Locomotor flexibility of Polypterus senegalus across various aquatic and terrestrial substrates

Terrestrial locomotion characteristics of climbing perch (Anabas testudineus)

The evolution and utilization of limbs facilitated terrestrial vertebrate movement on land, but little is known about how other lateral structures enhance terrestrial locomotion in amphibian fishes without terrestrialized limb structures. Climbing perch (Anabas testudineus) exhibit sustained terrestrial locomotion using uniaxial rotating gill covers instead of appendages. To investigate the role of such simple lateral structures in terrestrial locomotion and the motion-generating mechanism of the corresponding locomotor structure configuration (gill covers and body undulation), we measured the terrestrial kinematics of climbing perch and quantitatively analysed its motion characteristics. The digitized locomotor kinematics showed a unique body postural adjustment ability that enables the regulation of the posture of the caudal peduncle for converting lateral bending force into propulsion. An analysis of the coordination characteristics demonstrated that the motion of the gill cover is kinematically independent of axial undulation, suggesting that the gill cover functions as an anchored simple support pole while axial undulation actively mediates body posture and produces propulsive force. The two identified feature shapes explained more than 87% of the complex lateral undulation in multistage locomotion. The kinematic characteristics enhance our understanding of the underlying coordinating mechanism corresponding to locomotor configurations. Our work provides quantitative insight into the terrestrial locomotor adaptation of climbing perch and sheds light on terrestrial motion potential of locomotor configurations containing a typical aquatic body and restricted lateral structure.

Maneuvering on non-Newtonian fluidic terrain: a survey of animal and bio-inspired robot locomotion techniques on soft yielding grounds

Frictionally yielding media are a particular type of non-Newtonian fluids that significantly deform under stress and do not recover their original shape. For example, mud, snow, soil, leaf litters, or sand are such substrates because they flow when stress is applied but do not bounce back when released. Some robots have been designed to move on those substrates. However, compared to moving on solid ground, significantly fewer prototypes have been developed and only a few prototypes have been demonstrated outside of the research laboratory. This paper surveys the existing biology and robotics literature to analyze principles of physics facilitating motion on yielding substrates. We categorize animal and robot locomotion based on the mechanical principles and then further on the nature of the contact: discrete contact, continuous contact above the material, or through the medium. Then, we extract different hardware solutions and motion strategies enabling different robots and animals to progress. The result reveals which design principles are more widely used and which may represent research gaps for robotics. We also discuss that higher level of abstraction helps transferring the solutions to the robotics domain also when the robot is not explicitly meant to be bio-inspired. The contribution of this paper is a review of the biology and robotics literature for identifying locomotion principles that can be applied for future robot design in yielding environments, as well as a catalog of existing solutions either in nature or man-made, to enable locomotion on yielding grounds.

Behaviour and muscle activity across the aquatic-terrestrial transition in Polypterus senegalus

Amphibious fishes moving from water to land experience continuous changes in environmental forces. How these subtle changes impact behavioural transitions cannot be resolved by comparisons of aquatic and terrestrial locomotion. For example, aquatic and terrestrial locomotion appear distinct in the actinopterygian fish Polypterus senegalus; however, it is unclear how gradual water level changes influence the transition between these locomotor behaviours. We tested the hypothesis in P. senegalus that swimming and walking are part of an incremental continuum of behaviour and muscle activity across the environmental transition from water to land rather than two discrete behaviours, as proposed by previous literature. We exposed P. senegalus to discrete environments from fully aquatic to fully terrestrial while recording body and pectoral fin kinematics and muscle activity. Anterior axial red muscle effort increases as water depth decreases; however, a typical swimming-like anterior-to-posterior wave of axial red muscle activity is always present, even during terrestrial locomotion, indicating gradual motor control changes. Thus, walking appears to be based on swimming-like axial muscle activity whereas kinematic differences between swimming and walking appear to be due to mechanical constraints. A discrete change in left-right pectoral fin coordination from in-phase to out-of-phase at 0.7 body depths relies on adductor muscle activity with a similar duty factor and adductor muscle effort that increases gradually as water depth decreases. Thus, despite distinct changes in kinematic timing, neuromuscular patterning is similar across the water depth continuum. As the observed, gradual increases in axial muscle effort reflect muscle activity changes between aquatic and terrestrial environments observed in other elongate fishes, a modified, swimming-like axial muscle activity pattern for terrestrial locomotion may be common among elongate amphibious fishes.

