Hormonal correlates of environmentally induced metamorphosis in the Western spadefoot toad, Scaphiopus hammondii

Early development of the glucocorticoid stress response in poison frog tadpoles

In vertebrates, the glucocorticoid response through the hypothalamic-pituitary-adrenal (HPA) axis controls many essential functions, including behavior, metabolism, and ontogenetic transitions. However, there are tradeoffs associated with high levels of glucocorticoids, including reduced growth rate and lowered immunity. These tradeoffs drive variation in the timing of the development of the HPA axis across taxa. In anurans (frogs and toads), corticosterone has critical roles in development and behavior, and concentrations can fluctuate in response to environmental stressors. Given the role of corticosterone in ontogenetic changes and behaviors, we hypothesized that species with immediate habitat transitions and challenges would develop an HPA axis early in development. To test this hypothesis, we studied tadpoles of the dyeing poison frog ( Dendrobates tinctorius ), a species in which tadpoles hatch terrestrially and are transported to pools of water by their parent. We measured the excretion rate and whole-body concentration of corticosterone and the corticosterone response to adrenocorticotropic hormone (ACTH). We found no significant differences in excretion rates and whole-body concentration of corticosterone, nor physiological response to ACTH injection across tadpole development. These findings indicate that the glucocorticoid response is developed early in ontogeny. These findings generally differ from those found in other species of tadpoles, which may suggest the unique ecological pressures of D. tinctorius has shaped the development of its HPA axis. More broadly, this study illustrates how life history strategies and tradeoffs of glucocorticoids impact the timing of the development of the HPA axis.

Highlights

The timing of HPA axis development differs across species. We studied the HPA axis across tadpole development in Dendrobates tinctorius . No difference in corticosterone concentration across development.No difference in corticosterone response to ACTH across development.Results suggest an early developed HPA axis is essential for their life history.

Niche construction and the environmental term of the price equation: How natural selection changes when organisms alter their environments

Organisms construct their own environments and phenotypes through the adaptive processes of habitat choice, habitat construction, and phenotypic plasticity. We examine how these processes affect the dynamics of mean fitness change through the environmental change term of the Price Equation. This tends to be ignored in evolutionary theory, owing to the emphasis on the first term describing the effect of natural selection on mean fitness (the additive genetic variance for fitness of Fisher's Fundamental Theorem). Using population genetic models and the Price Equation, we show how adaptive niche constructing traits favorably alter the distribution of environments that organisms encounter and thereby increase population mean fitness. Because niche-constructing traits increase the frequency of higher-fitness environments, selection favors their evolution. Furthermore, their alteration of the actual or experienced environmental distribution creates selective feedback between niche constructing traits and other traits, especially those with genotype-by-environment interaction for fitness. By altering the distribution of experienced environments, niche constructing traits can increase the additive genetic variance for such traits. This effect accelerates the process of overall adaption to the niche-constructed environmental distribution and can contribute to the rapid refinement of alternative phenotypic adaptations to different environments. Our findings suggest that evolutionary biologists revisit and reevaluate the environmental term of the Price Equation: owing to adaptive niche construction, it contributes directly to positive change in mean fitness; its magnitude can be comparable to that of natural selection; and, when there is fitness G × E, it increases the additive genetic variance for fitness, the much-celebrated first term.

Exploring water-borne corticosterone collection as a non-invasive tool in amphibian conservation physiology: benefits, limitations and future perspectives

Global change exposes wildlife to a variety of environmental stressors and is affecting biodiversity worldwide, with amphibian population declines being at the forefront of the global biodiversity crisis. The use of non-invasive methods to determine the physiological state in response to environmental stressors is therefore an important advance in the field of conservation physiology. The glucocorticoid hormone corticosterone (CORT) is one useful biomarker to assess physiological stress in amphibians, and sampling water-borne (WB) CORT is a novel, non-invasive collection technique. Here, we tested whether WB CORT can serve as a valid proxy of organismal levels of CORT in larvae of the common frog (Rana temporaria). We evaluated the association between tissue and WB CORT levels sampled from the same individuals across ontogenetic stages, ranging from newly hatched larvae to froglets at 10 days after metamorphosis. We also investigated how both tissue and WB CORT change throughout ontogeny. We found that WB CORT is a valid method in pro-metamorphic larvae as values for both methods were highly correlated. In contrast, there was no correlation between tissue and WB CORT in newly hatched, pre-metamorphic larvae, metamorphs or post-metamorphic froglets probably due to ontogenetic changes in respiratory and skin morphology and physiology affecting the transdermal CORT release. Both collection methods consistently revealed a non-linear pattern of ontogenetic change in CORT with a peak at metamorphic climax. Thus, our results indicate that WB CORT sampling is a promising, non-invasive conservation tool for studies on late-stage amphibian larvae. However, we suggest considering that different contexts might affect the reliability of WB CORT and consequently urge future studies to validate this method whenever it is used in new approaches. We conclude proposing some recommendations and perspectives on the use of WB CORT that will aid in broadening its application as a non-invasive tool in amphibian conservation physiology.

Hormonal and pheromonal studies on amphibians with special reference to metamorphosis and reproductive behavior

In this article, we review studies which have been conducted to investigate the hormonal influence on metamorphosis in bullfrog (Rana catesbeiana) and Japanese toad (Bufo japonicus) larvae, in addition to studies conducted on the hormonal and pheromonal control of reproductive behavior in red-bellied newts (Cynops pyrrhogaster). Metamorphosis was studied with an emphasis on the roles of prolactin (PRL) and thyrotropin (TSH). The release of PRL was shown to be regulated by thyrotropin-releasing hormone (TRH) and that of TSH was evidenced to be regulated by corticotropin-releasing factor. The significance of the fact that the neuropeptide that controls the secretion of TSH is different from those encountered in mammals is discussed in consideration of the observation that the release of TRH, which stimulates the release of PRL, is enhanced when the animals are subjected to a cold temperature. Findings that were made by using melanin-rich cells of Bufo embryos and larvae, such as the determination of the origin of the adenohypophyseal primordium, identification of the pancreatic chitinase, and involvement of the rostral preoptic recess organ as the hypothalamic inhibitory center of α-melanocyte-stimulating hormone (α-MSH) secretion, are mentioned in this article. In addition, the involvement of hormones in eliciting courtship behavior in male red-bellied newts and the discovery of the peptide sex pheromones and hormonal control of their secretion are also discussed in the present article.

Integrating morphology and physiology of the key endocrine organ during tadpole development: The interrenal gland

Indirect development is widespread in anurans and is considered an ancestral condition. The metamorphosis of larvae into juveniles involves highly coordinated morphological, physiological, biochemical, and behavioral changes, promoted by the thyroid hormone and interrenal corticosteroids. Stress response to environmental changes is also mediated by corticosteroids, affecting the timing and rate of metamorphosis and leading to great developmental plasticity in tadpoles. Given the potential effect of interrenal gland ontogeny alterations on metamorphosis and the lack of studies addressing both the morphology and endocrinology of this gland in tadpoles, we present corticosterone (CORT) production and histological changes through the ontogeny of interrenal gland in the generalized pond-type tadpole of Rhinella arenarum (Anura, Bufonidae). This species shows the highest concentration of whole-body CORT by the early climax when drastic metamorphic changes begin. This is coincident with the morphological differentiation of steroidogenic cells and the formation of interrenal cords. By this stage, steroidogenic cells have a shrunken cytoplasm, with a significantly higher nucleus-to-cell diameter ratio. The lowest CORT concentration during premetamorphosis and late climax is associated with small undifferentiated cells with lipid inclusions surrounding large blood vessels between kidneys, and with cords of differentiated steroidogenic cells with a significantly lower nucleus-to-cell diameter ratio, respectively. Our study characterizes the morphological and physiological pattern of interrenal gland development, showing an association between certain histological and morphometric characteristics and CORT levels. Variations in this morpho-physiological pattern should be considered when studying the phenotypic plasticity or variable growth rates of tadpoles.

