Search for: All records

The shape and relative size of an ocular lens affect the focal length of the eye, with consequences for visual acuity and sensitivity. Lenses are typically spherical in aquatic animals with camera-type eyes and axially flattened in terrestrial species to facilitate vision in optical media with different refractive indices. Frogs and toads (Amphibia: Anura) are ecologically diverse, with many species shifting from aquatic to terrestrial ecologies during metamorphosis. We quantified lens shape and relative size using 179 micro X-ray computed tomography scans of 126 biphasic anuran species and tested for correlations with life stage, environmental transitions, adult habits and adult activity patterns. Across broad phylogenetic diversity, tadpole lenses are more spherical than those of adults. Biphasic species with aquatic larvae and terrestrial adults typically undergo ontogenetic changes in lens shape, whereas species that remain aquatic as adults tend to retain more spherical lenses after metamorphosis. Further, adult lens shape is influenced by adult habit; notably, fossorial adults tend to retain spherical lenses following metamorphosis. Finally, lens size relative to eye size is smaller in aquatic and semiaquatic species than other adult ecologies. Our study demonstrates how ecology shapes visual systems, and the power of non-invasive imaging of museum specimens for studying sensory evolution. 

Abstract Pupil constriction has important functional consequences for animal vision, yet the evolutionary mechanisms underlying diverse pupil sizes and shapes are poorly understood. We aimed to quantify the diversity and evolution of pupil shapes among amphibians and to test for potential correlations to ecology based on functional hypotheses. Using photographs, we surveyed pupil shape across adults of 1294 amphibian species, 74 families and three orders, and additionally for larval stages for all families of frogs and salamanders with a biphasic ontogeny. For amphibians with a biphasic life history, pupil shape changed in many species that occupy distinct habitats before and after metamorphosis. In addition, non-elongated (circular or diamond) constricted pupils were associated with species inhabiting aquatic or underground environments, and elongated pupils (with vertical or horizontal long axes) were more common in species with larger absolute eye sizes. We propose that amphibians provide a valuable group within which to explore the anatomical, physiological, optical and ecological mechanisms underlying the evolution of pupil shape. 

Differences in morphology, ecology, and behavior through ontogeny can result in opposing selective pressures at different life stages. Most animals, however, transition through two or more distinct phenotypic phases, which is hypothesized to allow each life stage to adapt more freely to its ecological niche. How this applies to sensory systems, and in particular how sensory systems adapt across life stages at the molecular level, is not well understood. Here, we used whole-eye transcriptomes to investigate differences in gene expression between tadpole and juvenile southern leopard frogs (Lithobates sphenocephalus), which rely on vision in aquatic and terrestrial light environments, respectively. Because visual physiology changes with light levels, we also tested the effect of light and dark exposure.

We found 42% of genes were differentially expressed in the eyes of tadpoles versus juveniles and 5% for light/dark exposure. Analyses targeting a curated subset of visual genes revealed significant differential expression of genes that control aspects of visual function and development, including spectral sensitivity and lens composition. Finally, microspectrophotometry of photoreceptors confirmed shifts in spectral sensitivity predicted by the expression results, consistent with adaptation to distinct light environments.

Overall, we identified extensive expression-level differences in the eyes of tadpoles and juveniles related to observed morphological and physiological changes through metamorphosis and corresponding adaptive shifts to improve vision in the distinct aquatic and terrestrial light environments these frogs inhabit during their life cycle. More broadly, these results suggest that decoupling of gene expression can mediate the opposing selection pressures experienced by organisms with complex life cycles that inhabit different environmental conditions throughout ontogeny.

 

No abstract available. 

Cryptogenic species are those whose native and introduced ranges are unknown. The extent and long history of human migration rendered numerous species cryptogenic. Incomplete knowledge regarding the origin and native habitat of a species poses problems for conservation management and may confound ecological and evolutionary studies. The Lesser Antilles pose a particular challenge with regard to cryptogenic species because these islands have been anthropogenically connected since before recorded history. Here, we use population genetic and phylogeographic tools in an attempt to determine the origin ofEleutherodactylus johnstonei, a frog species with a potentially widespread introduced range and whose native range within the Lesser Antilles is unknown. Based on elevated estimates of genetic diversity and within-island geographic structure not present elsewhere in the range, we identify Montserrat as the native island ofE. johnstonei. We also document two major clades withinE. johnstonei, only one of which is the primary source of introduced populations throughout the Americas. Our results demonstrate the utility of genetic tools for resolving cryptogenic species problems and highlightE. johnstoneias a potential system for understanding differences in invasive potential among sister lineages.

