Groundwater invertebrates and droughts: resistance in stygobiont isopods and planarians

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