Constraints and Opportunities for the Evolution of Metamorphic Organisms in a Changing Climate

We argue that developmental hormones facilitate the evolution of novel phenotypic innovations and timing of life history events by genetic accommodation. Within an individual’s life cycle, metamorphic hormones respond readily to environmental conditions and alter adult phenotypes. Across generations, the many effects of hormones can bias and at times constrain the evolution of traits during metamorphosis; yet, hormonal systems can overcome constraints through shifts in timing of, and acquisition of tissue specific responses to, endocrine regulation. Because of these actions of hormones, metamorphic hormones can shape the evolution of metamorphic organisms. We present a model called a developmental goblet, which provides a visual representation of how metamorphic organisms might evolve. In addition, because developmental hormones often respond to environmental changes, we discuss how endocrine regulation of postembryonic development may impact how organisms evolve in response to climate change. Thus, we propose that developmental hormones may provide a mechanistic link between climate change and organismal adaptation.

Sensitivity of Amphibians to Copper

Amphibian populations are declining globally. Major drivers of these global declines are known. However, the contribution of these major drivers to population declines varies by the presence or absence and the interactive effect of drivers, thus creating local challenges for conservation of populations. Studies have determined that environmental contaminants contribute to amphibian population declines. However, there is a disagreement over the use of amphibians as sentinel species in ecotoxicological testing rather than the traditional taxa used, fish and invertebrates. Reviews of ecotoxicological studies have demonstrated that amphibians are generally less sensitive than fish and invertebrates to different groups of contaminants. Nonetheless, because of the distinct nature and mechanism of toxicity of various contaminants, it is necessary to study contaminants individually to be able to come to any conclusion on the relative sensitivity of amphibians. Copper is one of the most studied environmental contaminants. We conducted a literature review of Cu toxicity to amphibians and the relative sensitivity of amphibians to other aquatic animals. The available data suggest that although amphibians may be tolerant of acute Cu exposure, they are relatively sensitive to chronic exposure (i.e., 100-fold greater sensitivity to chronic compared to acute exposure). In addition, ecologically relevant endpoints specific to amphibians (e.g., duration of metamorphosis and behavior) are shown to provide a better understanding of their sensitivity compared to traditional endpoints (e.g., survival and growth). Our current knowledge on amphibian sensitivity is far from complete. Considering the current status of this globally threatened class of animals, it is necessary to fill the knowledge gaps regarding their sensitivity to individual contaminants, beginning with Cu. Environ Toxicol Chem 2021;40:1810-1821. © 2021 SETAC.

Stress hormones mediate developmental plasticity in vertebrates with complex life cycles

The environment experienced by developing organisms can shape the timing and character of developmental processes, generating different phenotypes from the same genotype, each with different probabilities of survival and performance as adults. Chordates have two basic modes of development, indirect and direct. Species with indirect development, which includes most fishes and amphibians, have a complex life cycle with a free-swimming larva that is typically a growth stage, followed by a metamorphosis into the adult form. Species with direct development, which is an evolutionarily derived developmental mode, develop directly from embryo to the juvenile without an intervening larval stage. Among the best studied species with complex life cycles are the amphibians, especially the anurans (frogs and toads). Amphibian tadpoles are exposed to diverse biotic and abiotic factors in their developmental habitat. They have extensive capacity for developmental plasticity, which can lead to the expression of different, adaptive morphologies as tadpoles (polyphenism), variation in the timing of and size at metamorphosis, and carry-over effects on the phenotype of the juvenile/adult. The neuroendocrine stress axis plays a pivotal role in mediating environmental effects on amphibian development. Before initiating metamorphosis, if tadpoles are exposed to predators they upregulate production of the stress hormone corticosterone (CORT), which acts directly on the tail to cause it to grow, thereby increasing escape performance. When tadpoles reach a minimum body size to initiate metamorphosis they can vary the timing of transformation in relation to growth opportunity or mortality risk in the larval habitat. They do this by modulating the production of thyroid hormone (TH), the primary inducer of metamorphosis, and CORT, which synergizes with TH to promote tissue transformation. Hypophysiotropic neurons that release the stress neurohormone corticotropin-releasing factor (CRF) are activated in response to environmental stress (e.g., pond drying, food restriction, etc.), and CRF accelerates metamorphosis by directly inducing secretion of pituitary thyrotropin and corticotropin, thereby increasing secretion of TH and CORT. Although activation of the neuroendocrine stress axis promotes immediate survival in a deteriorating larval habitat, costs may be incurred such as reduced tadpole growth and size at metamorphosis. Small size at transformation can impair performance of the adult, reducing probability of survival in the terrestrial habitat, or fecundity. Furthermore, elevations in CORT in the tadpole caused by environmental stressors cause long term, stable changes in neuroendocrine function, behavior and physiology of the adult, which can affect fitness. Comparative studies show that the roles of stress hormones in developmental plasticity are conserved across vertebrate taxa including humans.

Development of the hypothalamo-hypophyseal system in amphibians with special reference to metamorphosis

In this review article, topics of the embryonic origin of the adenohypophysis and hypothalamus and the development of the hypothalamo-hypophyseal system for the completion of metamorphosis in amphibians are included. The primordium of the adenohypophysis as well as the primordium of the hypothalamus in amphibians is of neural origin as shown in other vertebrates, and both are closely associated with each other at the earliest stage of development. Metamorphosis progresses via the interaction of thyroid hormone and adrenal corticosteroids, of which secretion is enhanced by thyrotropin and corticotropin, respectively. However, unlike in mammals, the hypothalamic releasing factor for thyrotropin is not thyrotropin-releasing hormone (TRH), but corticotropin-releasing factor (CRF) and the major releasing factor for corticotropin is arginine vasotocin (AVT). Prolactin, the release of which is profoundly enhanced by TRH at the metamorphic climax, is another pituitary hormone involved in metamorphosis. Prolactin has a dual role: modulation of the metamorphic speed and the development of organs for adult life. The secretory activities of the pituitary cells containing the three above-mentioned pituitary hormones are elevated toward the metamorphic climax in parallel with the activities of the CRF, AVT, and TRH neurons.

Stress hormone-mediated antipredator morphology improves escape performance in amphibian tadpoles

Complete functional descriptions of the induction sequences of phenotypically plastic traits (perception to physiological regulation to response to outcome) should help us to clarify how plastic responses develop and operate. Ranid tadpoles express several plastic antipredator traits mediated by the stress hormone corticosterone, but how they influence outcomes remains uncertain. We investigated how predator-induced changes in the tail morphology of wood frog (Rana sylvatica) tadpoles influenced their escape performance over a sequence of time points when attacked by larval dragonflies (Anax junius). Tadpoles were raised with no predator exposure, chemical cues of dragonflies added once per day, or constant exposure to caged dragonflies crossed with no exogenous hormone added (vehicle control only), exogenous corticosterone, or metyrapone (a corticosteroid synthesis inhibitor). During predation trials, we detected no differences after four days, but after eight days, tadpoles exposed to larval dragonflies and exogenous corticosterone had developed deeper tail muscles and exhibited improved escape performance compared to controls. Treatment with metyrapone blocked the development of a deeper tail muscle and resulted in no difference in escape success. Our findings further link the predator-induced physiological stress response of ranid tadpoles to the development of an antipredator tail morphology that confers performance benefits.

