A Light in the Dark: The Discovery of Another Fluorescent Frog in the Costa Rican Rainforests

Abstract

Fluorescence, or the re-emittance of longer wavelength light under shorter wavelength light, is a recurrent phenomenon in many animal species. Mounting evidence suggests that fluorescence is likely to be more ubiquitous than once thought.1Prötzel D. Heß M. Scherz M.D. Schwager M. Padje AV. Glaw F. Widespread bone-based fluorescence in chameleons.Scientific Reports. 2018; 8: 698Crossref PubMed Scopus (32) Google Scholar Fluorescence is thought to play a role in recognition of conspecifics and interspecific interaction.1Prötzel D. Heß M. Scherz M.D. Schwager M. Padje AV. Glaw F. Widespread bone-based fluorescence in chameleons.Scientific Reports. 2018; 8: 698Crossref PubMed Scopus (32) Google Scholar, 2Sparks J.S. Schelly R.C. Smith W.L. Davis M.P. Tchernov D. Pieribone V.A. et al.The covert world of fish biofluorescence: a phylogenetically widespread and phenotypically variable phenomenon.PLoS One. 2014; 9: e83259Crossref PubMed Scopus (105) Google Scholar, 3Lim M.L. Land M.F. Li D. Sex-specific UV and fluorescence signals in jumping spiders.Science. 2007; 315: 481Crossref PubMed Scopus (113) Google Scholar Recently it has been shown that the bony tubercules of quite a few chameleon species fluoresce under ultraviolet (UV) A light, which could be related to signaling cues between the sexes.1Prötzel D. Heß M. Scherz M.D. Schwager M. Padje AV. Glaw F. Widespread bone-based fluorescence in chameleons.Scientific Reports. 2018; 8: 698Crossref PubMed Scopus (32) Google Scholar Other species that fluoresce with excitation of UV or near UV light include many fish taxa,2Sparks J.S. Schelly R.C. Smith W.L. Davis M.P. Tchernov D. Pieribone V.A. et al.The covert world of fish biofluorescence: a phylogenetically widespread and phenotypically variable phenomenon.PLoS One. 2014; 9: e83259Crossref PubMed Scopus (105) Google Scholar jumping spiders,3Lim M.L. Land M.F. Li D. Sex-specific UV and fluorescence signals in jumping spiders.Science. 2007; 315: 481Crossref PubMed Scopus (113) Google Scholar birds,4Arnold K.E. Owens I.P. Marshall N.J. Fluorescent signaling in parrots.Science. 2002; 295: 92Crossref PubMed Scopus (128) Google Scholar mantis shrimp,5Mazel C. Cronin T. Caldwell R. Marshall N. Fluorescent enhancement of signaling in a mantis shrimp.Science. 2004; 303: 51Crossref PubMed Scopus (77) Google Scholar and at least 1 frog species, Hypsiboas punctatus.6Taboada C. Brunetti A.E. Pedron F.N. Carnevale Neto F. Estrin D.A. Bari S.E. et al.Naturally occurring fluorescence in frogs.Proc Natl Acad Sci USA. 2017; 114: 3672-3677Crossref PubMed Scopus (64) Google Scholar Equipped with a high quality UV light (Convoy S2+) and a digital single-lens reflex camera, we observed and photographed several individuals of another frog species that exhibit fluorescence when exposed to light with a peak wavelength of 365 nm. Boana rufitelus is a close relative of H punctatus6Taboada C. Brunetti A.E. Pedron F.N. Carnevale Neto F. Estrin D.A. Bari S.E. et al.Naturally occurring fluorescence in frogs.Proc Natl Acad Sci USA. 2017; 114: 3672-3677Crossref PubMed Scopus (64) Google Scholar and can be found from southern Nicaragua into the northern part of Panama. We found the species on low, broad-leafed vegetation near small bodies of water at the edge of Braulio Carrillo National Park. The close match between sensitivity of 2 retinal rods in low-light conditions in several frog species, especially Hyla cinerea, and the fluorescence emission spectra in H punctatus suggest a role for fluorescence in identifying conspecifics at night. Taboada et al have estimated the contribution of the reflectance spectrum of fluorescence of H punctatus to the total emerging light to range from 18.5±2.6% on a full moon night to 29.6±3.2% during twilight.7Andrews K. Reed S.M. Masta S.E. Spiders fluoresce variably across many taxa.Biol Lett. 2007; 3: 265-267Crossref PubMed Scopus (38) Google Scholar It is likely that the way in which some nocturnal frogs see their environment renders emitted fluorescence more obvious to them than to the human eye. Interestingly, some predators (birds) have vision that extends into the UV spectrum, in which fluorophores of some spiders fluoresce most intensely.7Andrews K. Reed S.M. Masta S.E. Spiders fluoresce variably across many taxa.Biol Lett. 2007; 3: 265-267Crossref PubMed Scopus (38) Google Scholar This could have elaborate consequences concerning the selective pressures on some prey animals such as spiders and frogs. Further research could investigate whether predators of fluorescent frogs are able to exploit this “lighting up in the dark” feature with tailored receptor cells in their retina. Unfortunately, many frog species are under increasing threat of extinction. Frogs have been discussed as a sentinel group for environmental degradation because they often are the first taxonomic group to disappear under increasing human impact. Photograph by Pablo Deschepper, MSc.