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Amphibians aglow

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Until recently, the scientific world knew of only a handful of biofluorescent amphibians. Today, scientists are wondering whether all amphibians glow.

Herpetologist Jennifer Lamb and ichthyologist Matthew Davis led a group of researchers in a study of 32 species of amphibians, with a particular focus on salamanders. Prior to the study, only one species of salamander had been identified to have biofluorescent properties. Now, the team suspects every amphibian may biofluoresce, possibly at all stages of life, sometimes down to their bones.

“Every amphibian species and life stage we examined, including aquatic larvae, is biofluorescent,” the authors wrote in their paper, published in Nature.

White-light images depicting biofluorescence in three species of salamander (Ambystoma tigrinum, Pseudobranchus striatus, and Icthyosaura alpestris). Salamanders with bold patterns and colors (such as the A. tigrinum) fluoresce brightly, and dorsal surfaces often fluoresce less intensely than ventral ones, depending on patterning (see the I. alpestris). In some salamanders (such as the Ambystoma opacum), bones that are otherwise not visible under white light (dentary and digits) fluoresce distinctly, as does the cloacal region. Courtesy of Jennifer Y. Lamb and Matthew P. Davis.


White-light images depicting biofluorescence in three species of salamander (Ambystoma tigrinum, Pseudobranchus striatus, and Icthyosaura alpestris). Salamanders with bold patterns and colors (such as the A. tigrinum) fluoresce brightly, and dorsal surfaces often fluoresce less intensely than ventral ones, depending on patterning (see the I. alpestris). In some salamanders (such as the Ambystoma opacum), bones that are otherwise not visible under white light (dentary and digits) fluoresce distinctly, as does the cloacal region. Courtesy of Jennifer Y. Lamb and Matthew P. Davis.

The jury is still mostly out on what purpose biofluorescence serves for salamanders and other amphibians.

“Hypothesized functions for biofluorescence include communication, sexual selection, camouflage, and improved visual acuity, to perhaps no function at all in some lineages,” the authors wrote.

Amphibians’ eyes contain specialized rods that provide greater sensitivity to red and green light. As many species are crepuscular or nocturnal, the researchers theorized that the patterns may help amphibians to identify one another during the night, when blue light is the predominant source of illumination.

The researchers found that amphibians fluoresce green to yellow in response to blue (440 to 460 nm) and ultraviolet (360 to 380 nm) excitation lights. Biofluorescent light under blue excitation, the researchers found, is more intense than when excited by ultraviolet light.

To document the reaction, the researchers illuminated the subjects — one species of frog and two species of salamanders — with a blue excitation light at 440 to 460 nm and imaged them through a 500-nm long-pass filter. To gauge the response to ultraviolet light, they used 360- to 380-nm lights and imaged through a 415-nm long-pass filter. The researchers used a spectrometer to measure the spectra of each species’ biofluorescent emissions, collecting 10 independent spectra recordings per specimen. The readings were then averaged to generate spectra profiles for each amphibian.

Biofluorescent emissions were measured with a FLAME spectrometer through a yellow long pass filter (500 nm) from either the dorsal or ventral surfaces, but we did not focus on any specific part of the anatomy.   Courtesy of Jennifer Y. Lamb and Matthew P. Davis.


Biofluorescent emissions were measured with a FLAME spectrometer through a yellow long pass filter (500 nm) from either the dorsal or ventral surfaces, but we did not focus on any specific part of the anatomy. Courtesy of Jennifer Y. Lamb and Matthew P. Davis. Davis.

Biofluorescent patterns differ substantially among amphibians. In some salamanders, according to the researchers, the ventral (front) surface fluoresces more intensely than the dorsal (back) surface. In other amphibians, specific parts of the anatomy or secretions also fluoresce. Bones in the digits and cloacal (pelvic) regions of the marbled salamander, and in the cloacal regions of caecilians (legless amphibians), exhibited prominent fluorescence.

Until further study is done, the exact purposes of biofluorescence in amphibious creatures, and how widespread it is, will remain a mystery. The research, however, provides a roadmap for future efforts. Sample sizes ranged from one to five specimens per species, so further study will be needed to determine differences between sexes and individuals. The researchers hypothesized that all salamanders likely possess biofluorescent properties, and they suggest the possibility that the phenomenon is prevalent throughout amphibians in general.

Photonics Spectra
Apr 2020
Lighter Side

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