A Step Forward: Functional Diversity and Emerging Themes of Slow-Speed Locomotion in Vertebrates

Walking can be defined broadly as a slow-speed movement produced when appendages interact with the ground to generate forward propulsion. Until recently, most studies of walking have focused on humans and a handful of domesticated vertebrates moving at a steady rate over highly simplified, static surfaces, which may bias our understanding of the unifying principles that underlie vertebrate locomotion. In the last few decades, studies have expanded to include a range of environmental contexts (e.g., uneven terrain, perturbations, deformable substrates) and greater phylogenetic breadth (e.g., non-domesticated species, small and/or ectothermic tetrapods and fishes); these studies have revealed that even a gait as superficially simple as walking is far more complex than previously thought. In addition, technological advances and accessibility of imaging systems and computational power have recently expanded our capabilities to test hypotheses about the locomotor movements of extant and extinct organisms in silico. In this symposium, scientists showcased diverse taxa (from extant fishes to extinct dinosaurs) moving through a range of variable conditions (speed perturbations, inclines, and deformable substrates) to address the causes and consequences of functional diversity in locomotor systems and discuss nascent research areas and techniques. From the symposium contributions, several themes emerged: (1) slow-speed, appendage-based movements in fishes are best described as walking-like movements rather than true walking gaits, (2) environmental variation (e.g., deformable substrates) and dynamic stimuli (e.g., perturbations) trigger kinematic and neuromuscular changes in animals that make defining a single gait or the transition between gaits more complicated than originally thought, and (3) computational advances have increased the ability to process large data sets, emulate the 3D motions of extant and extinct taxa, and even model species interactions in ancient ecosystems. Although this symposium allowed us to make great strides forward in our understanding of vertebrate walking, much ground remains to be covered. First, there is a much greater range of vertebrate appendage-based locomotor behaviors than has been previously recognized and existing terminology fails to accurately capture and describe this diversity. Second, despite recent efforts, the mechanisms that vertebrates use modify locomotor behaviors in response to predictable and unpredictable locomotor challenges are still poorly understood. Third, while computer-based models and simulations facilitate a greater understanding of the kinetics and kinematics of movement in both extant and extinct animals, a universal, one-size-fits-all, predictive model of appendage-based movement in vertebrates remains elusive.

Kinematic comparisons between mudskipper fins and salamander limbs during terrestrial locomotion

Terrestrial and aquatic environments present drastically different challenges, yet amphibious behaviors evolved multiple times in vertebrates. Terrestrial salamanders are often used to model the locomotion of crownward stem tetrapods, but amphibious fishes may model earlier evolutionary stages as vertebrates became terrestrial. For instance, some early tetrapods may have moved on land with a mudskipper-like gait. Previously published kinetic data found that the ground reaction forces produced by the pectoral fins of mudskippers (Periophthalmus barbarus) were more medial than the limbs of tiger salamanders (Ambystoma tigrinum), which might elevate bending stresses in the fins. However, kinematic data are needed to explain these kinetic differences. Therefore, we quantified the three-dimensional kinematics of mudskipper pectoral fins and compared these to published data on tiger salamander limbs. We found that mudskipper pectoral fins generally remained more retracted, extended, and adducted compared to salamander limbs. Kinematic patterns in mudskipper pectoral fins were aligned with published kinetic data and shared a restricted range of motion found in early tetrapods. Our findings demonstrate that mudskipper pectoral fins provide weight support and propulsion but have lower mobility in the proximal versus distal elements, for which greater flexibility in the latter might compensate. Broadly, these data provide new insights into the biomechanics of using fins versus limbs for moving over land and factors that may favor the evolution of different terrestrial gaits.

