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

Abstract

SUMMARYMany 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. Supplementary material available online at

Locomotion in elongate fishes: A contact sport

“…When swimming, highly elongate fishes typically use anguilliform locomotion for forward propulsion; however, these studies tend to observe fishes swimming in open spaces (Gillis, 1998;Ellerby et al, 2001). The anguilliform movement pattern is modified when these fishes cross from an aquatic to a terrestrial environment; for example, wave amplitude is greater during terrestrial locomotion than during aquatic locomotion (Gillis, 1998;Ellerby et al, 2001;Pace and Gibb, 2011). Gillis (1998) noted that during terrestrial locomotion, A. rostrata (American eel) would push into the sand producing slight indentations.…”

“…Pectoral fin reduction has been suggested to constrain highly elongate fishes to using only the axial skeleton to produce movements on land (Pace and Gibb, 2014). Interestingly, while most fishes use axial-based movements for aquatic locomotion, highly elongate amphibious species from disparate clades appear to employ axial-based lateral undulations during terrestrial locomotion although kinematic differences exist between species (Gillis, 1998;Ellerby et al, 2001;Pace and Gibb, 2011).…”

“…All Polypteriformes are facultative air breathers (Abdel Magid, 1967;Pettit and Beitinger, 1981). Ropefish are known to make terrestrial excursions (Sacca and Burggren, 1982;Pace and Gibb, 2011) and a recent study showed how terrestrially raised Polypterus senegalus can use their pectoral fins to effectively locomote on land (Standen et al, 2014). Erpetoichthys calabaricus has approximately twice as many precaudal vertebrae (100) as P. senegalus (45), while both genera have similar caudal vertebral numbers (Ward and Brainerd, 2007;Ward and Kley, 2012).…”

Launches, squiggles and pounces, oh my! The water–land transition in mangrove rivulus (Kryptolebias marmoratus)

“…The movement, involving the splaying of the pectoral fin and yawing motions of the body axis, does not fit into any of the classic lateral undulation, sidewinding or concertina movements of snakes, opposing any description of the squiggle as a 'serpentine movement' (Jayne, 1986;Pace and Gibb, 2011;Huehner et al, 1985;Taylor, 1992). During the squiggle, K. marmoratus relies largely on its tail for propulsion while the planted pectoral fin acts as a pivot.…”

Lungfish Axial Muscle Function and the Vertebrate Water to Land Transition

“…However, some primarily aquatic organisms such as eels and rope fish occasionally move on land and do so with axial movements [6] and muscle activity [7], [8] that are grossly similar to the patterns used when swimming in water, including a posteriorly propagated wave of bending and motor activity, or ‘traveling’ wave.…”