Knott N A, Davis A R, Buttemer W A
School of Biological Sciences, University of Wollongong, Wollongong NSW 2522, Australia.
Biol Bull. 2004 Dec;207(3):217-24. doi: 10.2307/1543210.
Passive flow is believed to increase the gains and reduce the costs of active suspension feeding. We used a mixture of field and laboratory experiments to evaluate whether the unstalked intertidal ascidian Pyura stolonifera exploits passive flow. We predicted that its orientation to prevailing currents and the arrangement of its siphons would induce passive flow due to dynamic pressure at the inhalant siphon, as well as by the Bernoulli effect or viscous entrainment associated with different fluid velocities at each siphon, or by both mechanisms. The orientation of P. stolonifera at several locations along the Sydney-Illawarra coast (Australia) covering a wide range of wave exposures was nonrandom and revealed that the ascidians were consistently oriented with their inhalant siphons directed into the waves or backwash. Flume experiments using wax models demonstrated that the arrangement of the siphons could induce passive flow and that passive flow was greatest when the inhalant siphon was oriented into the flow. Field experiments using transplanted animals confirmed that such an orientation resulted in ascidians gaining food at greater rates, as measured by fecal production, than when oriented perpendicular to the wave direction. We conclude that P. stolonifera enhances suspension feeding by inducing passive flow and is, therefore, a facultatively active suspension feeder. Furthermore, we argue that it is likely that many other active suspension feeders utilize passive flow and, therefore, measurements of their clearance rates should be made under appropriate conditions of flow to gain ecologically relevant results.
被动水流被认为能够增加主动悬浮摄食的收益并降低成本。我们通过野外和实验室实验相结合的方式，来评估无柄潮间带海鞘Pyura stolonifera是否利用被动水流。我们预测，由于吸入虹吸管处的动压，以及每个虹吸管处不同流体速度相关的伯努利效应或粘性夹带，或者两种机制共同作用，其对主流的定向以及虹吸管的排列会诱导被动水流。在澳大利亚悉尼 - 伊拉瓦拉海岸沿线的几个地点，对Pyura stolonifera进行了广泛的波浪暴露测试，其定向并非随机，结果显示海鞘始终将其吸入虹吸管指向海浪或回流方向。使用蜡模型进行的水槽实验表明，虹吸管的排列能够诱导被动水流，并且当吸入虹吸管指向水流方向时，被动水流最大。使用移植动物进行的野外实验证实，通过粪便产量衡量，这种定向使得海鞘比垂直于波浪方向定向时获得食物的速率更高。我们得出结论，Pyura stolonifera通过诱导被动水流来增强悬浮摄食，因此是一种兼性主动悬浮摄食者。此外，我们认为很可能许多其他主动悬浮摄食者也利用被动水流，因此，应该在适当的水流条件下测量它们的清除率，以获得与生态相关的结果。