Shifts in sensitivity of amphibian metamorphosis to endocrine disruption: the common frog (Rana temporaria) as a case study

Effective conservation actions require knowledge on the sensitivity of species to pollution and other anthropogenic stressors. Many of these stressors are endocrine disruptors (EDs) that can impair the hypothalamus-pituitary-thyroid axis and thus alter thyroid hormone (TH) levels with physiological consequences to wildlife. Due to their specific habitat requirements, amphibians are often sentinels of environmental degradation. We investigated how altered TH levels affected the bioenergetics of growth and development (i.e. age, size, metabolism, cardiac function and energy stores) before, during and after metamorphosis in the European common frog (Rana temporaria). We also determined how ontogenetic stage affected susceptibility to endocrine disruption and estimated juvenile performance. TH levels significantly affected growth and energetics at all developmental stages. Tadpoles and froglets exposed to high TH levels were significantly younger, smaller and lighter at all stages compared to those in control and low TH groups, indicating increased developmental and reduced growth rates. Across all ontogenetic stages tested, physiological consequences were rapidly observed after exposure to EDs. High TH increased heart rate by an average of 86% and reduced energy stores (fat content) by 33% compared to controls. Effects of exposure were smallest after the completion of metamorphosis. Our results demonstrate that both morphological and physiological traits of the European common frog are strongly impacted by endocrine disruption and that ontogenetic stage modulates the sensitivity of this species to endocrine disruption. Since endocrine disruption during metamorphosis can impair the physiological stress response in later life stages, long-term studies examining carry-over effects will be an important contribution to the conservation physiology of amphibians.

Pesticide tolerance induced by a generalized stress response in wood frogs (Rana sylvatica)

Increasing evidence suggests that phenotypic plasticity can play a critical role in ecotoxicology. More specifically, induced pesticide tolerance, in which populations exposed to a contaminant show increased tolerance to the contaminants later, has been documented in multiple taxa. However, the physiological mechanisms of induced tolerance remain unclear. We hypothesized that induced pesticide tolerance is the result of a generalized stress response based on previous studies showing that both natural stressors and anthropogenic stressors can induce tolerance to pesticides. We tested this hypothesis by first exposing larval wood frogs (Rana sylvatica) to either an anthropogenic stressor (sublethal carbaryl concentration), a natural stressor (cues from a caged predator), or a simulated stressor via exogenous exposure to the stress hormone corticosterone (125 nM). We also included treatments that inhibited corticosterone synthesis with the compound metyrapone (MTP). We then exposed the larvae to a lethal carbaryl treatment to assess time to death. We found that prior exposure to 125 nM of exogenous CORT and predator cues induced tolerance to a lethal concentration of carbaryl through a slight delay in time to death. Pre-exposure to sublethal carbaryl, as well as MTP alone or in combination with predator cues, did not induce tolerance to the lethal carbaryl concentration relative to the ethanol vehicle control treatment. Our study provides evidence that pesticide tolerance can be induced by a generalized stress response both in the presence and absence (exogenous CORT) of specific cues and highlights the importance of considering physiological ecology and environmental context in ecotoxicology.

Identification of candidate loci for adaptive phenotypic plasticity in natural populations of spadefoot toads

Phenotypic plasticity allows organisms to alter their phenotype in direct response to changes in the environment. Despite growing recognition of plasticity's role in ecology and evolution, few studies have probed plasticity's molecular bases-especially using natural populations. We investigated the genetic basis of phenotypic plasticity in natural populations of spadefoot toads (Spea multiplicata). Spea tadpoles normally develop into an "omnivore" morph that is favored in long-lasting, low-density ponds. However, if tadpoles consume freshwater shrimp or other tadpoles, they can alternatively develop (via plasticity) into a "carnivore" morph that is favored in ephemeral, high-density ponds. By combining natural variation in pond ecology and morph production with population genetic approaches, we identified candidate loci associated with each morph (carnivores vs. omnivores) and loci associated with adaptive phenotypic plasticity (adaptive vs. maladaptive morph choice). Our candidate morph loci mapped to two genes, whereas our candidate plasticity loci mapped to 14 genes. In both cases, the identified genes tended to have functions related to their putative role in spadefoot tadpole biology. Our results thereby form the basis for future studies into the molecular mechanisms that mediate plasticity in spadefoots. More generally, these results illustrate how diverse loci might mediate adaptive plasticity.

Changes in physiology and microbial diversity in larval ornate chorus frogs are associated with habitat quality

Environmental change associated with anthropogenic disturbance can lower habitat quality, especially for sensitive species such as many amphibians. Variation in environmental quality may affect an organism's physiological health and, ultimately, survival and fitness. Using multiple health measures can aid in identifying populations at increased risk of declines. Our objective was to measure environmental variables at multiple spatial scales and their effect on three indicators of health in ornate chorus frog (Pseudacris ornata) tadpoles to identify potential correlates of population declines. To accomplish this, we measured a glucocorticoid hormone (corticosterone; CORT) profile associated with the stress response, as well as the skin mucosal immune function (combined function of skin secretions and skin bacterial community) and bacterial communities of tadpoles from multiple ponds. We found that water quality characteristics associated with environmental variation, including higher water temperature, conductivity and total dissolved solids, as well as percent developed land nearby, were associated with elevated CORT release rates. However, mucosal immune function, although highly variable, was not significantly associated with water quality or environmental factors. Finally, we examined skin bacterial diversity as it aids in immunity and is affected by environmental variation. We found that skin bacterial diversity differed between ponds and was affected by land cover type, canopy cover and pond proximity. Our results indicate that both local water quality and land cover characteristics are important determinants of population health for ornate chorus frogs. Moreover, using these proactive measures of health over time may aid in early identification of at-risk populations that could prevent further declines and aid in management decisions.

Altered thyroid hormone levels affect the capacity for temperature-induced developmental plasticity in larvae of Rana temporaria and Xenopus laevis

Anuran larvae show phenotypic plasticity in age and size at metamorphosis as a response to temperature variation. The capacity for temperature-induced developmental plasticity is determined by the thermal adaptation of a population. Multiple factors such as physiological responses to changing environmental conditions, however, might influence this capacity as well. In anuran larvae, thyroid hormone (TH) levels control growth and developmental rate and changes in TH status are a well-known stress response to sub-optimal environmental conditions. We investigated how chemically altered TH levels affect the capacity to exhibit temperature-induced developmental plasticity in larvae of the African clawed frog (Xenopus laevis) and the common frog (Rana temporaria). In both species, TH level influenced growth and developmental rate and modified the capacity for temperature-induced developmental plasticity. High TH levels reduced thermal sensitivity of metamorphic traits up to 57% (R. temporaria) and 36% (X. laevis). Rates of growth and development were more plastic in response to temperature in X. laevis (+30%) than in R. temporaria (+6%). Plasticity in rates of growth and development is beneficial to larvae in heterogeneous habitats as it allows a more rapid transition into the juvenile stage where rates of mortality are lower. Therefore, environmental stressors that increase endogenous TH levels and reduce temperature-dependent plasticity may increase risks and the vulnerability of anuran larvae. As TH status also influences metabolism, future studies should investigate whether reductions in physiological plasticity also increases the vulnerability of tadpoles to global change.

Glucocorticoid receptor is required for survival through metamorphosis in the frog Xenopus tropicalis

Stress hormones, also known as glucocorticoids, are critical for survival at birth in mammals due at least in part to their importance in lung maturation. However, because air breathing is not always required for amphibian survival and because stress hormones have no known developmental impact except to modulate the developmental actions of thyroid hormone (TH), the requirement for stress hormone signaling during metamorphosis is not well understoodi. Here, we produced a glucocorticoid receptor knockout (GRKO) Xenopus line with a frameshift mutation in the first exon of the glucocorticoid receptor. Induction by exogenous corticosterone (CORT, the frog stress hormone) of the CORT response genes, klf9 (Krüppel-like factor 9, also regulated by TH) and ush1g (Usher's syndrome 1G), was completely abrogated in GRKO tadpoles. Surprisingly, GRKO tadpoles developed faster than wild-type tadpoles until forelimb emergence and then developed more slowly until their death at the climax of metamorphosis. Growth rate was not affected in GRKO tadpoles, but they achieved a smaller maximum size. Gene expression analysis of the TH response genes, thrb (TH receptor beta) and klf9 showed reduced expression in the tail at metamorphic climax consistent with the reduced development rate. These results indicate that glucocorticoid receptor is required for survival through metamorphosis and support dual roles for GR signaling in control of developmental rate.

