In surface freshwater habitats water level can be strongly variable and benthic invertebrates are usually adapted to cope with hydrological variability. Groundwater habitats are usually more stable, even if in sites at the interface with the vadose zone, in epikarst streams and dripping pools hydroperiod may vary. Adaptations to survive droughts are thus likely to have been developed also by groundwater-dwelling animals. However very few studies have been performed to assess stygobiont resistance to dryness. They involve mainly the amphipods of the genus Niphargus and Stygobromus; one case is reported for a triclad of the genus Atrioplanaria. Here we describe cases of resistance to drying in stygobiont planarians of the genus Dendrocoelum and the in the isopod Monolistra pavani. Since 2016 we performed multiple surveys in 53 caves in Italy and Switzerland, sampling different stable and unstable groundwater environments. Stygobionts were searched visually and by distressing the substrate. When we detected stygobionts in dry sectors of a cave, we observed their features and behaviours, we pictured them, and we tried to rinse them. We rinsed M. pavani individuals in small plastic tanks and recorded the time of reactivation. During the surveys we recorded a Dendrocoelum planarian encapsulated in a thick mucus layer on the substrate of a dry subterranean stream. When placed in water the planarian started gliding slowly and reached a length of 18 mm. During the drought of January-April 2022, we detected individuals of M. pavani in dry areas of two different caves. 72% of the individuals were able to reactivate. Time to reactivate ranged between 0 s to 30 s. We recorded living M. pavani individuals that were able to reactivate themselves even after 39 days of drought. Our observations provide new insights into the natural history of groundwater-dwelling invertebrate taxa which global climatic changes increase the risk of drought occurrence in subterranean environments.

Benthic invertebrate communities are deeply influenced by hydrological variability in surface freshwater habitats. However, the responses of macroinvertebrates composing stygofauna in aquifers and unsaturated groundwater environments are not well understood nor specifically studied. Here, we present two new cases of resistance to drying in groundwater-dwelling animals: planarians of the genus Dendrocoelum and the isopod crustacean Monolistra pavani. Multiple surveys were performed in 53 caves of the Italian Alps and Apennines, sampling different stable and unstable subterranean