Developmental plasticity and the origin of tetrapods

Comprehensive review on collagen extraction from food by-products and waste as a value-added material

The consumption of animal products has witnessed a significant increase over the years, leading to a growing need for industries to adopt strict waste control measures to mitigate environmental impacts. The disposal of animal waste in landfill can result in diverse and potentially hazardous decomposition by-products. Animal by-products, derived from meat, poultry, seafood and fish industries, offer a substantial raw material source for collagen and gelatin production due to their high protein content. Collagen, being a major protein component of animal tissues, represents an abundant resource that finds application in various chemical and material industries. The demand for collagen-based products continues to grow, yet the availability of primary material remains limited and insufficient to meet projected needs. Consequently, repurposing waste materials that contain collagen provides an opportunity to meet this need while at the same time minimizing the amount of waste that is dumped. This review examines the potential to extract value from the collagen content present in animal-derived waste and by-products. It provides a systematic evaluation of different species groups and discusses various approaches for processing and fabricating repurposed collagen. This review specifically focuses on collagen-based research, encompassing an examination of its physical and chemical properties, as well as the potential for chemical modifications. We have detailed how the research and knowledge built on collagen structure and function will drive the new initiatives that will lead to the development of new products and opportunities in the future. Additionally, it highlights emerging approaches for extracting high-quality protein from waste and discusses efforts to fabricate collagen-based materials leading to the development of new and original products within the chemical, biomedical and physical science-based industries.

Comparisons of developmental processes of air-breathing organs among terrestrial isopods (Crustacea, Oniscidea): implications for their evolutionary origins

BACKGROUND

The acquisition of air-breathing organs is one of the key innovations for terrestrialization in animals. Terrestrial isopods, a crustacean lineage, can be interesting models to study the evolution of respiratory organs, as they exhibit varieties of air-breathing structures according to their habitats. However, the evolutionary processes and origins of these structures are unclear, due to the lack of information about their developmental processes. To understand the developmental mechanisms, we compared the developmental processes forming different respiratory structures in three isopod species, i.e., 'uncovered lungs' in Nagurus okinawaensis (Trachelipodidae), 'dorsal respiratory fields' in Alloniscus balssi (Alloniscidae), and pleopods without respiratory structures in Armadilloniscus cf. ellipticus (Detonidae).

RESULTS

In N. okinawaensis with uncovered lungs, epithelium and cuticle around the proximal hemolymph sinus developed into respiratory structures at post-manca juvenile stages. On the other hand, in Al. balssi with dorsal respiratory fields, the region for the future respiratory structure was already present at manca 1 stage, immediately after hatching, where the lateral protrusion of ventral epithelium occurred, forming the respiratory structure. Furthermore, on pleopods in Ar. cf. ellipticus, only thickened dorsal cuticle and the proximal hemolymph sinus developed during postembryonic development without special morphogenesis.

CONCLUSIONS

This study shows that the respiratory structures in terrestrial isopods develop primarily by postembryonic epithelial modifications, but the epithelial positions developing into respiratory structures differ between uncovered lungs and dorsal respiratory fields. This suggests that these two types of respiratory structures do not result from simple differences in the degree of development. Future analysis of molecular developmental mechanisms will help determine whether these are the result of heterotopic changes or have different evolutionary origins. Overall, this study provides fundamental information for evolutionary developmental studies of isopod respiratory organs.

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.

Agential autonomy and biological individuality

What is a biological individual? How are biological individuals individuated? How can we tell how many individuals there are in a given assemblage of biological entities? The individuation and differentiation of biological individuals are central to the scientific understanding of living beings. I propose a novel criterion of biological individuality according to which biological individuals are autonomous agents. First, I articulate an ecological-dynamical account of natural agency according to which, agency is the gross dynamical capacity of a goal-directed system to bias its repertoire to respond to its conditions as affordances. Then, I argue that agents or agential dynamical systems can be agentially dependent on, or agentially autonomous from, other agents and that this agential dependence/autonomy can be symmetrical or asymmetrical, strong or weak. Biological individuals, I propose, are all and only those agential dynamical systems that are strongly agentially autonomous. So, to determine how many individuals there are in a given multiagent aggregate, such as multicellular organism, a colony, symbiosis, or a swarm, we first have to identify how many agential dynamical systems there are, and then what their relations of agential dependence/autonomy are. I argue that this criterion is adequate to the extent that it vindicates the paradigmatic cases, and explains why the paradigmatic cases are paradigmatic, and why the problematic cases are problematic. Finally, I argue for the importance of distinguishing between agential and causal dependence and show the relevance of agential autonomy for understanding the explanatory structure of evolutionary developmental biology.

Promises and limits of an agency perspective in evolutionary developmental biology

An agent-based perspective in the study of complex systems is well established in diverse disciplines, yet is only beginning to be applied to evolutionary developmental biology. In this essay, we begin by defining agency and associated terminology formally. We then explore the assumptions and predictions of an agency perspective, apply these to select processes and key concept areas relevant to practitioners of evolutionary developmental biology, and consider the potential epistemic roles that an agency perspective might play in evo devo. Throughout, we discuss evidence supportive of agential dynamics in biological systems relevant to evo devo and explore where agency thinking may enrich the explanatory reach of research efforts in evolutionary developmental biology.

Early onset of urea synthesis and ammonia detoxification pathways in three terrestrially developing frogs

Frogs evolved terrestrial development multiple times, necessitating mechanisms to avoid ammonia toxicity at early stages. Urea synthesis from ammonia is a key adaptation that reduces water dependence after metamorphosis. We tested for early expression and plasticity of enzymatic mechanisms of ammonia detoxification in three terrestrial-breeding frogs: foam-nest-dwelling larvae of Leptodactylus fragilis (Lf) and arboreal embryos of Hyalinobatrachium fleischmanni (Hf) and Agalychnis callidryas (Ac). Activity of two ornithine-urea cycle (OUC) enzymes, arginase and CPSase, and levels of their products urea and CP in tissues were high in Lf regardless of nest hydration, but reduced in experimental low- vs. high-ammonia environments. High OUC activity in wet and dry nests, comparable to that under experimental high ammonia, suggests terrestrial Lf larvae maintain high capacity for urea excretion regardless of their immediate risk of ammonia toxicity. This may aid survival through unpredictably long waiting periods before rain enables their transition to water. Moderate levels of urea and CP were present in Hf and Ac tissues and enzymatic activities were lower than in Lf. In both species, embryos in drying clutches can hatch and enter the water early, behaviorally avoiding ammonia toxicity. Moreover, glutamine synthetase was active in early stages of all three species, condensing ammonia and glutamate to glutamine as another mechanism of detoxification. Enzyme activity appeared highest in Lf, although substrate and product levels were higher in Ac and Lf. Our results reveal that multiple biochemical mechanisms of ammonia detoxification occur in early life stages of anuran lineages that evolved terrestrial development.

