Fluorescent strips discovered on geckos

The desert gecko Pachydactylus rangei shows a neon green fluorescent side stripe and a ring around the eyes when illuminated by UV light (365 nm). (Image: David Prötzel)

The sides of the body and the eye area shine neon green: researchers have found an astonishingly strong biofluorescence in a desert gecko. They were able to show that the luminous effect can be traced back to special pigment cells in the skin that are stimulated to radiate by light of certain wavelengths. This is a previously unknown mechanism of fluorescence in terrestrial animals. The scientists say that the glow is probably used by nocturnal geckos for intra-species communication in moonlight.

Most people associate animal glow with the glow worm. However, this is what is known as bioluminescence, in which light is actively generated by the insect through biochemical processes. Biofluorescence, on the other hand, is a passive glow that differs significantly from reflection effects: Special biomolecules emit radiation of certain wavelengths when they are excited by light from the blue or ultraviolet spectrum. The fluorescent proteins of some jellyfish and corals, which shimmer green or red when illuminated by blue light, are known above all. These substances have gained great importance in biological and medical research: They are often used to mark certain tissues.

As if highlighted with a highlighter

Biofluorescence has long been known from marine organisms, but this phenomenon has also been increasingly discovered in land vertebrates in recent years. At certain wavelengths, which occur particularly at dusk, some amphibians, for example, emit a glow. In 2018, the researchers led by David Prötzel from the Munich State Zoological Collection also reported biofluorescence in chameleons. However, all these luminous effects previously known from reptiles and amphibians originate either from bony structures or from fluorescent molecules in the lymphatic fluid under the skin. But in the case that has now been discovered, the mechanism is different, the scientists report.

The gecko species Pachydactylus rangei was in their sights. The reptiles, up to 14 centimeters long, live in the deserts of southern Africa, where they look for small prey at night. On the other hand, they spend the hot day buried in the sand. As the researchers report, they found intense biofluorescence in captive Pachydactylus geckos: As if they had been edited with a highlighter, the geckos show strong neon green fluorescent stripes on the sides of the body and around the eyes under UV light. The areas are clearly visible from the gecko perspective and probably serve as a recognition signal. In the animals’ natural habitat, the blue part of the moonlight can produce the luminous effect, the researchers explain.

What’s up with the shimmer?

“It was immediately apparent that a previously unknown mechanism of biofluorescence had to be present in the desert geckos. Because the clearly neon green fluorescent patterns clearly came from the skin, ”says Prötzel. The researchers were able to confirm this through more detailed investigations: It was found that numerous special pigment cells, so-called iridophores, are embedded in the fluorescent areas of the skin, which are missing in the non-fluorescent areas. In addition to these fluorescent cells, the scientists also found iridophores that could not be activated in the fluorescent areas of the skin. As they explain, these may act like mirrors to enhance the lighting effect of the system.

Iridophores are already known as the coloring elements in the skin of geckos and other lizards. As has now been shown, some of these pigment cells can apparently also fluoresce, the scientists sum up. “This effect is much stronger than the bone-based fluorescence that we discovered in chameleons three years ago and is one of the strongest fluorescence phenomena that have been observed in terrestrial vertebrates,” says co-author Frank Glaw from the Munich State Zoological Collection. In further investigations, the scientists now want to get to the bottom of the secrets of the morphology and chemical structures of the system.

As far as the biological function of the optical effect in the desert geckos is concerned, the scientists assume that it is important in the context of intra-species communication. “In the desert gecko, on the other hand, the strength and the arrangement of the fluorescent areas around the eyes and on the sides of the flanks suggest that the fluorescence serves as a signal for conspecifics that can perhaps be perceived well from a distance,” says co-author Mark Scherz from the University of Potsdam.

Zoological State Collection Munich, specialist article: Scientific Reports, doi: 10.1038 / s41598-020-79706-z

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