Mudskippers modulate their locomotor kinematics when moving on deformable and inclined substrates

Many ecological factors influence animal movement, including properties of the media that they move on or through. Animals moving in terrestrial environments encounter conditions that can be challenging for generating propulsion and maintaining stability, such as inclines and deformable substrates that can cause slipping and sinking. In response, tetrapods tend to adopt a more crouched posture and lower their center of mass on inclines and increase the surface area of contact on deformable substrates, such as sand. Many amphibious fishes encounter the same challenges when moving on land, but how these finned animals modulate their locomotion with respect to different environmental conditions and how these modifications compare with those seen within tetrapods is relatively understudied. Mudskippers (Gobiidae: Oxudercinae) are a particularly noteworthy group of amphibious fishes in this context given that they navigate a wide range of environmental conditions, from flat mud to inclined mangrove trees. They use a unique form of terrestrial locomotion called 'crutching', where their pectoral fins synchronously lift and vault the front half of the body forward before landing on their pelvic fins while the lower half of the body and tail are kept straight. However, recent work has shown that mudskippers modify some aspects of their locomotion when crutching on deformable surfaces, particularly those at an incline. For example, on inclined dry sand, mudskippers bent their bodies laterally and curled and extended their tails to potentially act as a secondary propulsor and/or anti-slip device. In order to gain a more comprehensive understanding of the functional diversity and context-dependency of mudskipper crutching, we compared their kinematics on different combinations of substrate types (solid, mud, dry sand) and inclines (0°, 10°, 20°). In addition to increasing lateral bending on deformable and inclined substrates, we found that mudskippers increased the relative contact time and contact area of their paired fins while becoming more crouched, responses comparable to those seen in tetrapods and other amphibious fishes. Mudskippers on these substrates also exhibited previously undocumented behaviors, such as extending and adpressing the distal portions of their pectoral fins more anteriorly, dorsoventrally bending their trunk, "belly-flopping" on sand, and "gripping" the mud substrate with their pectoral fin rays. Our study highlights potential compensatory mechanisms shared among vertebrates in terrestrial environments while also illustrating that locomotor flexibility and even novelty can emerge when animals are challenged with environmental variation.

The Water to Land Transition Submerged: Multifunctional Design of Pectoral Fins for Use in Swimming and in Association with Underwater Substrate

Fins of fishes provide many examples of structures that are beautifully designed to power and control movement in water; however, some species also use their fins for substrate-associated behaviors where interactions with solid surfaces are key. Here, we examine how the pectoral fins of ray-finned fish with these multifunctional behavioral demands, in water and on solid surfaces, are structured and function. We subdivide fins used in swimming and substrate contact into two general morphological categories, regionalized vs. generalized fins. Regionalized fins have ventral rays that are free from connecting membrane or in which that membrane is reduced. Dorsally they maintain a more typical membranous fin. While all pectoral fins vary somewhat in their morphology from leading to trailing edge, generalized fins do not have the substantial membrane loss between rays that is seen in regionalized fins and the distal edge anatomy changes gradually along its margin. We add a new case study in regionalized fins with the dwarf hawkfish (Cirrhitichthys falco). Hawkfishes are most often found perching and moving on structures in their environments. During perching, the free ventral rays are in contact with the substrate and splayed. We found that unlike other fish with regionalized pectoral fins, hawkfish maintain use of the dorsal membranous region of its pectoral fin for rhythmic swimming. We found that typically hawkfish bend their ventral free rays under, toward the medial hemitrichs or hold them straight during substrate-associated postures. This appears also to be the case for the ventral free rays of other species with regionalized fins. Generalized fin use for substrate contact was reviewed in round gobies (Neogobius melanostomus). In addition, although their lobe fins are not representative of ray-finned fish anatomy, we explored fin contact on submerged substrates in the Senegal bichir (Polypterus senegalus), which has a generalized distal fin (no free fin rays or distinct membrane regions). Both groups use their pectoral fins for swimming. During substrate-based postures, unlike hawkfish, their distal rays generally bend outward toward the lateral hemitrichs and a large swath of the fin membrane can contact the surface. The alternative demands on multifunctional fins suggest specialization of the mechanosensory system. We review mechanosensation related to fin movement and surface contact. These alternative regionalized and generalized strategies for serving aquatic and substrate-based functions underwater provide opportunities to further investigate specializations, including sensory structures and systems, that accompany the evolution of substrate-based behaviors in vertebrates.