Darwin's agential materials: evolutionary implications of multiscale competency in developmental biology

A critical aspect of evolution is the layer of developmental physiology that operates between the genotype and the anatomical phenotype. While much work has addressed the evolution of developmental mechanisms and the evolvability of specific genetic architectures with emergent complexity, one aspect has not been sufficiently explored: the implications of morphogenetic problem-solving competencies for the evolutionary process itself. The cells that evolution works with are not passive components: rather, they have numerous capabilities for behavior because they derive from ancestral unicellular organisms with rich repertoires. In multicellular organisms, these capabilities must be tamed, and can be exploited, by the evolutionary process. Specifically, biological structures have a multiscale competency architecture where cells, tissues, and organs exhibit regulative plasticity-the ability to adjust to perturbations such as external injury or internal modifications and still accomplish specific adaptive tasks across metabolic, transcriptional, physiological, and anatomical problem spaces. Here, I review examples illustrating how physiological circuits guiding cellular collective behavior impart computational properties to the agential material that serves as substrate for the evolutionary process. I then explore the ways in which the collective intelligence of cells during morphogenesis affect evolution, providing a new perspective on the evolutionary search process. This key feature of the physiological software of life helps explain the remarkable speed and robustness of biological evolution, and sheds new light on the relationship between genomes and functional anatomical phenotypes.

Plastic loss of motile cilia in the gills of Polypterus in response to high CO2 or terrestrial environments

The evolutionary transition of vertebrates from water to land during the Devonian period was accompanied by major changes in animal respiratory systems in terms of physiology and morphology. Indeed, the fossil record of the early tetrapods has revealed the existence of internal gills, which are vestigial fish-like traits used underwater. However, the fossil record provides only limited data on the process of the evolutionary transition of gills from fish to early tetrapods. This study investigated the gills of Polypterus senegalus, a basal ray-finned/amphibious fish which shows many ancestral features of stem Osteichthyes. Based on scanning electron microscopy observations and transcriptome analysis, the existence of motile cilia in the gills was revealed which may create a flow on the gill surface leading to efficient ventilation or remove particles from the surface. Interestingly, these cilia were observed to disappear after rearing in terrestrial or high CO₂ environments, which mimics the environmental changes in the Devonian period. The cilia re-appeared after being returned to the original aquatic environment. The ability of plastic changes of gills in Polypterus revealed in this study may allow them to survive in fluctuating environments, such as shallow swamps. The ancestor of Osteichthyes is expected to have possessed such plasticity in the gills, which may be one of the driving forces behind the transition of vertebrates from water to land.

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.

Unique bone microanatomy reveals ancestry of subterranean specializations in mammals

Acquiring a subterranean lifestyle entails a substantial shift for many aspects of terrestrial vertebrates' biology. Although this lifestyle is associated with multiple instances of convergent evolution, the relative success of some subterranean lineages largely remains unexplained. Here, we focus on the mammalian transitions to life underground, quantifying bone microanatomy through high-resolution X-ray tomography. The true moles stand out in this dataset. Examination of this family's bone histology reveals that the highly fossorial moles acquired a unique phenotype involving large amounts of compacted coarse cancellous bone. This phenotype exceeds the adaptive optimum seemingly shared by several other subterranean mammals and can be traced back to some of the first known members of the family. This remarkable microanatomy was acquired early in the history of the group and evolved faster than the gross morphology innovations of true moles' forelimb. This echoes the pattern described for other lifestyle transitions, such as the acquisition of bone mass specializations in secondarily aquatic tetrapods. Highly plastic traits-such as those pertaining to bone structure-are hence involved in the early stages of different types of lifestyle transitions.

Rapid acquisition of microorganisms and microbial genes can help explain punctuated evolution

The punctuated mode of evolution posits that evolution occurs in rare bursts of rapid evolutionary change followed by long periods of genetic stability (stasis). The accepted cause for the rapid changes in punctuated evolution is special ecological circumstances – selection forces brought about by changes in the environment. This article presents a complementary explanation for punctuated evolution by the rapid formation of genetic variants in animals and plants by the acquisition of microorganisms from the environment into microbiomes and microbial genes into host genomes by horizontal gene transfer. Several examples of major evolutionary events driven by microorganisms are discussed, including the formation of the first eukaryotic cell, the ability of some animals to digest cellulose and other plant cell-wall complex polysaccharides, dynamics of root system architecture, and the formation of placental mammals. These changes by cooperation were quantum leaps in the evolutionary development of complex bilolgical systems and can contribute to an understanding of the mechanisms underlying punctuated evolution.

Bioinspired Design in Research: Evolution as Beta-Testing

Modern fishes represent over 400 million years of evolutionary processes that, in many cases, resulted in selection for phenotypes with particular performance advantages. While this certainly occurred without a trajectory for optimization, it cannot be denied that some morphologies allow organisms to be more effective than others at tasks like evading predation, securing food, and ultimately passing on their genes. In this way, evolution generates a series of iterative prototypes with varying but measurable success in accomplishing objectives. Therefore, careful analysis of fundamental properties underlying biological phenomena allow us to fast-track development of bioinspired technologies aiming to accomplish similar objectives. At the same time, bioinspired designs can be a way to explore evolutionary processes, by better understanding the performance space within which a given morphology operates. Through strong interdisciplinary collaborations, we can develop novel bioinspired technologies that not only excel as robotic devices but that teach us something about biology and the rules of life in the process.

