Locomotor behavior across an environmental transition in the ropefish,Erpetoichthys calabaricus

Cinnamon M Pace,Alice C Gibb

doi:10.1242/jeb.047902

Abstract

Many amphibious organisms undergo repeated aquatic to terrestrial transitions during their lifetime; limbless, elongate organisms that make such transitions must rely on axial-based locomotion in both habitats. How is the same anatomical structure employed to produce an effective behavior across such disparate habitats? Here, we examine an elongate amphibious fish, the ropefish (Erpetoichthys calabaricus), and ask: (1) how do locomotor movements change during the transition between aquatic and terrestrial environments and (2) do distantly related amphibious fishes demonstrate similar modes of terrestrial locomotion? Ropefish were examined moving in four experimental treatments (in which the water level was to lowered mimic the transition between environments) that varied from fully aquatic to fully terrestrial. Kinematic parameters (lateral excursion, wavelength, amplitude and frequency) were calculated for points along the midline of the body and compared across treatments. Terrestrial locomotion in the ropefish is characterized by long, slow, large-amplitude undulations down the length of the body; in contrast, aquatic locomotion is characterized by short-wavelength, small-amplitude, high-frequency undulations that gradually increase in an anterior to posterior direction. Experimental treatments with intermediate water levels were more similar to aquatic locomotion in that they demonstrated an anterior to posterior pattern of increasing lateral excursion and wave amplitude, but were more similar to terrestrial locomotion with regard to wavelength, which did not change in an anterior to posterior direction. Finally, the ropefish and another elongate amphibious fish, the eel, consistently exhibit movements characterized by 'path following' when moving on land, which suggests that elongate fishes exhibit functional convergence during terrestrial locomotion.

Highlights

Many organisms undergo transitions between aquatic and terrestrial environments – either during their lifetime or as a result of an amphibious habit
Elongate limbless vertebrates must employ the same propulsive structure, the axial skeleton and musculature, across both habitats. This raises the question: how is the same anatomical structure employed to produce an effective behavior across such disparate habitats? Despite the tractability and simplicity of elongate fish as a model system in which to address this question, few studies have directly compared axial-only locomotion in aquatic versus terrestrial environments
We examined locomotion in E. calabaricus in aquatic and terrestrial environments and across two transitional habitats

Summary

Introduction

Many organisms undergo transitions between aquatic and terrestrial environments – either during their lifetime or as a result of an amphibious habit Such transitions present a drastic change in physical conditions, as these environments differ in viscosity, density and gravitational effects (Vogel, 1994; Graham, 1997). Some such organisms employ different locomotor structures (or different combinations of structures) during aquatic and terrestrial locomotion. Elongate limbless vertebrates (or vertebrates with non-weight-bearing appendages) must employ the same propulsive structure, the axial skeleton and musculature, across both habitats This raises the question: how is the same anatomical structure employed to produce an effective behavior across such disparate habitats? Most such studies have considered only undulation frequency and/or overall animal velocity (Jayne, 1986; Shine and Shetty, 2001; Shine et al, 2003), with a much smaller subset encompassing other kinematic variables (Gillis, 1998; Ellerby et al, 2001)

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