Post-metamorphic carry-over effects of altered thyroid hormone level and developmental temperature: physiological plasticity and body condition at two life stages in Rana temporaria

Environmental stress induced by natural and anthropogenic processes including climate change may threaten the productivity of species and persistence of populations. Ectotherms can potentially cope with stressful conditions such as extremes in temperature by exhibiting physiological plasticity. Amphibian larvae experiencing stressful environments display altered thyroid hormone (TH) status with potential implications for physiological traits and acclimation capacity. We investigated how developmental temperature (T_dev) and altered TH levels (simulating proximate effects of environmental stress) influence the standard metabolic rate (SMR), body condition (BC), and thermal tolerance in metamorphic and post-metamorphic anuran larvae of the common frog (Rana temporaria) reared at five constant temperatures (14-28 °C). At metamorphosis, larvae that developed at higher temperatures had higher maximum thermal limits but narrower ranges in thermal tolerance. Mean CT_max was 37.63 °C ± 0.14 (low TH), 36.49 °C ± 0.31 (control), and 36.43 °C ± 0.68 (high TH) in larvae acclimated to different temperatures. Larvae were able to acclimate to higher T_dev by adjusting their thermal tolerance, but not their SMR, and this effect was not impaired by altered TH levels. BC was reduced by 80% (metamorphic) and by 85% (post-metamorphic) at highest T_dev. The effect of stressful larval conditions (i.e., different developmental temperatures and, to some extent, altered TH levels) on SMR and particularly on BC at the onset of metamorphosis was carried over to froglets at the end of metamorphic climax. This has far reaching consequences, since body condition at metamorphosis is known to determine metamorphic success and, thus, is indirectly linked to individual fitness in later life stages.

Cortisol is the predominant glucocorticoid in the giant paedomorphic hellbender salamander (Cryptobranchus alleganiensis)

Corticosterone is widely regarded to be the predominant glucocorticoid produced in amphibians. However, we recently described unusually low baseline and stress-induced corticosterone profiles in eastern hellbenders (Cryptobranchus alleganiensis alleganiensis), a giant, fully aquatic salamander. Here, we hypothesized that hellbenders might also produce cortisol, the predominant glucocorticoid used by fishes and non-rodent mammals. To test our hypothesis, we collected plasma samples in two field experiments and analyzed them using multiple analytical techniques to determine how plasma concentrations of cortisol and corticosterone co-varied after 1) physical restraint and 2) injection with adrenocorticotropic hormone (ACTH), the pituitary hormone responsible for triggering the release of glucocorticoids from amphibian interrenal glands. Using liquid chromatography-mass spectrometry, we found that baseline and restraint-induced plasma concentrations of cortisol were more than five times those of corticosterone. We then demonstrated that plasma concentrations of both glucocorticoids increased in response to ACTH in a dose-dependent manner, but cortisol concentrations were consistently higher (up to 10-fold) than corticosterone. Cortisol and corticosterone concentrations were not correlated with one another at basal or induced conditions. The extremely low plasma concentrations of corticosterone in hellbenders suggests that corticosterone could simply be a byproduct of cortisol production, and raises questions as to whether corticosterone has any distinct physiological function in hellbenders. Our results indicate that hellbenders produce cortisol as their predominant glucocorticoid, supporting a small and inconclusive body of literature indicating that some other amphibians may produce appreciable quantities of cortisol. We hypothesize that the use of cortisol by hellbenders could be an adaptation to their fully aquatic life history due to cortisol's ability to fulfill both mineralocorticoid and glucocorticoid functions, similar to its functions in fishes. Given the large number of amphibian species that are fully aquatic or have aquatic life stages, we suggest that the broadly held assumption that corticosterone is the predominant glucocorticoid in all amphibians requires further scrutiny. Ultimately, multi-species tests of this assumption will reveal the ecological factors that influenced the evolution of endocrine adaptations among amphibian lineages, and may provide insight into convergent evolution of endocrine traits in paedomorphic species.

Characterizing the composition, metabolism and physiological functions of the fatty liver in Rana omeimontis tadpoles

Background

Fat storage is required for the life cycle of many organisms. The primary fat depot for most vertebrates is white adipose tissue. However, in primitive vertebrates (e.g., agnathan group and elasmobranchs), the liver is usually responsible for fat storage. Among the vertebrates, amphibians have a unique status, as their larvae live in the water and exhibit some primitive traits that are similar to fish. Although it has been recognized that adult frogs use their abdominal white adipose tissue as a primary fat depot, how tadpoles store their fat is still inconclusive. The metabolic traits and physiological functions of primitive fat depots may have wide-ranging implications on the pathology of abnormal lipid deposition in mammals and the evolution of fat storage.

Results

Rana omeimontis tadpoles used their liver as the primary fat depot. In sufficiently fed tadpoles at stage 30-31, the hepatosomatic index (HSI) reached up to 7%, and triglycerides (TG) accounted for 15% of liver weight. Their liver resembled white adipose tissue in histological morphology, characterized by polygonal hepatocytes filled with fat. Their liver metabolic composition was unique, characterized by the dominance of maltotriose, arachidonic acid and dipeptides in soluble carbohydrates, free fatty acids and amino acids. Hepatic fat was the major metabolic fuel of fasted R. omeimontis tadpoles, which had similar reserve mobilization and allocation patterns as mammals. From a developmental perspective, hepatic fat was important to fuel late metamorphic climax. Interestingly, starvation induced accelerated metamorphosis in tadpoles with high HSI (4.96 ± 0.21%). However, this phenomenon was not observed in tadpoles with low HSI (2.71 ± 0.16%), even though they had similar initial body weight and developmental stage. Hepatic fat abundance was the most prominent difference between the two groups.

Conclusion

To the best of our knowledge, this is the first report that liver can be the primary fat depot in vertebrates with higher evolutionary status than bony fish. The unique hepatic histological and metabolic traits likely either guard their liver against lipotoxicity or make their hepatocytes adapt to fat accumulation. This fatty liver could be a primitive counterpart of mammalian white adipose tissue (WAT). In addition, our study showed that the hepatic reserves of tadpoles, especially TG content, may provide body condition signals to modulate metamorphosis.

Embryonic ontogeny of three species of Horned Frogs, with a review of early development in Ceratophryidae

Horned Frogs of the family Ceratophryidae are conspicuous anurans represented by three endemic South American genera. Most ceratophryids inhabit semiarid environments, but three species of Ceratophrys occupy tropical or temperate humid areas. Several morphological and behavioral characters of larvae and adults are conserved across the family. Based on examination of specimens and accounts in the literature, the embryonic development of C. ornata, C. cranwelli, and the monotypic genus Chacophrys are described and compared with that of species of Lepidobatrachus. Ceratophryid embryos share a suite of morphological features and heterochronic shifts during development. Most features, such as gill structure, ciliation, early hatching, and precocious differentiation of the gut and hind limbs, are shared by all the species regardless the differences in the habitats that occupy. This is consistent with previous observations of some adult characters, and likely supports the hypothesis of an early diversification of ceratophryids in semiarid environments. Other embryonic features, such as the morphology and ontogeny of the oral disc and digestive tract, are correlated with larval feeding habits and vary within the family. The evolutionary and ecological significance of some conserved characters (e.g., gastrulation pattern, Type-A adhesive glands) and other taxon-specific features (e.g., nasal appendix) remain to be explored in the group.

