Yuan Jinzhou, Raizen David M, Bau Haim H
Department of Mechanical Engineering and Applied Mechanics and.
Department of Neurology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104.
Proc Natl Acad Sci U S A. 2015 Mar 24;112(12):3606-11. doi: 10.1073/pnas.1424962112. Epub 2015 Mar 9.
The ability to orient oneself in response to environmental cues is crucial to the survival and function of diverse organisms. One such orientation behavior is the alignment of aquatic organisms with (negative rheotaxis) or against (positive rheotaxis) fluid current. The questions of whether low-Reynolds-number, undulatory swimmers, such as worms, rheotax and whether rheotaxis is a deliberate or an involuntary response to mechanical forces have been the subject of conflicting reports. To address these questions, we use Caenorhabditis elegans as a model undulatory swimmer and examine, in experiment and theory, the orientation of C. elegans in the presence of flow. We find that when close to a stationary surface the animal aligns itself against the direction of the flow. We elucidate for the first time to our knowledge the mechanisms of rheotaxis in worms and show that rheotaxis can be explained solely by mechanical forces and does not require sensory input or deliberate action. The interaction between the flow field induced by the swimmer and a nearby surface causes the swimmer to tilt toward the surface and the velocity gradient associated with the flow rotates the animal to face upstream. Fluid mechanical computer simulations faithfully mimic the behavior observed in experiments, supporting the notion that rheotaxis behavior can be fully explained by hydrodynamics. Our study highlights the important role of hydrodynamics in the behavior of small undulating swimmers and may assist in developing control strategies to affect the animals' life cycles.
生物体根据环境线索确定自身方向的能力对于其生存和各种功能至关重要。一种这样的定向行为是水生生物与水流方向一致（负趋流性）或相反（正趋流性）排列。诸如蠕虫等低雷诺数波动游动者是否具有趋流性以及趋流性是对机械力的有意还是非自愿反应等问题一直是相互矛盾的报道的主题。为了解决这些问题，我们以秀丽隐杆线虫作为波动游动的模型，并通过实验和理论研究了秀丽隐杆线虫在有水流情况下的定向。我们发现，当靠近静止表面时，动物会使其自身与水流方向相反排列。据我们所知，我们首次阐明了蠕虫趋流性的机制，并表明趋流性可以仅由机械力来解释，不需要感官输入或有意行动。游动者诱导的流场与附近表面之间的相互作用导致游动者向表面倾斜，与水流相关的速度梯度使动物旋转以面向上游。流体力学计算机模拟忠实地模拟了实验中观察到的行为，支持了趋流性行为可以由流体动力学完全解释的观点。我们的研究突出了流体力学在小型波动游动者行为中的重要作用，并可能有助于制定影响动物生命周期的控制策略。