When the end modifies its means: the origins of novelty and the evolution of innovation

The origin of novel complex traits constitutes a central yet largely unresolved challenge in evolutionary biology. Intriguingly, many of the most promising breakthroughs in understanding the genesis of evolutionary novelty in recent years have occurred not in evolutionary biology itself, but through the comparative study of development and, more recently, the interface of developmental biology and ecology. Here, I discuss how these insights are changing our understanding of what matters in the origin of novel, complex traits in ontogeny and evolution. Specifically, my essay has two major objectives. First, I discuss how the nature of developmental systems biases the production of phenotypic variation in the face of novel or stressful environments toward functional, integrated and, possibly, adaptive variants. This, in turn, allows the production of novel phenotypes to precede (rather than follow) changes in genotype and allows developmental processes that are the product of past evolution to shape evolutionary change that has yet to occur. Second, I explore how this nature of developmental systems has itself evolved over time, increasing the repertoire of ontogenies to pursue a wider range of objectives across an expanding range of conditions, thereby creating an increasingly extensive affordance landscape in development and developmental evolution. Developmental systems and their evolution can thus be viewed as dynamic processes that modify their own means across ontogeny and phylogeny. The study of these dynamics necessitates more than the strict reductionist approach that currently dominates the fields of developmental and evolutionary developmental biology.

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.

EcoPhysioMechanics: Integrating energetics and biomechanics to understand fish locomotion under climate change

Ecological physiologists and biomechanists have been broadly investigating swimming performance in a diversity of fishes, however the connection between form, function and energetics of locomotion has been rarely evaluated in the same system and under climate change scenarios. In this perspective I argue that working within the framework of 'EcoPhysioMechanics', i.e., integrating energetics and biomechanics tools, to measure locomotor performance and behavior under different abiotic factors, improves our understanding of the mechanisms, limits and costs of movement. To demonstrate how ecophysiomechanics can be applied to locomotor studies, I outline how linking biomechanics and physiology allows us to understand how fishes may modulate their movement to achieve high speeds or reduce the costs of locomotion. I also discuss how the framework is necessary to quantify swimming capacity under climate change scenarios. Finally, I discuss current dearth of integrative studies and gaps in empirical datasets that are necessary to understand fish swimming under changing environments.

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.

Skeletal repatterning enhances the protective capacity of the shell in African hinge-back tortoises (Kinixys)

Changes in the structural association of skeletal traits are crucial to the evolution of novel forms and functions. In vertebrates, such rearrangements often occur gradually and may precede or coincide with the functional activation of skeletal traits. To illustrate this process, we examined the ontogeny of African hinge-back tortoises (Kinixys spp.). Kinixys species feature a moveable "hinge" on the dorsal shell (carapace) that enables shell closure (kinesis) when the hind limbs are withdrawn. This hinge, however, is absent in juveniles. Herein, we describe how this unusual phenotype arises via alterations in the tissue configuration and shape of the carapace. The ontogenetic repatterning of osseous and keratinous tissue coincided with shifts in morphological integration and the establishment of anterior (static) and posterior (kinetic) carapacial modules. Based on ex vivo skeletal movement and raw anatomy, we propose that Kinixys employs a "sliding hinge" shell-closing system that overcomes thoracic rigidity and enhances the protective capacity of the carapace. Universal properties of the vertebrate skeleton, such as plasticity, modularity, and secondary maturation processes, contributed to adaptive evolutionary change in Kinixys. We discuss a hypothetical model to explain the delayed emergence of skeletal traits and its relevance to the origins of novel form-to-function relationships.

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.

Terrestrial force production by the limbs of a semi-aquatic salamander provides insight into the evolution of terrestrial locomotor mechanics

Amphibious fishes and salamanders are valuable functional analogs for vertebrates that spanned the water-to-land transition. However, investigations of walking mechanics have focused on terrestrial salamanders and, thus, may better reflect the capabilities of stem tetrapods that were already terrestrial. The earliest tetrapods were likely aquatic, so salamanders that are not primarily terrestrial may yield more appropriate data for modelling the incipient stages of terrestrial locomotion. In the present study, locomotor biomechanics were quantified from semi-aquatic Pleurodeles waltl, a salamander that spends most of its adult life in water, and then compared to a primarily terrestrial salamander (Ambystoma tigrinum) and semi-aquatic fish (Periophthalmus barbarus) to evaluate whether terrestrial locomotion was more comparable between species with ecological versus phylogenetic similarities. Ground reaction forces (GRFs) from individual limbs or fins indicated that the pectoral appendages of each taxon had distinct patterns of force production, but GRFs from the hind limbs were comparable between the salamander species. The rate that force is produced can affect musculoskeletal function, so we also calculated ‘yank’ (first time derivative of force) to quantify the dynamics of GRF production. Yank was sometimes slower in P. waltl but there were some similarities between the three species. Finally, the semi-aquatic taxa (P. waltl and P. barbarus) had a more medial inclination of the GRF compared to terrestrial salamanders, potentially elevating bone stresses among more aquatic taxa and limiting their excursions onto land.

Reproductive colonization of land by frogs: Embryos and larvae excrete urea to avoid ammonia toxicity

Vertebrate colonization of land has occurred multiple times, including over 50 origins of terrestrial eggs in frogs. Some environmental factors and phenotypic responses that facilitated these transitions are known, but responses to water constraints and risk of ammonia toxicity during early development are poorly understood. We tested if ammonia accumulation and dehydration risk induce a shift from ammonia to urea excretion during early stages of four anurans, from three origins of terrestrial development. We quantified ammonia and urea concentrations during early development on land, under well-hydrated and dry conditions. Where we found urea excretion, we tested for a plastic increase under dry conditions and with ammonia accumulation in developmental environments. We assessed the potential adaptive role of urea excretion by comparing ammonia tolerance measured in 96h-LC50 tests with ammonia levels in developmental environments. Ammonia accumulated in foam nests and perivitelline fluid, increasing over development and reaching higher concentrations under dry conditions. All four species showed high ammonia tolerance, compared to fishes and aquatic-breeding frogs. Both nest-dwelling larvae of Leptodactylus fragilis and late embryos of Hyalinobatrachium fleischmanni excreted urea, showing a plastic increase under dry conditions. These two species can develop the longest on land and urea excretion appears adaptive, preventing their exposure to potentially lethal levels of ammonia. Neither late embryos of Agalychnis callidryas nor nest-dwelling larvae of Engystomops pustulosus experienced toxic ammonia levels under dry conditions, and neither excreted urea. Our results suggest that an early onset of urea excretion, its increase under dry conditions, and elevated ammonia tolerance can all help prevent ammonia toxicity during terrestrial development. High ammonia represents a general risk for development which may be exacerbated as climate change increases dehydration risk for terrestrial-breeding frogs. It may also be a cue that elicits adaptive physiological responses during early development.