 

Surprisingly, little is known about body‐size evolution within the most diverse amphibian order, anurans (frogs and toads), despite known effects of body size on the physiological, ecological and life‐history traits of animals more generally. Here, we examined anuran body‐size evolution among 2,434 species with over 200 million years of shared evolutionary history. We found clade‐specific evolutionary shifts to new body‐size optima along with numerous independent transitions to gigantic and miniature body sizes, despite the upper limits of anuran body size remaining quite consistent throughout the fossil record. We found a weak, positive correlation between a species’ body size and maximum latitude and elevation, including a dearth of small species at higher elevations and broader latitudinal and elevational ranges in larger anurans. Although we found modest differences in mean anuran body size among microhabitats, there was extensive overlap in the range of body sizes across microhabitats. Finally, we found that larger anurans are more likely to consume vertebrate prey than smaller anurans are and that species with a free‐swimming larval phase during development are larger on average than those in which development into a froglet occurs within the egg. Overall, anuran body size does not conform to geographic and ecological patterns observed in other tetrapods but is perhaps more notable for variation in body size within geographic regions, ecologies and life histories. Here, we document this variation and propose target clades for detailed studies aimed at disentangling how and why variation in body size was generated and is maintained in anurans.

 

Biodiversity loss is one major outcome of human-mediated ecosystem disturbance. One way that humans have triggered wildlife declines is by transporting disease-causing agents to remote areas of the world. Amphibians have been hit particularly hard by disease due in part to a globally distributed pathogenic chytrid fungus ( Batrachochytrium dendrobatidis [ Bd ]). Prior research has revealed important insights into the biology and distribution of Bd ; however, there are still many outstanding questions in this system. Although we know that there are multiple divergent lineages of Bd that differ in pathogenicity, we know little about how these lineages are distributed around the world and where lineages may be coming into contact. Here, we implement a custom genotyping method for a global set of Bd samples. This method is optimized to amplify and sequence degraded DNA from noninvasive skin swab samples. We describe a divergent lineage of Bd , which we call Bd ASIA3, that appears to be widespread in Southeast Asia. This lineage co-occurs with the global panzootic lineage ( Bd GPL) in multiple localities. Additionally, we shed light on the global distribution of Bd GPL and highlight the expanded range of another lineage, Bd CAPE. Finally, we argue that more monitoring needs to take place where Bd lineages are coming into contact and where we know little about Bd lineage diversity. Monitoring need not use expensive or difficult field techniques but can use archived swab samples to further explore the history—and predict the future impacts—of this devastating pathogen. 

The spectral characteristics of vertebrate ocular lenses affect the image of the world that is projected onto the retina, and thus help shape diverse visual capabilities. Here, we tested whether amphibian lens transmission is driven by adaptation to diurnal activity (bright light) and/or scansorial habits (complex visual environments).

Spectral transmission through the lenses of 79 species of frogs and six species of salamanders was measured, and data for 29 additional frog species compiled from published literature. Phylogenetic comparative methods were used to test ecological explanations of variation in lens transmission and to test for selection across traits.

Lenses of diurnal (day‐active) and scansorial (climbing) frogs transmitted significantly less shortwave light than those of non‐diurnal or non‐scansorial amphibians, and evolutionary modelling suggested that these differences have resulted from differential selection.

The presence of shortwave‐transparent lenses was common among the sampled amphibians, which implies that many are sensitive to shortwave light to some degree even in the absence of visual pigments maximally sensitive in the UV. This suggests that shortwave light, including UV, could play an important role in amphibian behaviour and ecology.

Shortwave‐absorbing lens pigments likely provide higher visual acuity to diurnally active frogs of multiple ecologies and to nocturnally active scansorial frogs. This new mechanistic understanding of amphibian visual systems suggests that shortwave‐filtering lenses are adaptive not only in daylight conditions but also in those scotopic conditions where high acuity is advantageous.

Read the freePlain Language Summaryfor this article on the Journal blog.