Resistance of groundwater invertebrates to droughts: Two new cases in planarians and isopods

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 aquatic environments. Planarians and crustaceans were searched visually and by distressing the substrate. When we detected individuals in dry sectors, we photographed them, observed their features, and released them in pools or streams of the same cave. We rinsed dry individuals of M. pavani in small tanks and recorded the time of reactivation. In a dried subterranean stream, we observed a Dendrocoelum planarian encapsulated in a thick mucus layer, which after a total of 5 min started gliding slowly and reached a length of 18 mm. During the drought of January–April 2022, we detected several individuals of M. pavani in dry areas of two caves. In total, 72% of the tested individuals were able to reactivate. This percentage decreased to 42% after 20 days of dryness. Time to reactivate ranged between 0 and 30 s. Living individuals were observed even after 39 days of drought. Our observations provide new insights into the natural history of groundwater-dwelling invertebrate taxa that are often neglected in the studies of freshwater ecology. Resistance of groundwater-dwelling animals to droughts has several implications for global climatic changes and could be species- and site-specific, as previously observed in surface macroinvertebrates.