Salamanders and other amphibians are aglow with biofluorescence

Abstract

Biofluorescence is the absorption of electromagnetic radiation (light) at one wavelength followed by its reemission at a lower energy and longer wavelength by a living organism. Previous studies have documented the widespread presence of biofluorescence in some animals, including cnidarians, arthropods, and cartilaginous and ray-finned fishes. Many studies on biofluorescence have focused on marine animals (cnidarians, cartilaginous and ray-finned fishes) but we know comparatively little about the presence of biofluorescence in tetrapods. We show for the first time that biofluorescence is widespread across Amphibia, with a focus on salamanders (Caudata), which are a diverse group with a primarily Holarctic distribution. We find that biofluorescence is not restricted to any particular family of salamanders, there is striking variation in their fluorescent patterning, and the primary wavelengths emitted in response to blue excitation light are within the spectrum of green light. Widespread biofluorescence across the amphibian radiation is a previously undocumented phenomenon that could have significant ramifications for the ecology and evolution of these diverse and declining vertebrates. Our results provide a roadmap for future studies on the characterization of molecular mechanisms of biofluorescence in amphibians, as well as directions for investigations into the potential impact of biofluorescence on the visual ecology and behavior of biofluorescent amphibians.

Figure 1

Biofluorescent patterns are variable across salamander diversity and anatomy. We observed biofluorescence across the salamander radiation. Salamanders with bold patterns and colors fluoresce brightly (e.g., Ambystoma tigrinum), and dorsal surfaces often fluoresce less intensely than ventral surfaces (e.g., Icthyosaura alpestris) depending on patterning. In some salamanders (e.g., Ambystoma opacum) bones (e.g., dentary, digits) that are otherwise not visible under white light fluoresce distinctly, as does the cloacal region. Included are white light images of three species (A. tigrinum, Pseudobranchus striatus, I. alpestris) above images depicting biofluorescence. Biofluorescence was imaged by exposing individuals to blue light (440–460 nm) and viewing them through a yellow long pass filter (500 nm).

Figure 2

Amphibian biofluorescent emission spectra. The peak emission wavelengths of the biofluorescent light emitted by salamanders, frogs (Ceratophrys cranwelli), and caecelians (Typhlonectes natans) are green to greenish-yellow. Biofluorescent emissions were measured with a FLAME spectrometer through a yellow long pass filter (500 nm) from either the dorsal (Ceratophrys, Ambystoma, Dicamptodon) or ventral surfaces (Typhlonectes, Amphiuma, Cynops, Icthyosaura), but we did not focus on any specific part of the anatomy (see Methods for further detail). Relative intensities of these biofluorescent emissions varied substantially across taxa. Pictured are white light images of T. natans and C. cranwelli above images of these species biofluorescing. Biofluorescence was imaged by exposing individuals to blue light (440–460 nm) and viewing them through a yellow long pass filter (500 nm).

Figure 3

Distribution of fluorescence across amphibians. The evolutionary relationships among Amphibians with an emphasis on salamander (Caudata) families are presented based on the hypothesis of relationships from Pyron and Wiens inferred from gene-fragment data. Green branches and terminals indicate lineages where taxonomic representatives have been confirmed to have biofluorescence in this study. White terminals and branches indicate that biofluorescence is currently unknown in those taxa.