Osteohistology of Greererpeton provides insight into the life history of an early Carboniferous tetrapod

The vertebrate transition to land is one of the most consequential, yet poorly understood periods in tetrapod evolution. Despite the importance of the water-land transition in establishing modern ecosystems, we still know very little about the life histories of the earliest tetrapods. Bone histology provides an exceptional opportunity to study the biology of early tetrapods and has the potential to reveal new insights into their life histories. Here, we examine the femoral bone histology from an ontogenetic series of Greererpeton, an early tetrapod from the Middle-Late Mississippian (early Carboniferous) of North America. Thin-sections and micro-CT data show a moderately paced rate of bone deposition with significant cortical thickening through development. An interruption to regular bone deposition, as indicated by a zone of avascular tissue and growth marks, is notable at the same late juvenile stage of development throughout our sample. This suggests that an inherent aspect to the life history of juvenile Greererpeton resulted in a temporary reduction in bone deposition. We review several possible life history correlates for this bony signature including metamorphosis, an extended juvenile phase, environmental stress, and movement (migration/dispersal) between habitats. We argue that given the anatomy of Greererpeton, it is unlikely that events related to polymorphism (metamorphosis, extended juvenile phase) can explain the bony signature observed in our sample. Furthermore, the ubiquity of this signal in our sample indicates a taxon-level rather than a population-level trait, which is expected for an environmental stress. We conclude that movement via dispersal represents a likely correlate, as such events are a common life history strategy of aquatically bound vertebrates.

Bridging the explanatory gaps: What can we learn from a biological agency perspective?

We begin this article by delineating the explanatory gaps left by prevailing gene-focused approaches in our understanding of phenotype determination, inheritance, and the origin of novel traits. We aim not to diminish the value of these approaches but to highlight where their implementation, despite best efforts, has encountered persistent limitations. We then discuss how each of these explanatory gaps can be addressed by expanding research foci to take into account biological agency-the capacity of living systems at various levels to participate in their own development, maintenance, and function by regulating their structures and activities in response to conditions they encounter. Here we aim to define formally what agency and agents are and-just as importantly-what they are not, emphasizing that agency is an empirical property connoting neither intention nor consciousness. Lastly, we discuss how incorporating agency helps to bridge explanatory gaps left by conventional approaches, highlight scientific fields in which implicit agency approaches are already proving valuable, and assess the opportunities and challenges of more systematically incorporating biological agency into research programs.

More than Breathing Air: Evolutionary Drivers and Physiological Implications of an Amphibious Lifestyle in Fishes

Amphibious and aquatic air-breathing fishes both exchange respiratory gasses with the atmosphere, but these fishes differ in physiology, ecology, and possibly evolutionary origins. We introduce a scoring system to characterize interspecific variation in amphibiousness and use this system to highlight important unanswered questions about the evolutionary physiology of amphibious fishes.

Recombination facilitates genetic assimilation of new traits in gene regulatory networks

A new phenotypic variant may appear first in organisms through plasticity, that is, as a response to an environmental signal or other nongenetic perturbation. If such trait is beneficial, selection may increase the frequency of alleles that enable and facilitate its development. Thus, genes may take control of such traits, decreasing dependence on nongenetic disturbances, in a process called genetic assimilation. Despite an increasing amount of empirical studies supporting genetic assimilation, its significance is still controversial. Whether genetic assimilation is widespread depends, to a great extent, on how easily mutation and recombination reduce the trait's dependence on nongenetic perturbations. Previous research suggests that this is the case for mutations. Here we use simulations of gene regulatory network dynamics to address this issue with respect to recombination. We find that recombinant offspring of parents that produce a new phenotype through plasticity are more likely to produce the same phenotype without requiring any perturbation. They are also prone to preserve the ability to produce that phenotype after genetic and nongenetic perturbations. Our work also suggests that ancestral plasticity can play an important role for setting the course that evolution takes. In sum, our results indicate that the manner in which phenotypic variation maps unto genetic variation facilitates evolution through genetic assimilation in gene regulatory networks. Thus, we contend that the importance of this evolutionary mechanism should not be easily neglected.

The Evolution of Appendicular Muscles During the Fin-to-Limb Transition: Possible Insights Through Studies of Soft Tissues, a Perspective

The evolution of the appendages during the fin-to-limb transition has been extensively studied, yet the majority of studies focused on the skeleton and the fossil record. Whereas the evolution of the anatomy of the appendicular musculature has been studied, the changes in the muscular architecture during the fin-to-limb transition remain largely unstudied, yet may provide important new insights. The fin-to-limb transition is associated with the appearance of a new mode of locomotion and the associated shift from pectoral to pelvic dominance. Here, we propose ways to investigate this question and review data on muscle mass and muscle architecture of the pectoral and pelvic muscles in extant vertebrates. We explore whether changes in appendage type are associated with changes in the muscular architecture and the relative investment in different muscle groups. These preliminary data show a general increase in the muscle mass of the appendages relative to the body mass during the fin-to-limb transition. The locomotor shift suggested to occur during the fin-to-limb transition appears supported by our preliminary data since in “fish” the pectoral fins are heavier than the pelvic fins, whereas in tetrapods, the forelimb muscles are less developed than the hind limb muscles. Finally, a shift in the investment in different muscle groups with an increase of the contribution of the superficial groups in tetrapods compared to “fish” appears to take place. Our study highlights the potential of investigating quantitative features of the locomotor muscles, yet also demonstrates the lack of quantitative data allowing to test these ideas.

