Naturally occurring fluorescence protects the eutardigrade sp. from ultraviolet radiation.

Naturally occurring fluorescence has been observed in multiple species ranging from bacteria to birds. In macroscopic animals such as birds, fluorescence provides a visual communication signal. However, the functional significance of this phenomenon is unknown in most cases. Though photoprotection is attributed to fluorescence under ultraviolet (UV) light in some organisms, it lacks direct experimental evidence. Here, we demonstrate naturally occurring fluorescence under UV light in a eutardigrade belonging to the genus . Using a natural variant that lacks fluorescence, we show that the fluorescence confers tolerance to lethal UV radiation. Remarkably, the fluorescent extract from sp. could protect the UV-sensitive tardigrade and nematode from germicidal UV radiation. We propose that sp. possess a protective fluorescent shield that absorbs harmful UV radiation and emits harmless blue light.