Endocrine Disruption Alters Developmental Energy Allocation and Performance in Rana temporaria

Environmental change exposes wildlife to a wide array of environmental stressors that arise from both anthropogenic and natural sources. Many environmental stressors with the ability to alter endocrine function are known as endocrine disruptors, which may impair the hypothalamus-pituitary-thyroid axis resulting in physiological consequences to wildlife. In this study, we investigated how the alteration of thyroid hormone (TH) levels due to exposure to the environmentally relevant endocrine disruptor sodium perchlorate (SP; inhibitory) and exogenous L-thyroxin (T4; stimulatory) affects metabolic costs and energy allocation during and after metamorphosis in a common amphibian (Rana temporaria). We further tested for possible carry-over effects of endocrine disruption during larval stage on juvenile performance. Energy allocated to development was negatively related to metabolic rate and thus, tadpoles exposed to T4 could allocate 24% less energy to development during metamorphic climax than control animals. Therefore, the energy available for metamorphosis was reduced in tadpoles with increased TH level by exposure to T4. We suggest that differences in metabolic rate caused by altered TH levels during metamorphic climax and energy allocation to maintenance costs might have contributed to a reduced energetic efficiency in tadpoles with high TH levels. Differences in size and energetics persisted beyond the metamorphic boundary and impacted on juvenile performance. Performance differences are mainly related to strong size-effects, as altered TH levels by exposure to T4 and SP significantly affected growth and developmental rate. Nevertheless, we assume that juvenile performance is influenced by a size-independent effect of achieved TH. Energetic efficiency varied between treatments due to differences in size allocation of internal macronutrient stores. Altered TH levels as caused by several environmental stressors lead to persisting effects on metamorphic traits and energetics and, thus, caused carry-over effects on performance of froglets. We demonstrate the mechanisms through which alterations in abiotic and biotic environmental factors can alter phenotypes at metamorphosis and reduce lifetime fitness in these and likely other amphibians.

Early ontogeny and sequence heterochronies in Leiuperinae frogs (Anura: Leptodactylidae)

The study of early development in Neotropical Leiuperinae frogs (Anura, Leptodactylidae) has been addressed by several works in recent times. However, a comparative developmental approach under a phylogenetic context was not available. Herein we analyzed the morphological and ontogenetic diversity of embryos belonging to 22 species of the three largest genera in Leiuperinae. We find that in most cases, variations fit with the phylogeny at the inter- and intrageneric levels. Embryo kyphosis and whitish color are synapomorphies for the clade grouping Physalaemus and Engystomops. The presence of a third lower tooth row on the oral disc is plesiomorphic for Leiuperinae, only changing in derived clades. The configurations and developmental trajectories of the lower lip are exceptionally diverse. The developmental sequences optimized on the phylogenetic tree recover an early differentiated first lower tooth row a synapomorphy of Pseudopaludicola and Physalaemus, and an early differentiated second row as synapomorphy of Pleurodema. On the other hand, few features are highly conserved in the subfamily, such as the adhesive glands universally present in a type-C configuration. Our results also suggest that the morphology and ontogeny of embryos is in some cases associated to the environment where they develop. A large body size, poorly developed transient respiratory structures, large yolk provision and delayed development of the digestive tract occur convergently in embryos inhabiting cold, oxygenated environments. Embryos that develop in warmer water bodies in xeric environments show more complex and persistent transient respiratory structures and an early onset of hind limbs development. Our survey highlights that morphology and early development of anurans can be a valuable source of information for phylogenetic studies, and provide fundamental bases to explore and discuss how evolutionary changes can be shaped by environmental conditions.

Contaminant and Environmental Influences on Thyroid Hormone Action in Amphibian Metamorphosis

Aquatic and terrestrial environments are increasingly contaminated by anthropogenic sources that include pharmaceuticals, personal care products, and industrial and agricultural chemicals (i. e., pesticides). Many of these substances have the potential to disrupt endocrine function, yet their effect on thyroid hormone (TH) action has garnered relatively little attention. Anuran postembryonic metamorphosis is strictly dependent on TH and perturbation of this process can serve as a sensitive barometer for the detection and mechanistic elucidation of TH disrupting activities of chemical contaminants and their complex mixtures. The ecological threats posed by these contaminants are further exacerbated by changing environmental conditions such as temperature, photoperiod, pond drying, food restriction, and ultraviolet radiation. We review the current knowledge of several chemical and environmental factors that disrupt TH-dependent metamorphosis in amphibian tadpoles as assessed by morphological, thyroid histology, behavioral, and molecular endpoints. Although the molecular mechanisms for TH disruption have yet to be determined for many chemical and environmental factors, several affect TH synthesis, transport or metabolism with subsequent downstream effects. As molecular dysfunction typically precedes phenotypic or histological pathologies, sensitive assays that detect changes in transcript, protein, or metabolite abundance are indispensable for the timely detection of TH disruption. The emergence and application of 'omics techniques-genomics, transcriptomics, proteomics, metabolomics, and epigenomics-on metamorphosing tadpoles are powerful emerging assets for the rapid, proxy assessment of toxicant or environmental damage for all vertebrates including humans. Moreover, these highly informative 'omics techniques will complement morphological, behavioral, and histological assessments, thereby providing a comprehensive understanding of how TH-dependent signal disruption is propagated by environmental contaminants and factors.

Disease and the Drying Pond: Examining Possible Links among Drought, Immune Function, and Disease Development in Amphibians

Drought can heavily impact aquatic ecosystems. For amphibian species that rely on water availability for larval development, drought can have direct and indirect effects on larval survival and postmetamorphic fitness. Some amphibian species can accelerate the timing of metamorphosis to escape drying habitats through developmental plasticity. However, trade-offs associated with premature metamorphosis, such as reduced body size and altered immune function in the recently metamorphosed individual, may have downstream effects on susceptibility to disease. Here, we review the physiological mechanisms driving patterns in larval amphibian development under low water conditions. Specifically, we discuss drought-induced accelerated metamorphosis and how it may alter immune function, predisposing juvenile amphibians to infectious disease. In addition, we consider how these physiological and immunological adjustments could play out in a lethal disease system, amphibian chytridiomycosis. Last, we propose avenues for future research that adopt an ecoimmunological approach to evaluate the combined threats of drought and disease for amphibian populations.

Evolutionary Conservation of Thyroid Hormone Receptor and Deiodinase Expression Dynamics in ovo in a Direct-Developing Frog, Eleutherodactylus coqui

Direct development is a reproductive mode in amphibians that has evolved independently from the ancestral biphasic life history in at least a dozen anuran lineages. Most direct-developing frogs, including the Puerto Rican coquí, Eleutherodactylus coqui, lack a free-living aquatic larva and instead hatch from terrestrial eggs as miniature adults. Their embryonic development includes the transient formation of many larval-specific features and the formation of adult-specific features that typically form postembryonically-during metamorphosis-in indirect-developing frogs. We found that pre-hatching developmental patterns of thyroid hormone receptors alpha (thra) and beta (thrb) and deiodinases type II (dio2) and type III (dio3) mRNAs in E. coqui limb and tail are conserved relative to those seen during metamorphosis in indirect-developing frogs. Additionally, thra, thrb, and dio2 mRNAs are expressed in the limb before formation of the embryonic thyroid gland. Liquid-chromatography mass-spectrometry revealed that maternally derived thyroid hormone is present throughout early embryogenesis, including stages of digit formation that occur prior to the increase in embryonically produced thyroid hormone. Eleutherodactylus coqui embryos take up much less 3,5,3'-triiodothyronine (T₃) from the environment compared with X. tropicalis tadpoles. However, E. coqui tissue explants mount robust and direct gene expression responses to exogenous T₃ similar to those seen in metamorphosing species. The presence of key components of the thyroid axis in the limb and the ability of limb tissue to respond to T₃ suggest that thyroid hormone-mediated limb development may begin prior to thyroid gland formation. Thyroid hormone-dependent limb development and tail resorption characteristic of metamorphosis in indirect-developing anurans are evolutionarily conserved, but they occur instead in ovo in E. coqui.

