌性个体表现出强烈的负趋光性（negative phototaxis）且游泳速度显著提升，而雄性个体则呈现较弱的正趋光性（positive phototaxis）；龟甲轮虫与矩形龟甲轮虫的响应程度相对更弱。本研究揭示了轮虫对UVR的物种特异性行为响应。此外，我们还证实了萼花臂尾轮虫的雌雄二态性（sexual dimorphism）不仅存在于形态与运动特征中，还体现在行为性状层面。本研究通过对一类主要浮游动物分类类群的UVR响应机制进行阐释，为理解浮游动物群落动态提供了理论支撑。