Terrestrial capabilities of invasive fishes and their management implications

Amphibious fishes have many adaptations that make them successful in a wide variety of conditions, including air-breathing, terrestrial locomotor capabilities, and extreme tolerance of poor water quality. However, the traits that make them highly adaptable may allow these fishes to successfully establish themselves outside of their native regions. In particular, the terrestrial capabilities of invasive amphibious fishes allow them to disperse overland, unlike fully aquatic invasive fishes, making their management more complicated. Despite numerous amphibious fish introductions around the world, ecological risk assessments and management plans often fail to adequately account for their terrestrial behaviors. In this review, I discuss the diversity of invasive amphibious fishes and what we currently know about why they emerge onto land, how they move around terrestrial environments, and how they orient while on land. In doing so, I use case studies of the performance and motivations of nonnative amphibious fishes in terrestrial environments to propose management solutions that factor in their complete natural history. Because of their terrestrial capabilities, we may need to manage amphibious fishes more like amphibians than fully aquatic fishes, but to do so, we need to learn more about how these species perform in a wide range of terrestrial environments and conditions.

Patterns and processes in amphibious fish: biomechanics and neural control of fish terrestrial locomotion

Amphibiousness in fishes spans the actinopterygian tree from the earliest to the most recently derived species. The land environment requires locomotor force production different from that in water, and a diversity of locomotor modes have evolved across the actinopterygian tree. To compare locomotor mode between species, we mapped biomechanical traits on an established amphibious fish phylogeny. Although the diversity of fish that can move over land is large, we noted several patterns, including the rarity of morphological and locomotor specialization, correlations between body shape and locomotor mode, and an overall tendency for amphibious fish to be small. We suggest two idealized empirical metrics to consider when gauging terrestrial 'success' in fishes and discuss patterns of terrestriality in fishes considering biomechanical scaling, physical consequences of shape, and tissue plasticity. Finally, we suggest four ways in which neural control could change in response to a novel environment, highlighting the importance and challenges of deciphering when these control mechanisms are used. We aim to provide an overview of the diversity of successful amphibious locomotion strategies and suggest several frameworks that can guide the study of amphibious fish and their locomotion.

They like to move it (move it): walking kinematics of balitorid loaches of Thailand

Balitorid loaches are a family of fishes that exhibit morphological adaptations to living in fast flowing water, including an enlarged sacral rib that creates a 'hip'-like skeletal connection between the pelvis and the axial skeleton. The presence of this sacral rib, the robustness of which varies across the family, is hypothesized to facilitate terrestrial locomotion seen in the family. Terrestrial locomotion in balitorids is unlike that of any known fish: the locomotion resembles that of terrestrial tetrapods. Emergence and convergence of terrestrial locomotion from water to land has been studied in fossils; however, studying balitorid walking provides a present-day natural laboratory to examine the convergent evolution of walking movements. We tested the hypothesis that balitorid species with more robust connections between the pelvic and axial skeleton (M3 morphotype) are more effective at walking than species with reduced connectivity (M1 morphotype). We predicted that robust connections would facilitate travel per step and increase mass support during movement. We collected high-speed video of walking in seven balitorid species to analyze kinematic variables. The connection between internal anatomy and locomotion on land are revealed herein with digitized video analysis, μCT scans, and in the context of the phylogenetic history of this family of fishes. Our species sampling covered the extremes of previously identified sacral rib morphotypes, M1 and M3. Although we hypothesized the robustness of the sacral rib to have a strong influence on walking performance, there was not a large reduction in walking ability in the species with the least modified rib (M1). Instead, walking kinematics varied between the two balitorid subfamilies with a generally more 'walk-like' behavior in the Balitorinae and more 'swim-like' behavior in the Homalopteroidinae. The type of terrestrial locomotion displayed in balitorids is unique among living fishes and aids in our understanding of the extent to which a sacral connection facilitates terrestrial walking.

