Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907, United States.
Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138, United States.
Integr Comp Biol. 2024 Sep 17;64(2):480-495. doi: 10.1093/icb/icae066.
The scales and skin mucus of bony fishes are both proposed to have a role in beneficially modifying the hydrodynamics of water flow over the body surface. However, it has been challenging to provide direct experimental evidence that tests how mucus and fish scales change the boundary layer in part due to the difficulties in working with live animal tissue and difficulty directly imaging the boundary layer. In this manuscript, we use direct imaging and flow tracking within the boundary layer to compare boundary layer dynamics over surfaces of fish skin with mucus, without mucus, and a flat control surface. Our direct measurements of boundary layer flows for these three different conditions are repeated for two different species, bluegill sunfish (Lepomis macrochirus) and blue tilapia (Oreochromis aureus). Our goals are to understand if mucus and scales reduce drag, shed light on mechanisms underlying drag reduction, compare these results between species, and evaluate the relative contributions to hydrodynamic function for both mucus and scales. We use our measurements of boundary layer flow to calculate shear stress (proportional to friction drag), and we find that mucus reduces drag overall by reducing the velocity gradient near the skin surface. Both bluegill and tilapia showed similar patterns of surface velocity reduction. We also note that scales alone do not appear to reduce drag, but that mucus may reduce friction drag up to 50% compared to scaled surfaces without mucus or flat controls.
硬骨鱼类的鳞片和皮肤黏液都被认为对有益地改变水流在体表的动力学特性具有一定作用。然而，由于难以直接对活体动物组织进行操作，以及难以直接对边界层进行成像，因此很难提供直接的实验证据来测试黏液和鱼类鳞片如何改变边界层。在本手稿中，我们使用边界层内的直接成像和流动跟踪，比较了有黏液、无黏液和光滑对照表面上的鱼类皮肤边界层动力学。我们对这三种不同条件下的边界层流进行了直接测量，并对两种不同的鱼类重复了测量，分别是蓝鳃太阳鱼（Lepomis macrochirus）和奥利亚罗非鱼（Oreochromis aureus）。我们的目标是了解黏液和鳞片是否可以减少阻力，揭示阻力减少的潜在机制，比较这些结果在不同物种之间的差异，并评估黏液和鳞片对水动力功能的相对贡献。我们使用边界层流的测量结果来计算剪切应力（与摩擦阻力成正比），发现黏液通过减少皮肤表面附近的速度梯度来整体减少阻力。蓝鳃太阳鱼和奥利亚罗非鱼都表现出类似的表面速度降低模式。我们还注意到，单独的鳞片似乎不会减少阻力，但与有黏液或光滑对照的无鳞片表面相比，黏液可能会减少高达 50%的摩擦阻力。