The More, the Merrier? Multiple Myoglobin Genes in Fish Species, Especially in Gray Bichir (Polypterus senegalus) and Reedfish (Erpetoichthys calabaricus)

The members of the globin superfamily are a classical model system to investigate gene evolution and their fates as well as the diversity of protein function. One of the best-known globins is myoglobin (Mb), which is mainly expressed in heart muscle and transports oxygen from the sarcolemma to the mitochondria. Most vertebrates harbor a single copy of the myoglobin gene, but some fish species have multiple myoglobin genes. Phylogenetic analyses indicate an independent emergence of multiple myoglobin genes, whereby the origin is mostly the last common ancestor of each order. By analyzing different transcriptome data sets, we found at least 15 multiple myoglobin genes in the polypterid gray bichir (Polypterus senegalus) and reedfish (Erpetoichthys calabaricus). In reedfish, the myoglobin genes are expressed in a broad range of tissues but show very different expression values. In contrast, the Mb genes of the gray bichir show a rather scattered expression pattern; only a few Mb genes were found expressed in the analyzed tissues. Both, gray bichir and reedfish possess lungs which enable them to inhabit shallow and swampy waters throughout tropical Africa with frequently fluctuating and low oxygen concentrations. The myoglobin repertoire probably reflects the molecular adaptation to these conditions. The sequence divergence, the substitution rate, and the different expression pattern of multiple myoglobin genes in gray bichir and reedfish imply different functions, probably through sub- and neofunctionalization during evolution.

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.

Long lasting effects of early temperature exposure on the swimming performance and skeleton development of metamorphosing Gilthead seabream (Sparus aurata L.) larvae

Temperatures experienced during early ontogeny significantly influence fish phenotypes, with clear consequences for the wild and reared stocks. We examined the effect of temperature (17, 20, or 23 °C) during the short embryonic and yolk-sac larval period, on the swimming performance and skeleton of metamorphosing Gilthead seabream larvae. In the following ontogenetic period, all fish were subjected to common temperature (20 °C). The critical swimming speed of metamorphosing larvae was significantly decreased from 9.7 ± 0.6 TL/s (total length per second) at 17 °C developmental temperature (DT) to 8.7 ± 0.6 and 8.8 ± 0.7 TL/s at 20 and 23 °C DT respectively (p < 0.05). Swimming performance was significantly correlated with fish body shape (p < 0.05). Compared with the rest groups, fish of 17 °C DT presented a slender body shape, longer caudal peduncle, terminal mouth and ventrally transposed pectoral fins. Moreover, DT significantly affected the relative depth of heart ventricle (VD/TL_, p < 0.05), which was comparatively increased at 17 °C DT. Finally, the incidence of caudal-fin abnormalities significantly decreased (p < 0.05) with the increase of DT. To our knowledge, this is the first evidence for the significant effect of DT during the short embryonic and yolk-sac larval period on the swimming performance of the later stages.

Waxing and Waning of Wings

A major challenge to Darwinian evolution is explaining 'rudimentary' organs. This is particularly relevant to birds: rudimentary wings occur in fossils, as well as in developing, molting, and flight-impaired birds. Evidence shows that young birds flap small wings to improve locomotion and transition to flight. Although small wings also occur in adults, their potential role in locomotion is rarely considered. Here we describe the prevalence of rudimentary wings in extant birds, and how wings wax and wane on many timescales. This waxing and waning is integral to the avian clade and offers a rich arena for exploring links between form, function, performance, behavior, ecology, and evolution. Although our understanding is nascent, birds clearly show that rudimentary structures can enhance performance and survival.

Models of benthic bipedalism

Walking is a common bipedal and quadrupedal gait and is often associated with terrestrial and aquatic organisms. Inspired by recent evidence of the neural underpinnings of primitive aquatic walking in the little skate Leucoraja erinacea, we introduce a theoretical model of aquatic walking that reveals robust and efficient gaits with modest requirements for body morphology and control. The model predicts undulatory behaviour of the system body with a regular foot placement pattern, which is also observed in the animal, and additionally predicts the existence of gait bistability between two states, one with a large energetic cost for locomotion and another associated with almost no energetic cost. We show that these can be discovered using a simple reinforcement learning scheme. To test these theoretical frameworks, we built a bipedal robot and show that its behaviours are similar to those of our minimal model: its gait is also periodic and exhibits bistability, with a low efficiency mode separated from a high efficiency mode by a 'jump' transition. Overall, our study highlights the physical constraints on the evolution of walking and provides a guide for the design of efficient biomimetic robots.

Tides: A key environmental driver of osteichthyan evolution and the fish-tetrapod transition?

Tides are a major component of the interaction between the marine and terrestrial environments, and thus play an important part in shaping the environmental context for the evolution of shallow marine and coastal organisms. Here, we use a dedicated tidal model and palaeogeographic reconstructions from the Late Silurian to early Late Devonian (420 Ma, 400 Ma and 380 Ma, Ma = millions of years ago) to explore the potential significance of tides for the evolution of osteichthyans (bony fish) and tetrapods (land vertebrates). The earliest members of the osteichthyan crown-group date to the Late Silurian, approximately 425 Ma, while the earliest evidence for tetrapods is provided by trackways from the Middle Devonian, dated to approximately 393 Ma, and the oldest tetrapod body fossils are Late Devonian, approximately 373 Ma. Large tidal ranges could have fostered both the evolution of air-breathing organs in osteichthyans to facilitate breathing in oxygen-depleted tidal pools, and the development of weight-bearing tetrapod limbs to aid navigation within the intertidal zones. We find that tidal ranges over 4 m were present around areas of evolutionary significance for the origin of osteichthyans and the fish-tetrapod transition, highlighting the possible importance of tidal dynamics as a driver for these evolutionary processes.

Population and organismal perspectives on trait origins

Some biologists and philosophers of biology claim selection can "create" novel traits. Others claim creativity is to be found only in development. I here endorse the former claim, but take seriously and address the concerns that underlie the latter. My discussion of these issues is informed by recent work that champions the "return of the organism" to mainstream evolutionary biology, and I suggest how population and organismal perspectives on trait origins can be reconciled.