How Temperature, Pond-Drying, and Nutrients Influence Parasite Infection and Pathology

The rapid pace of environmental change is driving multi-faceted shifts in abiotic factors that influence parasite transmission. However, cumulative effects of these factors on wildlife diseases remain poorly understood. Here we used an information-theoretic approach to compare the relative influence of abiotic factors (temperature, diurnal temperature range, nutrients and pond-drying), on infection of snail and amphibian hosts by two trematode parasites (Ribeiroia ondatrae and Echinostoma spp.). A temperature shift from 20 to 25 °C was associated with an increase in infected snail prevalence of 10-20%, while overall snail densities declined by a factor of 6. Trematode infection abundance in frogs was best predicted by infected snail density, while Ribeiroia infection specifically also declined by half for each 10% reduction in pond perimeter, despite no effect of perimeter on the per snail release rate of cercariae. Both nutrient concentrations and Ribeiroia infection positively predicted amphibian deformities, potentially owing to reduced host tolerance or increased parasite virulence in more productive environments. For both parasites, temperature, pond-drying, and nutrients were influential at different points in the transmission cycle, highlighting the importance of detailed seasonal field studies that capture the importance of multiple drivers of infection dynamics and the mechanisms through which they operate.

Genetic accommodation via modified endocrine signalling explains phenotypic divergence among spadefoot toad species

Phenotypic differences among species may evolve through genetic accommodation, but mechanisms accounting for this process are poorly understood. Here we compare hormonal variation underlying differences in the timing of metamorphosis among three spadefoot toads with different larval periods and responsiveness to pond drying. We find that, in response to pond drying, Pelobates cultripes and Spea multiplicata accelerate metamorphosis, increase standard metabolic rate (SMR), and elevate whole-body content of thyroid hormone (the primary morphogen controlling metamorphosis) and corticosterone (a stress hormone acting synergistically with thyroid hormone to accelerate metamorphosis). In contrast, Scaphiopus couchii has the shortest larval period, highest whole-body thyroid hormone and corticosterone content, and highest SMR, and these trait values are least affected by pond drying among the three species. Our findings support that the atypically rapid and canalized development of S. couchii evolved by genetic accommodation of endocrine pathways controlling metamorphosis, showing how phenotypic plasticity within species may evolve into trait variation among species.

Genetic accommodation is a potential mechanism for the phenotypic divergence of species. Here, Kulkarni et al. compare endocrine responses of three spadefoot toad species to pond drying and suggest how evolution of mechanisms of developmental plasticity may account for trait variation among species.

Amphibian immunity-stress, disease, and climate change

Like all other vertebrate groups, amphibian responses to the environment are mediated through the brain (hypothalamic)-pituitary-adrenal/interrenal (HPA/I) axis and the sympathetic nervous system. Amphibians are facing historically unprecedented environmental stress due to climate change that will involve unpredictable temperature and rainfall regimes and possible nutritional deficits due to extremes of temperature and drought. At the same time, amphibians in all parts of the world are experiencing unprecedented declines due to the emerging diseases, chytridiomycosis (caused by Batrachochytrium dendrobatidis and Batrachochytrium salamandrivorans) and ranavirus diseases due to viruses of the genus Ranavirus in the family Iridoviridae. Other pathogens and parasites also afflict amphibians, but here I will limit myself to a review of recent literature linking stress and these emerging diseases (chytridiomycosis and ranavirus disease) in order to better predict how environmental stressors and disease will affect global amphibian populations.

Warmer temperature modifies effects of polybrominated diphenyl ethers on hormone profiles in leopard frog tadpoles (Lithobates pipiens)

Amphibian populations have been declining, and climate change and exposure to environmental contaminants are thought to be involved. Higher water temperature accelerates larval development; however, its combined effects with contaminants and their influence on hormones during metamorphosis are poorly understood. The authors investigated changes in whole-body triiodothyronine (T3) and corticosterone concentrations in developing leopard frogs reared at 23 °C and 28 °C on diets with 0 ng g^-1 , 6 ng g^-1 , and 37 ng g^-1 of a technical mixture of polybrominated diphenyl ethers (PBDE; DE-71) from 10 d to 44 d (premetamorphosis to late climax; Gosner Stages 28 to 46). Unlike controls, PBDE-exposed tadpoles (6 ng g^-1 ) reared at 23 °C failed to show any increase in T3 concentrations throughout metamorphosis, and exposed tadpoles reared at 28 °C showed a lower peak at climax compared to controls. Corticosterone levels progressively increased throughout metamorphosis, but the levels were higher in PBDE-exposed tadpoles compared to controls at both temperatures. At the warmer temperature, corticosterone increase occurred earlier (at early climax) in controls and exposed tadpoles compared to tadpoles reared at the cooler temperature (late climax), coinciding with the faster development observed at 28 °C. Tadpoles reared at 28 °C were longer and developed faster than tadpoles reared at 23 °C. At both temperatures, PBDE exposure decreased T3 and increased corticosterone concentrations, which can potentially impair developing tadpoles. Environ Toxicol Chem 2017;36:120-127. © 2016 SETAC.

Home alone-The effects of isolation on uptake of a pharmaceutical contaminant in a social fish

A wide range of biologically active pharmaceutical residues is present in aquatic systems worldwide. As uptake potential and the risk of effects in aquatic wildlife are directly coupled, the aim of this study was to investigate the relationships between stress by isolation, uptake and effects of the psychiatric pharmaceutical oxazepam in fish. To do this, we measured cortisol levels, behavioral stress responses, and oxazepam uptake under different stress and social conditions, in juvenile perch (Perca fluviatilis) that were either exposed (1.03μgl^-1) or not exposed to oxazepam. We found single exposed individuals to take up more oxazepam than individuals exposed in groups, likely as a result of stress caused by isolation. Furthermore, the bioconcentration factor (BCF) was significantly negatively correlated with fish weight in both social treatments. We found no effect of oxazepam exposure on body cortisol concentration or behavioral stress response. Most laboratory experiments, including standardized bioconcentration assays, are designed to minimize stress for the test organisms, however wild animals experience stress naturally. Hence, differences in stress levels between laboratory and natural environments can be one of the reasons why predictions from artificial laboratory experiments largely underestimate uptake of oxazepam, and other pharmaceuticals, in the wild.

Effects of Water Loss on New Mexico Spadefoot Toad (Spea multiplicata) Development, Spleen Cellularity, and Corticosterone Levels

Amphibian metamorphosis is complex and larval morphology and physiology are completely restructured during this time. Amphibians that live in unpredictable environments are often exposed to stressors that can directly and indirectly alter physiological systems during development, with subsequent consequences (carryover effects) later in life. In this study, we investigated the effects of water level reduction on development rate, spleen size and cellularity, and examined the role of corticosterone levels in premetamorphic, metamorphic, and postmetamorphic New Mexico spadefoot toads (Spea multiplicata). Based on previous studies, we hypothesized that declining water level would increase tadpole developmental rate, but with the trade-off of increasing corticosterone to a level that would subsequently affect spleen size and cellularity, thus prolonging potential immunological suppression. Declining water levels increased developmental rate by 3 days; however, there were no significant body size effects. Corticosterone (CORT) was negatively correlated with total length, snout vent length, body weight, and spleen weight at metamorphosis, suggesting that size at metamorphosis and the immune system may be affected by excessive CORT levels. When compared to other studies, our results support the view that multiple factors may be acting as stressors in the field affecting amphibian responses, and simple pathways as tested in this study may not adequately represent field conditions.

