The East Asian common octopus Octopus sinensis is a bottom-dwelling species, but goes through a planktonic phase as ‘paralarvae’ during the first several weeks of life. As the swimming performance of the paralarvae is relatively poor, their dispersion relies primarily on ocean currents; however, vertical swimming may play an important role in their horizontal dispersion by controlling their buoyancy in the current. Within this context, ontogenic changes in vertical swimming in relation to the ambient water flow are key to understanding the dispersion properties of the paralarvae and the subsequent onset of settlement, yet this information remains largely unknown. Here, we investigated the effects of different water flows (downward × 4 velocities, stagnant and upward × 5 velocities) on the vertical swimming position in test columns using laboratory-raised O. sinensis paralarvae aged 0, 5, 10 and 15 days during the day and at night. Paralarvae younger than 10 days (< 3 mg in dry weight) tended to swim upwards regardless of the water flow conditions, but their swimming was disrupted under downward water flows, while they exhibited repeating upward swimming and downward pushing-back movements. Thus, for early paralarvae, an upward water flow may have a positive effect on dispersion by supporting their innate upward swimming, while a downward water flow may impact paralarvae negatively due to exhaustion through excess swimming activities. Paralarvae aged 15 days (> 5 mg) commenced short clinging behaviour, and their swimming positions dispersed downwards during daytime regardless of the water flow, indicating the onset of the transition to settlement. In contrast, their position shifted to near the upper limit at night by swimming upwards, even under downward water flows. Our results suggest that O. sinensis exhibits characteristic behaviour that alternates between diurnal descent dispersion with clinging (hyperbenthic trait) and nocturnal upward swimming (planktonic trait) during the transition to permanent benthic life.

中文翻译：

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不同水流条件下东亚普通章鱼中华章鱼幼虫垂直游泳的成因变化

东亚普通章鱼八达通是一种生活在底层的物种，但在生命的最初几周内经历了浮游阶段，称为“幼虫”。由于幼虫的游泳性能相对较差，它们的散布主要依靠洋流。但是，垂直游泳可能会通过控制海流的浮力在其水平散布中发挥重要作用。在这种情况下，垂直游泳相对于周围水流的个体发生变化是理解幼虫的弥散特性和随后的沉降开始的关键，但是该信息仍然未知。在这里，我们使用实验室升高的O来研究不同水流（向下×4速度，停滞和向上×5速度）对测试柱垂直游泳位置的影响。。中华白天和晚上，幼虫的年龄分别为0、5、10和15天。小于10天（干重<3 mg）的幼虫往往不顾水流情况向上游泳，但在向下的水流作用下它们的游泳受到干扰，而它们又表现出重复的向上游泳和向下推回运动。因此，对于早期的幼虫，向上的水流可以通过支持其先天的向上游泳而对分散产生积极的影响，而向下的水流则可能由于过度的游泳活动而耗尽体力，从而对幼虫产生负面影响。15天（> 5毫克）的幼虫开始出现短暂的粘着行为，无论水流如何，白天的游泳姿势都向下分散，这表明开始过渡到定居。相反，即使在水流向下的情况下，它们的位置也会在夜间游泳时向上移动到上限附​​近。我们的结果表明Ø。在向永久性底栖生物过渡的过程中，中华绒螯蟹的特征行为表现为昼夜下降并伴有粘着（高底栖性状）和夜间向上游泳（浮游性状）。