Skeletal and muscular pelvic morphology of hillstream loaches (Cypriniformes: Balitoridae)

The rheophilic hillstream loaches (Balitoridae) of South and Southeast Asia possess a range of pelvic girdle morphologies, which may be attributed to adaptations for locomotion against rapidly flowing water. Specifically, the connectivity of the pelvic plate (basipterygium) to the vertebral column via a sacral rib, and the relative size and shape of the sacral rib, fall within a spectrum of three discrete morphotypes: long, narrow rib that meets the basipterygium; thicker, slightly curved rib meeting the basipterygium; and robust crested rib interlocking with the basipterygium. Species in this third category with more robust sacral rib connections between the basipterygium and vertebral column are capable of walking out of water with a tetrapod-like lateral-sequence, diagonal-couplet gait. This behavior has not been observed in species lacking direct skeletal connection between the vertebrae and the pelvis. The phylogenetic positions of the morphotypes were visualized by matching the morphological features onto a novel hypothesis of relationships for the family Balitoridae. The morphotypes determined through skeletal morphology were correlated with patterns observed in the pelvic muscle morphology of these fishes. Transitions towards increasingly robust pelvic girdle attachment were coincident with a more anterior origin on the basipterygium and more lateral insertion of the muscles on the fin rays, along with a reduction of the superficial abductors and adductors with more posterior insertions. These modifications are expected to provide a mechanical advantage for generating force against the ground. Inclusion of the enigmatic cave-adapted balitorid Cryptotora thamicola into the most data-rich balitorid phylogeny reveals its closest relatives, providing insight into the origin of the skeletal connection between the axial skeleton and basipterygium.

Plasticity and evolutionary convergence in the locomotor skeleton of Greater Antillean Anolis lizards

Plasticity can put evolution on repeat if development causes species to generate similar morphologies in similar environments. Anolis lizards offer the opportunity to put this role of developmental plasticity to the test. Following colonization of the four Greater Antillean islands, Anolis lizards independently and repeatedly evolved six ecomorphs adapted to manoeuvring different microhabitats. By quantifying the morphology of the locomotor skeleton of 95 species, we demonstrate that ecomorphs on different islands have diverged along similar trajectories. However, microhabitat-induced morphological plasticity differed between species and did not consistently improve individual locomotor performance. Consistent with this decoupling between morphological plasticity and locomotor performance, highly plastic features did not show greater evolvability, and plastic responses to microhabitat were poorly aligned with evolutionary divergence between ecomorphs. The locomotor skeleton of Anolis may have evolved within a subset of possible morphologies that are highly accessible through genetic change, enabling adaptive convergence independently of plasticity.

Evolution of epithelial sodium channels: current concepts and hypotheses

The conquest of freshwater and terrestrial habitats was a key event during vertebrate evolution. Occupation of low-salinity and dry environments required significant osmoregulatory adaptations enabling stable ion and water homeostasis. Sodium is one of the most important ions within the extracellular liquid of vertebrates, and molecular machinery for urinary reabsorption of this electrolyte is critical for the maintenance of body osmoregulation. Key ion channels involved in the fine-tuning of sodium homeostasis in tetrapod vertebrates are epithelial sodium channels (ENaCs), which allow the selective influx of sodium ions across the apical membrane of epithelial cells lining the distal nephron or the colon. Furthermore, ENaC-mediated sodium absorption across tetrapod lung epithelia is crucial for the control of liquid volumes lining the pulmonary surfaces. ENaCs are vertebrate-specific members of the degenerin/ENaC family of cation channels; however, there is limited knowledge on the evolution of ENaC within this ion channel family. This review outlines current concepts and hypotheses on ENaC phylogeny and discusses the emergence of regulation-defining sequence motifs in the context of osmoregulatory adaptations during tetrapod terrestrialization. In light of the distinct regulation and expression of ENaC isoforms in tetrapod vertebrates, we discuss the potential significance of ENaC orthologs in osmoregulation of fishes as well as the putative fates of atypical channel isoforms in mammals. We hypothesize that ancestral proton-sensitive ENaC orthologs might have aided the osmoregulatory adaptation to freshwater environments whereas channel regulation by proteases evolved as a molecular adaptation to lung liquid homeostasis in terrestrial tetrapods.

nkx3.2 mutant zebrafish accommodate jaw joint loss through a phenocopy of the head shapes of Paleozoic jawless fish

The vertebrate jaw is a versatile feeding apparatus. To function, it requires a joint between the upper and lower jaws, so jaw joint defects are often highly disruptive and difficult to study. To describe the consequences of jaw joint dysfunction, we engineered two independent null alleles of a single jaw joint marker gene, nkx3.2, in zebrafish. These mutations caused zebrafish to become functionally jawless via fusion of the upper and lower jaw cartilages (ankylosis). Despite lacking jaw joints, nkx3.2 mutants survived to adulthood and accommodated this defect by: (a) having a remodeled skull with a fixed open gape, reduced snout and enlarged branchial region; and (b) performing ram feeding in the absence of jaw-generated suction. The late onset and broad extent of phenotypic changes in the mutants suggest that modifications to the skull are induced by functional agnathia, secondarily to nkx3.2 loss of function. Interestingly, nkx3.2 mutants superficially resemble ancient jawless vertebrates (anaspids and furcacaudiid thelodonts) in overall head shape. Because no homology exists in individual skull elements between these taxa, the adult nkx3.2 phenotype is not a reversal but rather a convergence due to similar functional requirements of feeding without moveable jaws. This remarkable analogy strongly suggests that jaw movements themselves dramatically influence the development of jawed vertebrate skulls. Thus, these mutants provide a unique model with which to: (a) investigate adaptive responses to perturbation in skeletal development; (b) re-evaluate evolutionarily inspired interpretations of phenocopies generated by gene knockdowns and knockouts; and (c) gain insight into feeding mechanics of the extinct agnathans.