Form and function of the caudal fin throughout the phylogeny of fishes

Fishes are the longest persisting living vertebrates and as such, display an incredible array of diversity. Variation in the tail, or caudal fin, is often a reflection of a fish's environment, and affects movement, predation, defense, and reproduction. Previous literature has discussed many aspects of caudal fin form and function in particular taxonomic groups; however, no previous work has synthesized these studies in order to detail how the caudal fin is structured, and what purpose this structure serves, throughout the phylogeny of fishes. This review examines the caudal fin throughout the main lineages of fish evolution, and highlights where changes in shape and usage have occurred. Such novelties in form and function tend to have far-reaching evolutionary consequences. Through integration of past and present work, this review creates a coherent picture of caudal fin evolution. Patterns and outliers that demonstrate how form and function of this appendage are intertwined can further inform hypotheses that fill critical gaps in knowledge concerning the caudal fin.

Snowflake morays, Echidna nebulosa, exhibit similar feeding kinematics in terrestrial and aquatic treatments

Some species of durophagous moray eels (Muraenidae) have been documented emerging from the marine environment to capture intertidal crabs but how they consume prey out of water is unknown. Here, we trained snowflake morays, Echidna nebulosa, to undulate out of the aquatic environment to feed on land. On land, snowflake morays remove prey from the substrate by biting and swallow prey using pharyngeal jaw enabled transport. Although snowflake morays exhibit smaller jaw rotation angles on land when apprehending their prey, transport kinematics involving dorsoventral flexion of the head to protract the pharyngeal jaws and overall feeding times did not differ between terrestrial and aquatic treatments. We suggest that their elongate body plan, ability to rotate their heads in the dorsoventral and lateral directions, and extreme pharyngeal movements all contribute to the ability of durophagous morays to feed in the terrestrial environment.

Summary: Body elongation and pharyngeal transport facilitates prey capture and swallowing on land for the snowflake moray, Echidna nebulosa.

Body and Tail Coordination in the Bluespot Salamander (Ambystoma laterale) During Limb Regeneration

Animals are incredibly good at adapting to changes in their environment, a trait envied by most roboticists. Many animals use different gaits to seamlessly transition between land and water and move through non-uniform terrains. In addition to adjusting to changes in their environment, animals can adjust their locomotion to deal with missing or regenerating limbs. Salamanders are an amphibious group of animals that can regenerate limbs, tails, and even parts of the spinal cord in some species. After the loss of a limb, the salamander successfully adjusts to constantly changing morphology as it regenerates the missing part. This quality is of particular interest to roboticists looking to design devices that can adapt to missing or malfunctioning components. While walking, an intact salamander uses its limbs, body, and tail to propel itself along the ground. Its body and tail are coordinated in a distinctive wave-like pattern. Understanding how their bending kinematics change as they regrow lost limbs would provide important information to roboticists designing amphibious machines meant to navigate through unpredictable and diverse terrain. We amputated both hindlimbs of blue-spotted salamanders (Ambystoma laterale) and measured their body and tail kinematics as the limbs regenerated. We quantified the change in the body wave over time and compared them to an amphibious fish species, Polypterus senegalus. We found that salamanders in the early stages of regeneration shift their kinematics, mostly around their pectoral girdle, where there is a local increase in undulation frequency. Amputated salamanders also show a reduced range of preferred walking speeds and an increase in the number of bending waves along the body. This work could assist roboticists working on terrestrial locomotion and water to land transitions.

