1. What is an amphibian?

‘What is an amphibian?’ provides an overview of amphibians, which zoologists divide into three very unequal sized Orders. The great majority are the frogs and toads, which make up the order Anura, a name that refers to the lack of a tail. The second amphibian order are the salamanders and newts, called the Urodela. Meanwhile, the third amphibian order, the Caecilia, are best known for being poorly known, thanks to their remote distribution and obscure habitat. There are four methods that amphibians can use to breathe: cutaneous respiration, bucco-pharyngeal respiration, pulmonary respiration, and branchial respiration.

Can Macroinvertebrate Traits Be Explored and Applied in Biomonitoring Riverine Systems Draining Forested Catchments?

Trait-based approach (TBA) in recent time has received tremendous attention as complementary tool over taxonomic-based approach in assessing ecological health of riverine systems in developed countries, but in the Afrotropical region the trait-based approach is still in its infancy. No trait-based approach has been developed for riverine systems draining forested catchment in the Afrotropical region. Hence, this study was conducted to explore and apply macroinvertebrates traits as potential biomonitoring tools in assessing ecological health of riverine systems draining forested catchments in the Niger Delta area of Nigeria. Selected physico-chemical variables were sampled together with macroinvertebrates in 18 stations of 10 riverine systems from 2008 to 2012. The 18 stations were classified into three ecological classes namely near natural stations (NNS), slightly disturbed stations (SDS), and moderately disturbed stations (MDS) using physico-chemical-based classification with the aid of principal component analysis (PCA). The results revealed traits such as possessions of hardshell body armouring, preferences for clear and transparent water and opaque water, climbing and crawling mobility mechanisms, large (>20–40 mm) body size, preferences for scrapping, shredding, and grazing feeding habits to be associated with NNS and SDS based on RLQ (R, physico-chemical variables; L, taxa; Q, traits) analysis performed. Thus, these traits were deemed to be sensitive to human impact in forested systems. Also, traits such as tegument/cutaneous respiration, soft and exposed body armouring, burrowing mobility mechanism, spherical body shape, preference for detritus [fine particulate organic materials (FPOM)] food materials, small (>5–10 mm) body size and preference for filter feeding mechanism were associated with MDS. Hence, they were deemed tolerant of human impact in forested systems. A fourth-corner test performed revealed tegumental/cutaneous respiration preference, soft and exposed body armouring and burrowing mobility mode, which were associated with the MDS on the RLQ ordination were also positively correlated to 5 day biochemical oxygen demand (BOD5); while preference for clear and transparent water, which were positively associated with MDS, were also positively correlated with pH and negatively correlated to dissolved oxygen (DO). Overall, this study affirmed that the TBA can be explored in biomonitoring riverine systems draining forested catchments. Nevertheless, we suggest the trait-based approach to be further explored, with a view to developing trait-informed indices for biomonitoring Afrotropical riverine systems.

Genomic and physiological mechanisms underlying skin plasticity during water to air transition in an amphibious fish

The terrestrial radiation of vertebrates required changes in skin that resolved the dual demands of maintaining a mechanical and physiological barrier while also facilitating ion and gas transport. Using the amphibious killifish Kryptolebias marmoratus, we found that transcriptional regulation of skin morphogenesis was quickly activated upon air exposure (1h). Rapid regulation of cell-cell adhesion complexes and pathways that regulate stratum corneum formation was consistent with barrier function and mechanical reinforcement. Unique blood vessel architecture and regulation of angiogenesis likely supported cutaneous respiration. Differences in ionoregulatory transcripts and ionocyte morphology were correlated with differences in salinity acclimation and resilience to air exposure. Evolutionary analyses reinforced the adaptive importance of these mechanisms. We conclude that rapid plasticity of barrier, respiratory, and ionoregulatory functions in skin evolved to support K. marmoratus’ amphibious lifestyle; similar processes may have facilitated the terrestrial radiation of other contemporary and ancient fishes.

From scales to armour: scale losses and trunk bony plate gains in ray-finned fishes

Actinopterygians (ray-finned fishes) are the most diversified group of vertebrates and are characterized by a variety of protective structures covering their tegument, the evolution of which has intrigued biologists for decades. Paleontological records showed that the first mineralized vertebrate skeleton was composed of dermal bony plates covering the body, including odontogenic and skeletogenic components. Later in evolution, the exoskeleton of actinopterygian’s trunk was composed of scale structures. Although scales are nowadays a widespread tegument cover, some contemporary lineages do not have scales but bony plates covering their trunk, whereas other lineages are devoid of any such structures. To understand the evolution of the tegument coverage and particularly the transition between different structures, we investigated the pattern of scale loss events along actinopterygian evolution and addressed the functional relationship between the scaleless phenotype and the ecology of fishes. Furthermore, we examined whether the emergence of trunk bony plates was dependent over the presence or absence of scales. To this aim, we used two recently published actinopterygian phylogenies, one including > 11,000 species, and by using stochastic mapping and Bayesian methods, we inferred scale loss events and trunk bony plate acquisitions. Our results reveal that a scaled tegument is the most frequent state in actinopterygians, but multiple independent scale loss events occurred along their phylogeny with essentially no scale re-acquisition. Based on linear mixed models, we found evidence supporting that after a scale loss event, fishes tend to change their ecology and adopt a benthic lifestyle. Furthermore, we show that trunk bony plates appeared independently multiple times along the phylogeny. By using fitted likelihood models for character evolution, we show that trunk bony plate acquisitions were dependent over a previous scale loss event. Overall, our findings support the hypothesis that tegument cover is a key evolutionary trait underlying actinopterygian radiation.Impact SummaryRay-finned fishes (actinopterygians) are the most diverse vertebrate group in the world. The majority of these fishes possess scales as a protective shield covering their trunk. However, several lineages display a body armour composed of trunk bony plates or are devoid of any protective structures. The diversity and the transitions between different tegument coverage types have not been previously studied in an evolutionary framework. Here, we investigate which structure was present at the origin of ray-finned fishes and how the different phenotypes emerged through time.We show that a scaled tegument was the most widespread sate along ray-finned fish evolution, yet scale losses occurred multiple independent times, while acquiring scales again almost never happened. Moreover, we reveal that scaleless teguments most probably led species to change their ecology and colonise the floors of oceans and water bodies. The functional advantages of a scaleless tegument in a benthic environment are yet to be demonstrated, but the increased cutaneous respiration could be an explanation. We show that trunk bony plates also emerged independently multiple times along the evolution of ray-finned fishes but these armours protecting the trunk can only appear after a scale loss event. Therefore, while the acquisitions of trunk bony plates are phylogenetically independent, they need a “common ground” to emerge. All together, our findings provide evidence that the various tegument covers have contributed to the outstanding diversification of ray-finned fishes.