Terrestrial Locomotion in American Eels (Anguilla rostrata): How Substrate and Incline Affect Movement Patterns

Fishes overcome a variety of challenges in order to invade the terrestrial environment. Terrestrial invasions by fish occur over a variety of environmental contexts. In order to advance their bodies on land, fishes capable of terrestrial excursions tend to use one of three different types of locomotor modes: axial-based, appendage-based, or axial-appendage-based. Elongate species with reduced appendages, such as the American eel, Anguilla rostrata, rely on axial based locomotion in water and on land. When eels move from water to land as part of their complex life cycle, they inevitably encounter a variety of substrates and must traverse variable degrees of incline. The aim of this study was to determine the effect of substrate and incline on the terrestrial locomotion of the American eel. In order to do this, eels were filmed from a dorsal view on three substrates and four inclines: sand, loose pebbles, and fixed (glued) pebbles at 0°, 5°, 10°, and 15°. We digitized 20 evenly spaced points along the body to examine the following characteristics of locomotion: velocity, distance ratio (DR), and wave parameters such as wave amplitude, frequency, and length and assessed whether substrate, incline, or body position affected these parameters. DR, our metric of movement efficiency, was highest on the flat sand condition and lowest on 15° pebble conditions. Efficiency also varied across the body. Velocity followed a similar pattern being highest on sand at 0° and lowest at the steepest inclines. Wave amplitude generally increased toward the tail but was similar across substrates and inclines. Wave frequency was relatively consistent across the body on both pebble substrates, but on sand, frequency was higher toward the head but decreased toward the tail. Wavelengths on sand were the longest at 0° near the head and shorter wavelengths were observed on steeper inclines. Both pebble substrates elicited lower wavelengths that were more similar across the body. Overall, A. rostrata were more effective in navigating compliant substrates but struggled at steeper inclines. Our findings provide insight into locomotor challenges that American eels may encounter as they move from and between bodies of water.

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.

Habitat transitions alter the adaptive landscape and shape phenotypic evolution in needlefishes (Belonidae)

Habitat occupancy can have a profound influence on macroevolutionary dynamics, and a switch in major habitat type may alter the evolutionary trajectory of a lineage. In this study, we investigate how evolutionary transitions between marine and freshwater habitats affect macroevolutionary adaptive landscapes, using needlefishes (Belonidae) as a model system. We examined the evolution of body shape and size in marine and freshwater needlefishes and tested for phenotypic change in response to transitions between habitats. Using micro-computed tomographic (µCT) scanning and geometric morphometrics, we quantified body shape, size, and vertebral counts of 31 belonid species. We then examined the pattern and tempo of body shape and size evolution using phylogenetic comparative methods. Our results show that transitions from marine to freshwater habitats have altered the adaptive landscape for needlefishes and expanded morphospace relative to marine taxa. We provide further evidence that freshwater taxa attain reduced sizes either through dwarfism (as inferred from axial skeletal reduction) or through developmental truncation (as inferred from axial skeletal loss). We propose that transitions to freshwater habitats produce morphological novelty in response to novel prey resources and changes in locomotor demands. We find that repeated invasions of different habitats have prompted predictable changes in morphology.

Adaptations of Interferon Regulatory Factor 3 with Transition from Terrestrial to Aquatic Life

Interferon regulatory factor 3 (IRF3) and IRF7 are closely related IRF members and the major factors for the induction of interferons, a key component in vertebrate innate immunity. However, there is limited knowledge regarding the evolution and adaptation of those IRFs to the environments. Two unique motifs in IRF3 and 7 were identified. One motif, GASSL, is highly conserved throughout the evolution of IRF3 and 7 and located in the signal response domain. Another motif, DPHK, is in the DNA-binding domain. The ancestral protein of IRF3 and 7 seemed to possess the DPHK motif. In the ray-finned fish lineage, while the DPHK is maintained in IRF7, the motif in IRF3 is changed to NPHK with a D → N amino acid substitution. The D → N substitution are also found in amphibian IRF3 but not in amphibian IRF7. Terrestrial animals such as reptiles and mammals predominantly use DPHK sequences in both IRF3 and 7. However, the D → N substitution in IRF3 DPHK is again found in cetaceans such as whales and dolphins as well as in marsupials. These observations suggest that the D → N substitutions in the IRF3 DPHK motif is likely to be associated with vertebrate's adaptations to aquatic environments and other environmental changes.

Evolutionary and cardio-respiratory physiology of air-breathing and amphibious fishes

Air-breathing and amphibious fishes are essential study organisms to shed insight into the required physiological shifts that supported the full transition from aquatic water-breathing fishes to terrestrial air-breathing tetrapods. While the origin of air-breathing in the evolutionary history of the tetrapods has received considerable focus, much less is known about the evolutionary physiology of air-breathing among fishes. This review summarizes recent advances within the field with specific emphasis on the cardiorespiratory regulation associated with air-breathing and terrestrial excursions, and how respiratory physiology of these living transitional forms are affected by development and personality. Finally, we provide a detailed and re-evaluated model of the evolution of air-breathing among fishes that serves as a framework for addressing new questions on the cardiorespiratory changes associated with it. This review highlights the importance of combining detailed studies on piscine air-breathing model species with comparative multi-species studies, to add an additional dimension to our understanding of the evolutionary physiology of air-breathing in vertebrates.

Elpistostege and the origin of the vertebrate hand

The evolution of fishes to tetrapods (four-limbed vertebrates) was one of the most important transformations in vertebrate evolution. Hypotheses of tetrapod origins rely heavily on the anatomy of a few tetrapod-like fish fossils from the Middle and Late Devonian period (393-359 million years ago)1. These taxa-known as elpistostegalians-include Panderichthys2, Elpistostege3,4 and Tiktaalik1,5, none of which has yet revealed the complete skeletal anatomy of the pectoral fin. Here we report a 1.57-metre-long articulated specimen of Elpistostege watsoni from the Upper Devonian period of Canada, which represents-to our knowledge-the most complete elpistostegalian yet found. High-energy computed tomography reveals that the skeleton of the pectoral fin has four proximodistal rows of radials (two of which include branched carpals) as well as two distal rows that are organized as digits and putative digits. Despite this skeletal pattern (which represents the most tetrapod-like arrangement of bones found in a pectoral fin to date), the fin retains lepidotrichia (fin rays) distal to the radials. We suggest that the vertebrate hand arose primarily from a skeletal pattern buried within the fairly typical aquatic pectoral fin of elpistostegalians. Elpistostege is potentially the sister taxon of all other tetrapods, and its appendages further blur the line between fish and land vertebrates.