Terrestrial acclimation and exercise lead to bone functional response in Polypterus senegalus pectoral fins

The ability of bones to sense and respond to mechanical loading is a central feature of vertebrate skeletons. However, the functional demands imposed on terrestrial and aquatic animals differ vastly. The pectoral girdle of the basal actinopterygian fish Polypterus senegalus was previously shown to exhibit plasticity following terrestrial acclimation, but the pectoral fin itself has yet to be examined. We investigated skeletal plasticity in the pectoral fins of P. senegalus after exposure to terrestrial loading. Juvenile fish were divided into three groups: a control group was kept under aquatic conditions without intervention, an exercised group was also kept in water but received daily exercise on land, and a terrestrial group was kept in a chronic semi-terrestrial condition. After 5 weeks, the pectoral fins were cleared and stained with Alcian Blue and Alizarin Red to visualize cartilage and bone, allowing measurements of bone length, bone width, ossification and curvature to be taken for the endochondral radial bones. Polypterus senegalus fin bones responded most strongly to chronic loading in the terrestrial condition. Fish that were reared in a terrestrial environment had significantly longer bones compared with those of aquatic controls, wider propterygia and metapterygia, and more ossified metapterygia and medial radials, and they showed changes in propterygial curvature. Exercised fish also had longer and more ossified medial radials compared with those of controls. Polypterus senegalus fin bones exhibit plasticity in response to novel terrestrial loading. Such plasticity could be relevant for transitions between water and land on evolutionary scales, but key differences between fish and tetrapod bone make direct comparisons challenging.

Decoding the essential interplay between central and peripheral control in adaptive locomotion of amphibious centipedes

Amphibious animals adapt their body coordination to compensate for changing substrate properties as they transition between terrestrial and aquatic environments. Using behavioural experiments and mathematical modelling of the amphibious centipede Scolopendra subspinipes mutilans, we reveal an interplay between descending command (brain), local pattern generation, and sensory feedback that controls the leg and body motion during swimming and walking. The elongated and segmented centipede body exhibits a gradual transition in the locomotor patterns as the animal crosses between land and water. Changing environmental conditions elicit a mechano-sensory feedback mechanism, inducing a gait change at the local segment level. The body segments operating downstream of a severed nerve cord (no descending control) can generate walking with mechano-sensory inputs alone while swimming behaviour is not recovered. Integrating the descending control for swimming initiation with the sensory feedback control for walking in a mathematical model successfully generates the adaptive behaviour of centipede locomotion, capturing the possible mechanism for flexible motor control in animals.

Emersion and Terrestrial Locomotion of the Northern Snakehead (Channa argus) on Multiple Substrates

Most fishes known for terrestrial locomotion are small and/or elongate. Northern snakeheads (Channa argus) are large, air-breathing piscivores anecdotally known for terrestrial behaviors. Our goals were to determine their environmental motivations for emersion, describe their terrestrial kinematics for fish 3.0-70.0 cm and compare kinematics among four substrates. For emersion experiments, C. argus was individually placed into aquatic containers with ramps extending through the surface of the water, and exposed to 15 ecologically-relevant environmental conditions. For kinematic experiments, fish were filmed moving on moist bench liner, grass, artificial turf, and a flat or tilted rubber boat deck. Videos were digitized for analysis in MATLAB and electromyography was used to measure muscular activity. Only the low pH (4.8), high salinity (30 ppt), and high dCO₂ (10% seltzer solution) treatments elicited emersion responses. While extreme, these conditions do occur in some of their native Asian swamps. Northern snakeheads >4.5 cm used a unique form of axial-appendage-based terrestrial locomotion involving cyclic oscillations of the axial body, paired with near-simultaneous movements of both pectoral fins. Individuals ≤3.5 cm used tail-flip jumps to travel on land. Northern snakeheads also moved more quickly on complex, three-dimensional substrates (e.g., grass) than on smooth substrates (e.g., bench liner), and when moving downslope. Release of snakeheads onto land by humans or accidentally by predators may be more common than voluntary emersion, but because northern snakeheads can respire air, it may be necessary to factor in the ability to spread overland into the management of this invasive species.

