Limnoterrestrial tardigrades are touted to be the toughest animals on earth. They can survive harsh, stressed environments, and are well-known for their long-term desiccation tolerance ability. Many bryophytes, the main habitats for herbivorous limnoterrestrial tardigrades, can also tolerate long-term desiccation. Environmental stresses like dehydration can lead to the synthesis of phytochemicals such as polyphenols in plants and according to the xenohormesis hypothesis, some heterotrophs can sense these as chemical cues of deteriorating environments, allowing them to prepare themselves for stressed environments. Bryophytes are known to produce flavonoids and the synthesis of these compounds upregulates under stressed conditions and we hypothesized that these may have an impact on their symbiotic tardigrades in line with the xenohormesis hypothesis. To test this hypothesis, we prepared water-soluble extracts from four moss samples known to harbor herbivorous tardigrade populations collected from rocks and tree trunks. High-performance liquid chromatography revealed the presence of selected flavonoids like apigenin and luteolin. Different concentrations of these extracts, as well as the two flavonoids in pure form, were then tested on both herbivorous heterotardigrades and carnivorous eutardigrades obtained from the same mosses. Our results show that herbivorous tardigrades react to these chemicals and enter desiccation-resistant states whilst carnivorous ones remain in their active state. Seeing as herbivorous heterotardigrades typically feed on the chloroplast and cytoplasm of moss tissues and tend to be much more host-species specific than the carnivorous ones, we conclude that these heterotatdigrades have potentially co-evolved a xenohormetic cue-use mechanism with their host mosses.
