起伏：桡足类逆转近体流以在水柱中游动
Ups and downs: Copepods reverse the near-body flow to cruise in the water column
AI总结：
本研究结合速度场与压力场实验，发现桡足类向上巡游时通过近体上游吸力在腿部恢复阶段补充推力，而向下游动时则依靠重力，揭示了朝向依赖的小尺度高效游动机制。
摘要：
桡足类作为海洋生态系统中的初级消费者，参与大规模的昼夜垂直迁移（DVM）。由于它们具有负浮力，重力有助于其向下巡游，但会阻碍其向上迁移。原则上，在向上游动时利用基于阻力的推进来克服这种阻滞力，会产生中性浮力或借助重力向下游动者从根本上避免的额外能量成本。这种增加的功率需求似乎使向上游动的效率显著低于向下游动。然而，我们发现了一种增强向上游动桡足类推力的补偿机制。利用实验获得的速度场和压力场，我们观察到桡足类在向上巡游时将水拉向身体前部，产生低于环境压力的梯度，从而诱发上游吸力，以补充腿部产生的推力。与阻力始终主导腿部恢复阶段的预期相反，结果表明，上游吸力在恢复划水约三分之一的时间内产生净推力。这种吸力—推力机制促进了鱼类和水母等较大动物的高性价比游动，但此前从未在小尺度上被报道。我们将这些结果与向下游动的桡足类进行比较，后者对水是推而非拉，并证明它们在恢复划水期间不会受益于增强推力的吸力效应。相反，向下巡游者利用重力效应游得更快。“拉动者”和“推动者”行为由腿部运动学的改变所驱动，表明这是对身体朝向的响应，而不是固定的生物体特征。这些结果为一种重要的游动机制提供了见解，可帮助我们理解中型浮游动物在驱动生物源水动力输运中的作用及其对海洋生物地球化学的影响。
英文摘要：
Copepods participate in large-scale diel vertical migrations (DVM) as primary consumers in marine ecosystems. Given that they are negatively buoyant, gravity facilitates their downward cruising but impedes their upward relocation. In principle, overcoming this retarding force using drag-based propulsion during upward swimming incurs extra energy costs that neutrally buoyant or gravity-assisted downward swimmers fundamentally avoid. This added power requirement seemingly renders upward swimming disproportionally less efficient than downward swimming. However, we found a compensatory mechanism enhancing thrust in upward-swimming copepods. Using experimentally derived velocity and pressure fields, we observed that copepods pull water to the anterior, generating sub-ambient pressure gradients when cruising upwards, thereby inducing an upstream suction force to complement the thrust produced by the legs. Contrary to expectations that drag forces always dominate the leg recovery phase, the results show that the upstream suction generates net thrust for about a third of the recovery stroke. Such a suction-thrust mechanism promotes cost-effective swimming in larger animals like fish and jellyfish. However, it has never been reported at small scales. We compare these results with downward-swimming copepods that push rather than pull on water and demonstrate they do not benefit from thrust-enhancing suction effects during the recovery stroke. Instead, downward cruisers leverage gravitational effects to swim faster. `Puller' and `pusher' behaviors are driven by alterations of the leg kinematics, indicating a response to the body orientation rather than a fixed organism trait. These results offer insights into an essential swimming mechanism that can inform us about the role of mesozooplankton in driving biogenic hydrodynamic transport and its effects on marine biogeochemistry.
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