Fluorescing frogs and neon newts

Amphibians, one of the world's most imperiled groups of vertebrates, can biofluoresce. Frogs, salamanders, and caecilians can absorb light from their environment and then emit it at different wavelengths, resulting in brilliantly bright greens and other colors.

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In the last decade scientists have made fascinating discoveries regarding interactions among organisms and light. Biofluorescence, a process in which a living organism absorbs light at one wavelength (e.g., blue) and then re-emits it at another wavelength (e.g., green), has been well documented in various marine invertebrates (e.g., corals and jellyfishes) and recently in marine vertebrate lineages such as sea turtles, ray-finned fishes (eels, wrasses, gobies), and cartilaginous fishes (sharks, skates, and rays). While some terrestrial animal lineages such as arthropods are well known to fluoresce, comparatively we still have much to discover regarding biofluorescence among terrestrial vertebrate lineages. 

In this study ("Salamanders and other amphibians are aglow with biofluoresence") we (Jennifer Lamb [@jenylamb] & Matthew Davis [@Bathypterois]) were interested in exploring the potential presence of biofluorescence among amphibians, a lineage that lives in a variety of habitats across the planet that has seen significant biodiversity declines in recent years. We began by focusing on a broad taxonomic survey of biofluorescence in salamanders, but our discovery of widespread biofluorescence in this lineage of amphibians led us to further examine frogs and caecilians for biofluorescence.

One of the most exciting aspects of this work to us was that with each species we examined we were always discovering something new that could bring novel insights into the life history and biology of amphibians worldwide. The Eastern tiger salamander (Ambystoma tigrinum) was the first salamander species that we surveyed for biofluorescence, and when we saw the bright, intense green light emitted from their yellow spots we each let out a collective “Woah!” At that point, we were captivated and we set out to investigate how prevalent biofluorescence was across amphibians and the extent of variation in their biofluorescent patterning. 

The primary wavelengths fluoresced by most species was in the range of green light.
An Eastern tiger salamander (Ambystoma tigrinum) fluorescing green was one of our first introductions to biofluoresence in amphibians.

One finding that really surprised us was that each species of amphibian we examined was fluorescent to some degree, including species that lacked bright or bold colors under white light. For example, the Rio Cauca caecilian (Typhlonectes natans) is a limbless, aquatic amphibian that is a muted gray under white light. When exposed to blue excitation lights, the reproductive cloacal disc of the male shone back at us like a bright green spotlight.

In some species, the cloaca fluoresced brightly.
The cloacal disc of a male Rio Cauca caecilian (Typhlonectes natans) fluoresced green while the rest of the body was not highly fluorescent.

The collaborative nature of the work and the thrill of discovery allowed for opportunities to arise due to the shared enthusiasm of colleagues. In order to survey some of the more difficult to encounter species of salamanders and other amphibians, scientists from the Field Museum (Caleb McMahan, Ichthyology Collection Manager) and the Shedd Aquarium (George Parsons, Senior Curator of Fishes) in Chicago (IL) allowed us to spend a night after hours at the Shedd Aquarium. We were able to peer into aquaria with our Nightsea excitation flashlights to examine and describe patterns of biofluorescence using a noninvasive method that involved blue flashlights (440 – 460 nm) and barrier filters for our eyes and cameras (which allow fluorescent light to pass through).

Some of the species we surveyed were residents at the Shedd Aquarium.
Some of the tropical frogs we surveyed, like this Amazon milk frog (Trachycephalus resinifictrix), fluoresced under blue light.

Overall, we demonstrate that biofluorescence is widespread across amphibian biodiversity in all three main living lineages of amphibians (caecilians, frogs, and salamanders) and that it occur across the life history of several species from tadpoles to adults. 

Some amphibians biofluoresce across their life history.
True frog (Family Ranidae) and tree frog (Family Hylidae) tadpoles also fluoresce.

Multiple mechanisms (e.g., pigments, components of mucous-like secretions, minerals in tissues or fluids) may contribute to amphibian fluorescence which may be used by amphibians to blend in with their environment, communicate with each other, or even aid in highlighting aposematic warning patterns to potential predators. We hope this work will serve as a roadmap for future explorations of the impact fluorescence may have on the biology and life history of amphibians and that it will help to identify the mechanisms and potential functions fluorescence may serve in amphibian ecology and evolution. This work also highlights the potential use of fluorescence in surveying amphibian biodiversity in the field, which would aid scientists in documenting and identifying amphibian biodiversity worldwide during critical biodiversity surveys.

We may be able to use biofluoresence to survey for amphibians in dense vegetation.
This fringe leaf frog (Cruziohyla craspedopus) fluoresces green against a red fluorescing plant. This is one of the species we surveyed at the Shedd Aquarium.

Photos copyright Jennifer Y. Lamb and Matthew P. Davis. Contact either author with questions or further inquiries about this work (jylamb@stcloudstate.edu; mpdavis@stcloudstate.edu).

Jennifer Lamb

Assistant Professor, St. Cloud State University