The evolution of dermal shield vascularization in Testudinata and Pseudosuchia: phylogenetic constraints versus ecophysiological adaptations

Studies on living turtles have demonstrated that shells are involved in the resistance to hypoxia during apnea via bone acidosis buffering; a process which is complemented with cutaneous respiration, transpharyngeal and cloacal gas exchanges in the soft-shell turtles. Bone acidosis buffering during apnea has also been identified in crocodylian osteoderms, which are also known to employ heat transfer when basking. Although diverse, many of these functions rely on one common trait: the vascularization of the dermal shield. Here, we test whether the above ecophysiological functions played an adaptive role in the evolutionary transitions between land and aquatic environments in both Pseudosuchia and Testudinata. To do so, we measured the bone porosity as a proxy for vascular density in a set of dermal plates before performing phylogenetic comparative analyses. For both lineages, the dermal plate porosity obviously varies depending on the animal lifestyle, but these variations prove to be highly driven by phylogenetic relationships. We argue that the complexity of multi-functional roles of the post-cranial dermal skeleton in both Pseudosuchia and Testudinata probably is the reason for a lack of obvious physiological signal, and we discuss the role of the dermal shield vascularization in the evolution of these groups. This article is part of the theme issue ‘Vertebrate palaeophysiology’.

The potential respiratory surfaces of a fish living in a historically polluted river

The acquisition of oxygen is fundamental for maintaining metabolic activity in fish, and most species obtain oxygen through aquatic gill respiration. Throughout evolution, different fish lineages have evolved secondary routes to obtain oxygen from atmospheric air, and the spinycheek sleeper, Eleotris pisonis, may be one example of such bimodal respiration. Gill structure and the top epithelium of the head of E. pisonis from the Subaé River (Santo Amaro, Bahia, Brazil) were evaluated morphometrically. The gills showed histopathologies, especially proliferation, which was prevalent on 35.6% of the filaments and on 13.7% of the lamellae, probably increasing the water-blood diffusion barrier of the lamellae to 5.48 ± 1.32 μm, similar to the air-blood diffusion distance of the skin (5.77 ± 1.72 μm). Quantitative morphometric analysis of histopathologies and diffusion distances could be considered as biomarkers, and also suggest the presence of cutaneous respiration as a possible strategy for aerial oxygen acquisition.

Evaluation of the combined temperature and relative humidity preferences of the Colombian terrestrial salamander Bolitoglossa ramosi (Amphibia: Plethodontidae)

Temperature and humidity are critical factors for terrestrial lungless salamanders, as their body temperatures are largely determined by the environmental temperature and require moisture to sustain cutaneous respiration. Herein, we evaluated the preference of Bolitoglossa ramosi Brame and Wake, 1972 between a high temperature and a high relative humidity (RH), the influence of temperature on RH preferences, and the influence of RH on the thermal preferences. This study was performed in a field location in the municipality of Líbano, Tolima, Colombia. There, on different nights, we collected 84 adult B. ramosi and carried out the preference experiments, using aluminum troughs with different thermal and RH gradients. We found that between high temperature and high RH, B. ramosi preferred high RH. However, B. ramosi selected high temperatures when the gradient had a high RH and low temperatures when the gradient had a low RH. These results show that B. ramosi is able to thermoregulate and hydroregulate. Nevertheless, hydroregulation seems to be more important than thermoregulation because B. ramosi always selected the high RH gradients, while their thermal selection relied on the hydric environment.

Upper limits to body size imposed by respiratory–structural trade-offs in Antarctic pycnogonids

Across metazoa, surfaces for respiratory gas exchange are diverse, and the size of those surfaces scales with body size. In vertebrates with lungs and gills, surface area and thickness of the respiratory barrier set upper limits to rates of metabolism. Conversely, some organisms and life stages rely on cutaneous respiration, where the respiratory surface (skin, cuticle, eggshell) serves two primary functions: gas exchange and structural support. The surface must be thin and porous enough to transport gases but strong enough to withstand external forces. Here, we measured the scaling of surface area and cuticle thickness in Antarctic pycnogonids, a group that relies on cutaneous respiration. Surface area and cuticle thickness scaled isometrically, which may reflect the dual roles of cuticle in gas exchange and structural support. Unlike in vertebrates, the combined scaling of these variables did not match the scaling of metabolism. To resolve this mismatch, larger pycnogonids maintain steeper oxygen gradients and higher effective diffusion coefficients of oxygen in the cuticle. Interactions among scaling components lead to hard upper limits in body size, which pycnogonids could evade only with some other evolutionary innovation in how they exchange gases.