Adaptations for low drag, downforce and eddy use in hillstream loaches

High in the hills steep gradients lead to raging torrents in the streams. Hillstream loaches are a group of fishes specialised to cope with these violent high-speed flows. These fishes have evolved an array of adaptations to reduce drag and to stick to the bottom, equivalent to the downforce generating wings and skirts of F1 racing cars, and drag-reducing riblet skin-architecture of Mako sharks. However, the conditions hillstream loaches live in are more severe than either Mako sharks or F1 cars experience. To cope with these conditions, hillstream loaches exhibit far more diverse flow-modifying morphological features than fast pelagic predators, suggesting as yet unknown down-force and drag reducing systems remain to be discovered.

In this project, we will study these drag reducing mechanisms, from both hydrodynamic and behavioural perspectives. The skin architectures of hillstream loaches appear similar to those that reduce drag in fast pelagic predators, but with the key difference that whereas the marine predators generate the flow (which is therefore predictable) the environment of the hillstream loaches produces highly turbulent unpredictable flows, and there are clear ‘design’ features on the hillstream loaches that appear to be appropriate to provide drag reduction in the chaotic flows they encounter. Hillstream loaches also exhibit striking adaptations to their body morphology that appear appropriate to generate substantial downforce. One very striking result is that you find a dead hillstream loach wherever you put it in the flow – they can hold station against high-speed flows entirely passively. Preliminary observations also suggest that the fish have behavioural adaptations that allow them to exploit variations in local flow regime, drafting like the Tour-de-France peloton, and exploiting favourable local eddies.

Balitorid loach of the genus 
Gastromyzon. The streamlined shape and flattened fins are clearly demonstrated. Fish is approx. 30mm TL. Credit: Prof. Kevin Foster.

We plan to throw the entire toolbox of aerodynamic investigation techniques at these little fish, treating them as if they were Formula 1 cars undergoing rigorous track testing. By doing so, we will be able to identify the drag reducing features and the downforce producing features that allow the hillstream loaches to stay put in strong, turbulent currents. 

Not all hillstream loaches live in the same sorts of habitats, or use those habitats in the same ways. Some spend all their time stuck onto rocks, while others spend more time swimming in open water. The rock-huggers do not seem to care which way the flow hits them, whereas the open water fishes are generally accurately aligned with the flow. We expect that this will lead to different types of drag reducing system – with different potential engineering applications, and evolutionary consequences. 

Understanding the essential elements of how hillstream loaches tackle extreme flow will open the door into further research into both evolutionary biomechanics and behaviour. Of course, the engineering potential of systems that reduce drag in high flows, and that are tolerant of a wide range of flow directions, and that work in turbulent flows, is massive.

Professor Adrian Thomas and Professor Theresa Burt de Perera
University of Oxford