Origin of the mechanism of phenotypic plasticity in satyrid butterfly eyespots

Plasticity is often regarded as a derived adaptation to help organisms survive in variable but predictable environments, however, we currently lack a rigorous, mechanistic examination of how plasticity evolves in a large comparative framework. Here, we show that phenotypic plasticity in eyespot size in response to environmental temperature observed in Bicyclus anynana satyrid butterflies is a complex derived adaptation of this lineage. By reconstructing the evolution of known physiological and molecular components of eyespot size plasticity in a comparative framework, we showed that 20E titer plasticity in response to temperature is a pre-adaptation shared by all butterfly species examined, whereas expression of EcR in eyespot centers, and eyespot sensitivity to 20E, are both derived traits found only in a subset of species with eyespots.

A well-known family of butterflies have circular patterns on their wings that look like eyes. These eye-like markings help deflect predators away from the butterfly’s body so they attack the outer edges of their wings. However, in certain seasons, such as the dry season in Africa, the best way for this family to survive is by not drawing any attention to their bodies. Thus, butterflies born during this season shrink the size of their eyespots so they can hide among the dry leaves.

How this family of butterflies are able to change the size of these eye-like spots has only been studied in the species Bicyclus anynana. During development low temperatures, which signify the beginning of the dry season, reduce the amount of a hormone called 20E circulating in the blood of this species. This changes the behavior of hormone-sensitive cells in the eyespots making them smaller in size. But it remains unclear how B. anynana evolved this remarkable tactic and whether its relatives have similar abilities.

Now, Bhardwaj et al. show that B. anynana is the only one of its relatives that can amend the size of its eyespots in response to temperature changes. In the experiments, 13 different species of butterflies, mostly from the family that has eyespots, were developed under two different temperatures. Low temperatures caused 20E hormone levels to decrease in all 13 species. However, most of these species did not develop smaller eyespots in response to this temperature change. This includes species that are known to have larger and smaller eyespots depending on the season.

Like B. anynana, four of the species studied have receptors for the 20E hormone at the center of their eyespots. However, changing 20E hormone levels in these species did not reduce eyespot size.

These results show that although temperature changes alter hormone levels in a number of species, only B. anynana have taken advantage of this mechanism to regulate eyespot size. In addition, Bhardwaj et al. found that this unique mechanism evolved from several genetic changes over millions of years. Other species likely use other environmental cues to trigger seasonal changes in the size of their eyespots.

Bone adaptation: Safety factors and load predictability in shaping skeletal form

Much is known about skeletal adaptation in relation to the mechanical functions that bones serve. This includes how bone adapts to mechanical loading during an individual's lifetime as well as over evolutionary time. Although controlled loading in animal models allows us to observe short-term bone adaptation (epigenetic mechanobiology), examining an assemblage of extant vertebrate bones or a group of fossils' bony structures can reveal the combined effects of long-term trends in loading history and the effects of natural selection. In this survey we examine adaptations that take place over both time scales and highlight a few of the extraordinary insights first published by John Currey. First, we provide a historical perspective on bone adaptation control mechanisms, followed by a discussion of safety factors in bone. We then summarize examples of structural- and material-level adaptations and mechanotransduction, and analyze the relationship between these structural- and material-level adaptations observed in situations where loading modes are either predictable or unpredictable. We argue that load predictability is a major consideration for bone adaptation broadly across an evolutionary timescale, but that its importance can also be seen during ontogenetic growth trajectories, which are subject to natural selection as well. Furthermore, we suggest that bones with highly predictable load patterns demonstrate more precise design with lower safety factors, while bones that experience less predictable loads or those that are less capable of repair and adaptation are designed with a higher safety factor. Finally, exposure to rare loading events with high potential costs of failure leads to design of structures with very high safety factor compared to everyday loading experience. Understanding bone adaptations at the structural and material levels, which take place over an individual's lifetime or over evolutionary time has numerous applications in translational and clinical research to understand and treat musculoskeletal diseases, as well as to permit the furthering of human extraterrestrial exploration in environments with altered gravity.

Phenotypic Switching Resulting From Developmental Plasticity: Fixed or Reversible?

The prevalent view of developmental phenotypic switching holds that phenotype modifications occurring during critical windows of development are "irreversible" - that is, once produced by environmental perturbation, the consequent juvenile and/or adult phenotypes are indelibly modified. Certainly, many such changes appear to be non-reversible later in life. Yet, whether animals with switched phenotypes during early development are unable to return to a normal range of adult phenotypes, or whether they do not experience the specific environmental conditions necessary for them to switch back to the normal range of adult phenotypes, remains an open question. Moreover, developmental critical windows are typically brief, early periods punctuating a much longer period of overall development. This leaves open additional developmental time for reversal (correction) of a switched phenotype resulting from an adverse environment early in development. Such reversal could occur from right after the critical window "closes," all the way into adulthood. In fact, examples abound of the capacity to return to normal adult phenotypes following phenotypic changes enabled by earlier developmental plasticity. Such examples include cold tolerance in the fruit fly, developmental switching of mouth formation in a nematode, organization of the spinal cord of larval zebrafish, camouflage pigmentation formation in larval newts, respiratory chemosensitivity in frogs, temperature-metabolism relations in turtles, development of vascular smooth muscle and kidney tissue in mammals, hatching/birth weight in numerous vertebrates,. More extreme cases of actual reversal (not just correction) occur in invertebrates (e.g., hydrozoans, barnacles) that actually 'backtrack' along normal developmental trajectories from adults back to earlier developmental stages. While developmental phenotypic switching is often viewed as a permanent deviation from the normal range of developmental plans, the concept of developmental phenotypic switching should be expanded to include sufficient plasticity allowing subsequent correction resulting in the normal adult phenotype.