Aerial and aquatic visual acuity of the grey bichir Polypterus senegalus, as estimated by optokinetic response

The present study assessed the aerial and aquatic visual abilities of juvenile grey bichir Polypterus senegalus, fish capable of terrestrial locomotion, by measuring the optokinetic response to stimuli of varying speed and spatial frequency. In water, fish tracked slow-moving (2° s^-1 ) stimuli moderately well and fast-moving stimuli very poorly. Spatial acuity was very low compared with many other species, with maximum response observed at 0.05-0.075 stimulus cycles per degree of visual arc; however, it should be noted that adult fish, with their larger eyes, are likely to have somewhat improved spatial acuity. Low spatial acuity and limited stimulus tracking ability might be expected in a nocturnal ambush predator such as P. senegalus, where gaze stabilization may be less crucial and other sensory inputs may have greater importance in perception of the environment. In air, spatial and temporal acuity were both poorer by every measure, but some visual ability persisted. As the eye shows no anatomical specialization for aerial vision, poor vision was expected; however, the large decrease in saccade velocity observed in air trials was unexpected. Stimulus parameters typically have little effect on the characteristics of the saccade, so this finding may suggest that the function of the reflex system itself could be compromised in the aerial vision of some fishes capable of terrestrial locomotion.

Where do fish go when stranded on land? Terrestrial orientation of the mangrove rivulus Kryptolebias marmoratus

The goal of the present study was to determine which sensory cues the mangrove rivulus Kryptolebias marmoratus, a quasi-amphibious, hermaphroditic fish, uses to orient in an unfamiliar terrestrial environment. In a laboratory setting, K. marmoratus were placed on a terrestrial test arena and were provided the opportunity to move toward reflective surfaces, water, dark colours v. light colours, and orange colouration. Compared with hermaphrodites, males moved more often toward an orange section of the test arena, suggesting that the response may be associated with camouflage or male-male competition, since only males display orange colouration. Younger individuals also moved more often toward the orange quadrant than older individuals, suggesting age-dependent orientation performance or behaviour. Sloped terrain also had a significant effect on orientation, with more movement downhill, suggesting the importance of the otolith-vestibular system in terrestrial orientation of K. marmoratus. By understanding the orientation of extant amphibious fishes, we may be able to infer how sensory biology and behaviour might have evolved to facilitate invasion of land by amphibious vertebrates millions of years ago.

A walking behavior generates functional overland movements in the tidepool sculpin, Oligocottus maculosus

Tidepool sculpins (Oligocottus maculosus) have been observed moving overland in the rocky intertidal, and we documented the terrestrial walking behavior that they use to accomplish this. We quantified the terrestrial movements of O. maculosus and compared them to (1) their aquatic locomotion, (2) terrestrial locomotion of closely-related subtidal species (Leptocottus armatus and Icelinus borealis), and (3) terrestrial movements of walking catfishes (Clarias spp.). We recorded sculpin movements (210 fps) on a terrestrial platform and in a water tank and tracked body landmarks for kinematic analysis. The axial-appendage-based terrestrial locomotion of O. maculosus is driven by cyclic lateral oscillations of the tail, synchronized with alternating rotations about the base of the pectoral fins, a behavior that appears similar to a military "army crawl." The pectoral fins do not provide propulsion, but act as stable points for the body to rotate around. In contrast, individuals of O. maculosus use primarily axial undulation during slow-speed swimming. The army crawl is a more effective terrestrial behavior (greater distance ratio) than the movements produced by L. armatus and I. borealis, which use rapid, cyclic oscillations of the tail, without coordinated pectoral fin movements. Relative to Clarias spp., O. maculosus rotated the body about the base of the pectoral fin, rather than the tip of the fin, which may cause O. maculosus to have a lower distance ratio. Since O. maculosus lack major morphological adaptations for terrestrial locomotion, instead relying on behavioral adaptations, we propose behavioral adaptations may evolutionarily predate morphological adaptations for terrestrial locomotion in vertebrates.

