Evidence for fluorescence-supported species recognition in syntopic harvestmen

Biofluorescence is reported as a widespread phenomenon in a variety of animal species, but in most cases its biological relevance is poorly understood. Possible functions of biofluorescence as visual signals for species recognition are disputed or have not been convincingly demonstrated. Here, we report on the discovery of a strongly fluorescent structure in largely crepuscular and nocturnal harvestmen. Five syntopic species of harvestmen from the family Cosmetidae were observed and collected in a lowland rainforest of Peruvian Amazonia. Their most prominent species-specific character is a whitish structure on the dorsal scutum, called equuleus. Its shape is extremely constant intraspecifically, but strongly differs among these closely related species. The equuleus shows strong fluorescence in the blue frequency spectrum, which is enhanced by a subjacent layer of guanine crystals, acting as a reflector for the incoming light, as well as for the emitted fluorescence. We hypothesize that the equuleus most likely provides a visual signal for intra- and inter-specific species recognition in dim visible light, which is amplified by UV-induced fluorescence, excited by moonlight.