Overcrowding-mediated stress alters cell proliferation in key neuroendocrine areas during larval development in Rhinella arenarum

Exposure to adverse environmental conditions can elicit a stress response, which results in an increase in endogenous corticosterone levels. In early life stages, it has been thoroughly demonstrated that amphibian larval growth and development is altered as a consequence of chronic stress by interfering with the metamorphic process, however, the underlying mechanisms involved have only been partially disentangled. We examined the effect of intraspecific competition on corticosterone levels during larval development of the toad Rhinella arenarum and its ultimate effects on cell proliferation in particular brain areas as well as the pituitary gland. While overcrowding altered the number of proliferating cells in the pituitary gland, hypothalamus, and third ventricle of the brain, no differences were observed in areas which are less associated with neuroendocrine processes, such as the first ventricle of the brain. Apoptosis was increased in hypothalamic regions but not in the pituitary. With regards to pituitary cell populations, thyrotrophs but not somatoatrophs and corticotrophs showed a decrease in the cell number in overcrowded larvae. Our study shows that alterations in growth and development, produced by stress, results from an imbalance in the neuroendocrine systems implicated in orchestrating the timing of metamorphosis.

Seminiferous Epithelium Cycle in Bombina orientalis

The purpose of the present study was to examine the seminiferous epithelium cycle of Bombina orientalis using a light microscope. The cycle was divided into a total of 10 stages, according to the morphological characteristics of the cells. The spermatogenetic cells included primary spermatogonia, secondary spermatogonia, primary spermatocytes, secondary spermatocytes, spermatid and sperm. At stage I, the primary spermatogonia was located closer to basal lamina of the seminiferous tubule without spermatocyst formations. Especially at the stage II, the secondary spermatogonia were located in the spermatocyst. The primary and secondary spermatocytes were found from stages III to VI. The secondary spermatocytes were smaller in size than the primary spermatocytes, but they had thicker nucleoplasm and smaller nuclei. The round-shaped, early sperm cells were formed in stage VII, and further divided at stage VIII to have more concentrated nucleoplasm before division to matured sperm cells. At stage X, the matured sperm cells emerged from the spermatocyst. Considering the above results, this study presented the special characteristics in the generation and type of sperm formation. The germ cell formation occurred in various stages, like the perspectives of Franca et al (1999), ultimately, providing taxonomically useful information.

Seminiferous Epithelium Cycle in Bombina orientalis

The purpose of the present study was to examine the seminiferous epithelium cycle of Bombina orientalis using a light microscope. The cycle was divided into a total of 10 stages, according to the morphological characteristics of the cells. The spermatogenetic cells included primary spermatogonia, secondary spermatogonia, primary spermatocytes, secondary spermatocytes, spermatid and sperm. At stage I, the primary spermatogonia was located closer to basal lamina of the seminiferous tubule without spermatocyst formations. Especially at the stage II, the secondary spermatogonia were located in the spermatocyst. The primary and secondary spermatocytes were found from stages III to VI. The secondary spermatocytes were smaller in size than the primary spermatocytes, but they had thicker nucleoplasm and smaller nuclei. The round-shaped, early sperm cells were formed in stage VII, and further divided at stage VIII to have more concentrated nucleoplasm before division to matured sperm cells. At stage X, the matured sperm cells emerged from the spermatocyst. Considering the above results, this study presented the special characteristics in the generation and type of sperm formation. The germ cell formation occurred in various stages, like the perspectives of Franca et al (1999), ultimately, providing taxonomically useful information.

Lessons from evolution: developmental plasticity in vertebrates with complex life cycles

Developmental plasticity is the property of a given genotype to produce different phenotypes in response to the environmental conditions experienced during development. Chordates have two basic modes of development, direct and indirect. Direct development (mode of humans) was derived evolutionarily from indirect development (mode of many amphibians), the major difference being the presence of a larval stage with indirect development; larvae undergo metamorphosis to the juvenile adult. In amphibians, environmental conditions experienced during the larval stage can lead to extreme plasticity in behaviour, morphology and the timing of metamorphosis and can cause variation in adult phenotypic expression (carry-over effects, or developmental programming). Hormones of the neuroendocrine stress axis play pivotal roles in mediating environmental effects on animal development. Stress hormones, produced in response to a deteriorating larval habitat, accelerate amphibian metamorphosis; in mammals, stress hormones hasten the onset of parturition and play an important role in pre-term birth caused by intra-uterine stress. While stress hormones can promote survival in a deteriorating larval or intra-uterine habitat, costs may be incurred, such as reduced growth and size at metamorphosis or birth. Furthermore, exposure to elevated stress hormones during the tadpole or foetal stage can cause permanent neurological changes, leading to altered physiology and behaviour later in life. The actions of stress hormones in animal development are evolutionarily conserved, and therefore amphibians can serve as important model organisms for research on the mechanisms of developmental plasticity.

Corticosteroid signaling in frog metamorphosis

Stress in fetal and larval life can impact later health and fitness in humans and wildlife. Long-term effects of early life stress are mediated by altered stress physiology induced during the process of relaying environmental effects on development. Amphibian metamorphosis has been an important model system to study the role of hormones in development in an environmental context. Thyroid hormone (TH) is necessary and sufficient to initiate the dramatic morphological and physiological changes of metamorphosis, but TH alone is insufficient to complete metamorphosis. Other hormones, importantly corticosteroid hormones (CSs), influence the timing and nature of post-embryonic development. Stressors or treatments with CSs delay or accelerate metamorphic change, depending on the developmental stage of treatment. Also, TH and CSs have synergistic, antagonistic, and independent effects on gene regulation. Importantly, the identity of the endogenous corticosteroid hormone or receptor underlying any gene induction or remodeling event has not been determined. Levels of both CSs, corticosterone and aldosterone, peak at metamorphic climax, and the corticosteroid receptors, glucocorticoid and mineralocorticoid receptors, have wide expression distribution among tadpole tissues. Conclusive experiments to identify the endogenous players have been elusive due to difficulties in experimental control of corticosteroid production and signaling. Current data are consistent with the hypothesis that the two CSs and their receptors serve largely overlapping functions in regulating metamorphosis and synergy with TH. Knowledge of the endogenous players is critical to understanding the basic mechanisms and significance of corticosteroid action in regulating post-embryonic development in environmental contexts.

Mechanisms and consequences of developmental acceleration in tadpoles responding to pond drying

Many amphibian species exploit temporary or even ephemeral aquatic habitats for reproduction by maximising larval growth under benign conditions but accelerating development to rapidly undergo metamorphosis when at risk of desiccation from pond drying. Here we determine mechanisms enabling developmental acceleration in response to decreased water levels in western spadefoot toad tadpoles (Pelobates cultripes), a species with long larval periods and large size at metamorphosis but with a high degree of developmental plasticity. We found that P. cultripes tadpoles can shorten their larval period by an average of 30% in response to reduced water levels. We show that such developmental acceleration was achieved via increased endogenous levels of corticosterone and thyroid hormone, which act synergistically to achieve metamorphosis, and also by increased expression of the thyroid hormone receptor TRΒ, which increases tissue sensitivity and responsivity to thyroid hormone. However, developmental acceleration had morphological and physiological consequences. In addition to resulting in smaller juveniles with proportionately shorter limbs, tadpoles exposed to decreased water levels incurred oxidative stress, indicated by increased activity of the antioxidant enzymes catalase, superoxide dismutase, and glutathione peroxidase. Such increases were apparently sufficient to neutralise the oxidative damage caused by presumed increased metabolic activity. Thus, developmental acceleration allows spadefoot toad tadpoles to evade drying ponds, but it comes at the expense of reduced size at metamorphosis and increased oxidative stress.

