Glowing signals in the rainforest: Harvestmen apparently use fluorescent patterns for species recognition

Biofluorescence—light emitted by body structures after excitation by short-wave light—is widespread in the animal kingdom. But its biological significance is often unclear. In many animal groups, fluorescence has so far been considered a trait without a clear function. Robust evidence for a role in communication is rare.

In the lowland rainforest of the Peruvian Amazon region, zoologists of the Bavarian State Collections of Natural History (SNSB) discovered five harvestman species from the same family during nighttime surveys using ultraviolet flashlights. The species use the same habitat at the same time. Although the animals are very similar in size and coloration, they differ clearly in the shape and size of a pale pattern on their backs. This feature is consistent within each species and differs markedly between species. Under ultraviolet light, this dorsal pattern glows an intense blue-green. This probably allows the animals to see each other well even in very low light conditions, such as moonlight and twilight, which is a decisive advantage for crepuscular and nocturnal species.

Microscopic analyses revealed that the fluorescence originates from the outer body covering. Directly beneathlies an additional multi-layered, mirror-like layer of  crystals. This acts as a natural reflector: incoming light is bounced back, as well as the emitted fluorescent light. This amplifies the signal and makes it particularly intense.

Previous studies on other species of harvestmen suggest that these animals can perceive both short-wave components of light and their own glow in the blue-green range. The new study shows that their eyes are simple, but sensitive enough to detect high-contrast patterns at close range. Since harvestmen move three-dimensionally in foliage, on dead wood, and on the ground, they can easily see the dorsal markings of their conspecifics.

For closely related species that share the same habitat and the same active hours, reliable recognition is particularly important to be able to distinguish between conspecifics and different species —for example during mate choice or to avoid mismatched pairings. „Our study provides strong evidence that fluorescence serves the specific purpose of highlighting species-specific traits. We suspect that the complex combination of a fluorescent surface and a reflective underlayer has a very specific biological function and is not just a random side effect”, says first author Stefan Friedrich from the Bavarian State Collections of Natural History (SNSB).

In the harvestmen studied, several key criteria point to a function of fluorescence for communication: the back patterns are clearly visible, clearly defined, and species-specific, and their visibility matches the lighting conditions in which the animals are active. In addition, the eyes of these arachnids are able to perceive the patterns. However, the researchers also emphasize that behavioral experiments would be necessary in a next step to definitively confirm the role of the fluorescent patterns.

The work involved researchers from the SNSB, LMU Munich and Munich University of Applied Sciences. Fieldwork took place at the Panguana Biological Research Station in Peru.