The hydrodynamics of swimming at intermediate Reynolds numbers in the water boatman (Corixidae).

Abstract

The fluid forces that govern propulsion determine the speed and energetic cost of swimming. These hydrodynamics are scale dependent and it is unclear what forces matter to the tremendous diversity of aquatic animals that are between a millimeter and a centimeter in length. Animals at this scale generally operate within the regime of intermediate Reynolds numbers, where both viscous and inertial fluid forces have the potential to play a role in propulsion. The present study aimed to resolve which forces create thrust and drag in the paddling of the water boatman (Corixidae), an animal that spans much of the intermediate regime (10<Re<200). By measuring the force generated by tethered water boatmen, we found that thrust is generated primarily by drag on the paddling appendages, with a negligible contribution from the acceleration reaction force. Based on these findings, we developed a forward-dynamic model of propulsion in free swimming that accurately predicted changes in the body's center of mass over time. For both tethered and free swimming, we used non-linear optimization algorithms to determine the force coefficients that best matched our measurements. With this approach, the drag coefficients on the body and paddle were found to be up to three times greater than on static structures in fully developed flow at the same Reynolds numbers. This is likely a partial consequence of unsteady interactions between the paddles or between the paddles and the body. In addition, the maximum values for these coefficients were inversely related to the Reynolds number, which suggests that viscous forces additionally play an important role in the hydrodynamics of small water boatmen. This understanding for the major forces that operate at intermediate Reynolds numbers offers a basis for interpreting the mechanics, energetics and functional morphology of swimming in many small aquatic animals.

New trace fossil evidence for eurypterid swimming behaviour

“…Following the propulsive backstroke, during which the paddle blade was held vertically, the pivot joint between podomeres 6 and 7 permitted the distal end of the appendage, including the blade, to be folded backward or collapsed horizontally during the recovery stroke. Selden's model is similar to rowing behaviour observed in some small aquatic insects, such as water boatmen (Corixidae; Ngo & McHenry ).…”

The comparative hydrodynamics of rapid rotation by predatory appendages

“…The diverse array of aquatic animals that rotate their appendages for locomotion and prey capture offers rich material for considering the role of hydrodynamics in morphological and kinematic diversification (Fish, 1984;Johansson and Lauder, 2004;Koehl, 1996;McHenry et al, 2003;Ngo and McHenry, 2014;Richards, 2010;Webb and Blake, 1985). Although the drag forces in terrestrial systems are often negligible, the aquatic appendages of dragonfly larvae, snapping shrimp and mantis shrimp, for example, necessarily incur substantial drag (Anker et al, 2006;McHenry et al, 2012;Tanaka and Hisada, 1980;Versluis et al, 2000).…”

Honeybees use their wings for water surface locomotion

“…The locomotion of the honeybee is also different from fully submerged aquatic insects that swim near the water surface, such as the whirligig beetle (23) and the water boatman (24). Both the whirligig beetle and water boatman use drag-based propulsion with their legs, which uses asymmetric power and recovery stroke speed and propulsive area to create net thrust.…”