Naturally occurring fluorescence protects the eutardigrade Paramacrobiotus 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 Paramacrobiotus. Using a natural variant that lacks fluorescence, we show that the fluorescence confers tolerance to lethal UV radiation. Remarkably, the fluorescent extract from Paramacrobiotus sp. could protect the UV-sensitive tardigrade Hypsibius exemplaris and nematode Caenorhabditis elegans from germicidal UV radiation. We propose that Paramacrobiotus sp. possess a protective fluorescent shield that absorbs harmful UV radiation and emits harmless blue light.