Host stress response is important for the pathogenesis of the deadly amphibian disease, Chytridiomycosis, in Litoria caerulea

Chytridiomycosis, a disease caused by Batrachochytrium dendrobatidis, has contributed to worldwide amphibian population declines; however, the pathogenesis of this disease is still somewhat unclear. Previous studies suggest that infection disrupts cutaneous sodium transport, which leads to hyponatremia and cardiac failure. However, infection is also correlated with unexplained effects on appetite, skin shedding, and white blood cell profiles. Glucocorticoid hormones may be the biochemical connection between these disparate effects, because they regulate ion homeostasis and can also influence appetite, skin shedding, and white blood cells. During a laboratory outbreak of B. dendrobatidis in Australian Green Tree Frogs, Litoria caerulea, we compared frogs showing clinical signs of chytridiomycosis to infected frogs showing no signs of disease and determined that diseased frogs had elevated baseline corticosterone, decreased plasma sodium and potassium, and altered WBC profiles. Diseased frogs also showed evidence of poorer body condition and elevated metabolic rates compared with frogs showing no signs of disease. Prior to displaying signs of disease, we also observed changes in appetite, body mass, and the presence of shed skin associated with infected but not yet diseased frogs. Collectively, these results suggest that elevated baseline corticosterone is associated with chytridiomycosis and correlates with some of the deleterious effects observed during disease development.

Stress hormones mediate predator-induced phenotypic plasticity in amphibian tadpoles

Amphibian tadpoles display extensive anti-predator phenotypic plasticity, reducing locomotory activity and, with chronic predator exposure, developing relatively smaller trunks and larger tails. In many vertebrates, predator exposure alters activity of the neuroendocrine stress axis. We investigated predator-induced effects on stress hormone production and the mechanistic link to anti-predator defences in Rana sylvatica tadpoles. Whole-body corticosterone (CORT) content was positively correlated with predator biomass in natural ponds. Exposure to caged predators in mesocosms caused a reduction in CORT by 4 hours, but increased CORT after 4 days. Tadpoles chronically exposed to exogenous CORT developed larger tails relative to their trunks, matching morphological changes induced by predator chemical cue; this predator effect was blocked by the corticosteroid biosynthesis inhibitor metyrapone. Tadpole tail explants treated in vitro with CORT increased tissue weight, suggesting that CORT acts directly on the tail. Short-term treatment of tadpoles with CORT increased predation mortality, likely due to increased locomotory activity. However, long-term CORT treatment enhanced survivorship, likely due to induced morphology. Our findings support the hypothesis that tadpole physiological and behavioural/morphological responses to predation are causally interrelated. Tadpoles initially suppress CORT and behaviour to avoid capture, but increase CORT with longer exposure, inducing adaptive phenotypic changes.

Individual variation in glucocorticoid stress responses in animals

When stimuli from the environment are perceived to be a threat or potential threat then animals initiate stress responses, with activation of the hypothalamo-pituitary-adrenal axis and secretion of glucocorticoid hormones (cortisol and corticosterone). Whilst standard deviation or standard error values are always reported, it is only when graphs of individual responses are shown that the extensive variation between animals is apparent. Some animals have little or no response to a stressor that evokes a relatively large response in others. Glucocorticoid responses of fish, amphibian, reptiles, birds, and mammals are considered in this review. Comparisons of responses between animals and groups of animals focused on responses to restraint or confinement as relatively standard stressors. Individual graphs could not be found in the literature for glucocorticoid responses to capture or restraint in fish or reptiles, with just one graph in mammals with the first sample was collected when animals were initially restrained. Coefficients of variation (CVs) calculated for parameters of glucocorticoid stress responses showed that the relative magnitudes of variation were similar in different vertebrate groups. The overall mean CV for glucocorticoid concentrations in initial (0 min) samples was 74.5%, and CVs for samples collected over various times up to 4 h were consistently between 50% and 60%. The factors that lead to the observed individual variation and the extent to which this variation is adaptive or non-adaptive are little known in most animals, and future studies of glucocorticoid responses in animals can focus on individual responses and their origins and significance.

Profile of cortisol, glycaemia, and blood parameters of American Bullfrog tadpoles Lithobates catesbeianus exposed to density and hypoxia stressors

The aim of this study was to evaluate alterations to the physiological profile (cortisol, glycaemia, and blood parameters) of Lithobates catesbeianus caused by the stressors density and hypoxia. The organisms were in the prometamorphosis stage and exposed to different tadpole densities: 1 tadpole/L (T1), 5 tadpoles/L (T2), and 10 tadpoles/L (T3) for 12 days. The blood was collected through the rupture of the caudal blood vessel and collected under normoxia (immediate collection) and hypoxia (after 15 minutes of air exposure) conditions. Cortisol levels rose on the fourth and eighth days of treatment and returned to basal levels by the end of the experiment. The stressor mechanisms tested did not affect glycaemia. White blood cells (total number of lymphocytes, neutrophils, and eosinophils) showed a significant difference at the twelfth day of the experiment when compared with the start of the experiment. We concluded that, under controlled conditions, a density of up to 10 tadpoles/L and air exposure for 15 minutes did not cause harmful physiological alterations during the experimental period. The answer to these stressors maybe was in another hormonal level (corticosterone).

Effect of starvation, body size, and temperature on the onset of metamorphosis in Japanese eel (Anguilla japonica)

We assessed the effects of starvation, body size, and water temperature on the onset of metamorphosis in leptocephali of Japanese eel ( Anguilla japonica Temminck and Schlegel, 1846) as determined by the morphological criteria of proportion, preanal length, and body depth to total length. Leptocephali of mean total length 55.6 mm that had been reared in captivity for 241 days from hatching were divided into unfed (n = 28) and fed (n = 30) groups in triplicate and reared for an additional 2 weeks. The mean percentage of larvae starting metamorphosis within 2 weeks was significantly higher in the unfed than in the fed groups (70% vs. 28.6%), suggesting that food deprivation acted as a cue for metamorphosis. The critical size for metamorphosis was a total length of 50–55 mm; smaller larvae did not start metamorphosis even in the absence of food, whereas larvae reaching that critical size were induced to undergo metamorphosis by starvation. The start of metamorphosis under unfed conditions was independent of diel-varying water temperature (day 23 °C; night 21–29 °C), suggesting a high plasticity in response to a wide range of environmental temperatures. These findings suggest methods for the efficient production of glass eels, as well as new insights into the mechanism of eel metamorphosis.

Examination of an amphibian metamorphosis assay under an individual-separated exposure system using Silurana tropicalis tadpoles

We examined the validity of an amphibian (Silurana tropicalis) metamorphosis assay (a 28-day semistatic test) under an individual-separated exposure system, where tadpoles were individually held in small glass beakers. We first conducted a comparative rearing experiment for 28 days between this exposure system and the traditional individual-grouped exposure system, both of which held 30 tadpoles (stages 49 and 50) in dechlorinated tap water (a control solution). The former system served to reduce interindividual variability in regard to three morphological measures (developmental stage, hind limb length, and total body length). Under this system, we tested thyroxine (T4, 1μg/L) and propylthiouracil (PTU, 75mg/L) for 28 days of exposure. The morphological data collected at 7-day intervals indicated that significant metamorphic acceleration and retardation were consistently induced in the tadpoles exposed to T4 and PTU, respectively. In addition, the thyroid glands of the tadpoles exposed to T4 and PTU clearly exhibited atrophy and hypertrophy accompanied with severe follicular cell hyperplasia, respectively. Our results are in agreement with the historical data generated from previous studies employing the traditional exposure system, thus indicating the validity of our alternative testing protocol.