Anguilliform Locomotion across a Natural Range of Swimming Speeds

Nils B Tack,Brad J Gemmell,Kevin T Du Clos

doi:10.3390/fluids6030127

Nils B Tack, Brad J Gemmell + Show 1 more

Open Access

https://doi.org/10.3390/fluids6030127

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Abstract

Eel-like fish can exhibit efficient swimming with comparatively low metabolic cost by utilizing sub-ambient pressure areas in the trough of body waves to generate thrust, effectively pulling themselves through the surrounding water. While this is understood at the fish’s preferred swimming speed, little is known about the mechanism over a full range of natural swimming speeds. We compared the swimming kinematics, hydrodynamics, and metabolic activity of juvenile coral catfish (Plotosus lineatus) across relative swimming speeds spanning two orders of magnitude from 0.2 to 2.0 body lengths (BL) per second. We used experimentally derived velocity fields to compute pressure fields and components of thrust along the body. At low speeds, thrust was primarily generated through positive pressure pushing forces. In contrast, increasing swimming speeds caused a shift in the recruitment of push and pull propulsive forces whereby sub-ambient pressure gradients contributed up to 87% of the total thrust produced during one tail-beat cycle past 0.5 BL s−1. This shift in thrust production corresponded to a sharp decline in the overall cost of transport and suggests that pull-dominated thrust in anguilliform swimmers is subject to a minimum threshold below which drag-based mechanisms are less effective.

Highlights

Anguilliform locomotion is widely regarded as an energetically efficient swimming mode due to the relatively low cost of transportation (COT) during steady swimming [1–3]
We investigated metabolic activity, swimming kinematics, and experimentally derived pressure fields and force vectors in an eel-like swimmer, the coral catfish (Plotosus lineatus), as the fish swam at different steady velocities spanning two orders of magnitude
The investigation of energetics, swimming kinematics, pressure fields, and force components of Plotosus lineatus swimming over a range of natural swimming speeds demonstrated that energy-efficient pull thrust served as a primary propulsive mechanism

Summary

Introduction

Anguilliform locomotion is widely regarded as an energetically efficient swimming mode due to the relatively low cost of transportation (COT) during steady swimming [1–3]. Since complex locomotor behaviors and the implementation of multiple control surfaces for swimming can make mathematical modelling efforts immensely challenging, direct empirical measurements of the flow around the body of a swimming fish represent a means to accurately determine the role of body kinematics and the resulting hydrodynamics in thrust production. This applies when trying to disentangle thrust from drag forces, involving the careful consideration of the partitioning of these forces

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