Thomas, A. L. R. | bioRxiv Science

bioRxiv2019

A new method for measuring fish swimming patterns in shallow water and a previously undescribed swimming gait

Hill stream loaches are fish which live their entire lives in close contact with rock. They have elaborate physical adaptations to fast flow, adherence to substrate, and movement in very shallow water. Here we describe a method for observing how they swim in detail. There are many similarly shaped rheophilic fish, insects, and amphibian larvae, which live in fast flowing water, and a method of observing their swimming modes has wide potential application. We measured the deflection of the water surface around a swimming fish by viewing a fixed pattern on the bottom of the tank through the water surface. This is a Schlieren method in which the movement or other physical properties of a medium are derived from the deflection of a pattern viewed through that medium. We used this method to describe a new type of swimming gait which is likely to be common among small rheophiles - pulse swimming mode - in which thrust is produced in a series of discrete impulses. The method of analysis described here is beneficial in that the fish is allowed to swim freely in relatively normal conditions without the use of intrusive equipment such as lasers, dyes, or additives to the water, and the pattern of thrust is viewed directly against the skin of the fish rather than being inferred from the wake pattern behind the fish. The method is also low cost and easily set up.

zoology↗

bioRxiv2019

The structure and function of the sucker systems of hill stream loaches

Hill stream loaches (family Balitoridae and Gastromyzontidae) are thumb-sized fish that effortlessly exploit environments where flow rates are so high that potential competitors would be washed away. To cope with these extreme flow rates hill stream loaches have evolved adaptations to stick to the bottom, equivalent to the downforce generating wings and skirts of F1 racing cars, and scale architecture reminiscent of the drag-reducing riblets of Mako sharks. Hill stream loaches exhibit far more diverse flow-modifying morphological features than fast pelagic predators, suggesting as yet unknown drag reducing systems remain to be discovered. Here we describe the skeletal structure of Sewellia lineolata and Gastromyzon punctulatus and contrast that with other fish that face similar hydrodynamic challenges. We identify a major structural variation within Balitoridae pelvic sucker attachment positions which may explain fundamental constraints on the parallel development of different genera and which has not been described before. We also use high speed video capture, CT scans and Frustrated Total Internal Reflection to image and measure the sucker system in live operation and describe how it functions on a familiar activity for hill stream loaches (climbing waterfalls). We show how they can drag 3 to 4 times their own bodyweight up a vertical glass waterfall. Adaptations to high flow rates are the inspiration for this study, because there are many engineering applications where the ability to deal with high flow rates are important - either by reducing drag, or by generating the forces needed to hold an animal in place.

zoology↗