Verhaegen Gerlien, Herzog Hendrik, Korsch Katrin, Kerth Gerald, Brede Martin, Haase Martin
1Vogelwarte, Zoologisches Institut und Museum, Universität Greifswald, Soldmannstraße 23, 17489 Greifswald, Germany.
2Institut für Zoologie, Rheinische Friedrich-Wilhelms-Universität Bonn, Meckenheimer Allee 169, 53115 Bonn, Germany.
Zoological Lett. 2019 Jan 18;5:5. doi: 10.1186/s40851-018-0119-6. eCollection 2019.
A major question in stream ecology is how invertebrates cope with flow. In aquatic gastropods, typically, larger and more globular shells with larger apertures are found in lotic (flowing water) versus lentic (stagnant water) habitats. This has been hypothetically linked to a larger foot, and thus attachment area, which has been suggested to be an adaptation against risk of dislodgement by current. Empirical evidence for this is scarce. Furthermore, these previous studies did not discuss the unavoidable increase in drag forces experienced by the snails as a consequence of the increased cross sectional area. Here, using as a study model, we integrated computational fluid dynamics simulations and a flow tank experiment with living snails to test whether 1) globular shell morphs are an adaptation against dislodgement through lift rather than drag forces, and 2) dislocation velocity is positively linked to foot size, and that the latter can be predicted by shell morphology. The drag forces experienced by the shells were always stronger compared to the lift and lateral forces. Drag and lift forces increased with shell height but not with globularity. Rotating the shells out of the flow direction increased the drag forces, but decreased lift. Our hypothesis that the controversial presence of globular shells in lotic environments could be explained by an adaptation against lift rather than drag forces was rejected. The foot size was only predicted by the size of the shell, not by shell shape or aperture size, showing that the assumed aperture/foot area correlation should be used with caution and cannot be generalized for all aquatic gastropod species. Finally, shell morphology and foot size were not related to the dislodgement speed in our flow tank experiment. We conclude that other traits must play a major role in decreasing dislodgement risk in stream gastropods, e.g., specific behaviours or pedal mucus stickiness. Although we did not find globular shells to be adaptations for reducing dislodgement risk, we cannot rule out that they are still flow-related adaptations. For instance, globular shells are more crush-resistant and therefore perhaps adaptive in terms of diminishing damage caused by tumbling after dislodgement or against lotic crush-type predators.
河流生态学中的一个主要问题是无脊椎动物如何应对水流。在水生腹足类动物中，通常在激流（流动水）栖息地与静水（死水）栖息地相比，会发现具有更大开口的更大且更球状的壳。这在假设上与更大的足部以及因此更大的附着面积有关，这被认为是一种针对被水流冲走风险的适应。对此的实证证据很少。此外，这些先前的研究没有讨论由于横截面积增加，蜗牛所经历的不可避免的阻力增加。在这里，以[具体物种]作为研究模型，我们将计算流体动力学模拟与对活体蜗牛的流水槽实验相结合，以测试：1）球状壳形态是否是通过升力而非阻力来防止被冲走的一种适应；2）脱位速度是否与足部大小呈正相关，并且后者可以通过壳形态来预测。与升力和侧向力相比，壳所经历的阻力总是更强。阻力和升力随着壳的高度增加而增加，但不随球状程度增加。将壳旋转出水流方向会增加阻力，但会降低升力。我们关于在激流环境中存在争议的球状壳可以通过针对升力而非阻力的适应来解释的假设被否定了。足部大小仅由壳的大小预测，而不是由壳的形状或开口大小预测，这表明所假设的开口/足部面积相关性应谨慎使用，不能推广到所有水生腹足类物种。最后，在我们的流水槽实验中，壳形态和足部大小与脱位速度无关。我们得出结论，其他特征必定在降低河流腹足类动物的脱位风险中起主要作用，例如特定行为或足部黏液黏性。虽然我们没有发现球状壳是降低脱位风险的适应，但我们不能排除它们仍然是与水流相关的适应。例如，球状壳更抗挤压，因此也许在减少脱位后翻滚造成的损害或对抗激流挤压型捕食者方面具有适应性。