Fin and body neuromuscular coordination changes during walking and swimming in Polypterus senegalus

The ability to modulate the function of muscle is integral to an animal's ability to function effectively in the face of widely disparate challenges. This modulation of function can manifest through short-term changes in neuromuscular control, but also through long-term changes in force profiles, fatiguability and architecture. However, the relative extent to which shorter-term modulation and longer-term plasticity govern locomotor flexibility remains unclear. Here, we obtain simultaneously recorded kinematic and muscle activity data of fin and body musculature of an amphibious fish, Polypterus senegalus After examining swimming and walking behaviour in aquatically raised individuals, we show that walking behaviour is characterized by greater absolute duration of muscle activity in most muscles when compared with swimming, but that the magnitude of recruitment during walking is only increased in the secondary bursts of fin muscle and in the primary burst of the mid-body point. This localized increase in intensity suggests that walking in P. senegalus is powered in a few key locations on the fish, contrasting with the more distributed, low intensity muscle force that characterizes the stroke cycle during swimming. Finally, the increased intensity in secondary, but not primary, bursts of the fin muscles when walking probably underscores the importance of antagonistic muscle activity to prevent fin collapse, add stabilization and increase body support. Understanding the principles that underlie the flexibility of muscle function can provide key insights into the sources of animal functional and behavioural diversity.

Oxygen drives skeletal muscle remodeling in an amphibious fish out of water

Skeletal muscle remodeling in response to terrestrial acclimation improves the locomotor performance of some amphibious fishes on land, but the cue for this remodeling is unknown. We tested the hypothesis that muscle remodeling in the amphibious Kryptolebias marmoratus on land is driven by higher O2 availability in atmospheric air, and the alternative hypothesis that remodeling is induced by a different environmental or physiological condition fish experience on land. Fish were acclimated to 28 days of air, aquatic hyperoxia, hypercapnia, hypoxia, elevated temperature, or fasting conditions. Air, fasting, and hyperoxic conditions increased (>25%) the size of oxidative fibers in K. marmoratus while hypoxia had the reverse effect (23% decrease). Surprisingly, hyperoxia-acclimation also resulted in a transformation of the musculature to include large bands of oxidative-like muscle. Our results show that K. marmoratus is highly responsive to environmental O2 levels and capitalize on O2-rich opportunities to enhance O2 utilization by skeletal muscle.

Phenotypic plasticity of muscle fiber type in the pectoral fins of Polypterus senegalus reared in a terrestrial environment

Muscle fiber types in the pectoral fins of fishes have rarely been examined, despite their morphological and functional diversity. Here, we describe the distribution of fast and slow muscle fibers in the pectoral fins of Polypterus senegalus, an amphibious, basal actinopterygian. Each of the four muscle groups examined using mATPase staining showed distinct fiber-type regionalization. Comparison between fish raised in aquatic and terrestrial environments revealed terrestrially reared fish possess 28% more fast muscle compared with aquatically reared fish. The pattern of proximal-distal variation in the abductors differed, with a relative decrease in fast muscle fibers near the pectoral girdle in aquatic fish compared with an increase in terrestrial fish. Terrestrially reared fish also possess a greater proportion of very small diameter fibers, suggesting that they undergo more growth via hyperplasia. These observations may be a further example of adaptive plasticity in Polypterus, allowing for greater bursts of power during terrestrial locomotion.

Fins into limbs: Recent insights from sarcopterygian fish

Limbs with digits evolved as sarcopterygian fish transitioned to a terrestrial life, giving rise to modern tetrapods. Since the Devonian, most of the sarcopterygian fish diversity became extinct, with the only surviving representatives being two coelacanth and six lungfish species. As the sister group of tetrapods, sarcopterygian fish constitute the ideal models to address questions regarding the transition of vertebrates from water to land. However, distantly related yet experimentally amenable teleost fish species have instead become the organisms of choice for geneticists and developmental biologists. Comparative studies using teleosts, such as zebrafish, have greatly enriched our knowledge on the genetic and regulatory mechanisms underlying fin development. However, their highly derived fin anatomy provides limited insights on the origin of limbs and the teleost-specific whole-genome duplication represents a further complication to comparisons of gene function and regulation. In recent years, refined morphological and behavioral studies and access to lungfish embryos and availability of genetic resources have underscored the value of nontetrapod sarcopterygians as model organisms. Here we review recent studies using lungfishes and coelacanths that shed light on various aspects of the evolution of the tetrapod limb, including the origin of the tetrapod pelvis, limb musculature, digits, locomotion, and regenerative capacity.