Salinity Shapes Groundwater Crustacean Assemblages on a Small Mediterranean Island

Groundwater ecosystems host unique and functionally important fauna, yet they are increasingly threatened by salinization driven by natural processes and human activities. Despite growing attention to groundwater quality, little is known about how salinity shapes subterranean crustacean assemblages on small carbonate islands. This study examines how groundwater salinity influences the distribution of stygobitic (i.e., highly specialized organisms that complete their whole life cycle in groundwater) and non-stygobitic crustaceans in groundwater habitats of the Pianosa Island (Tuscan Archipelago, Italy). We sampled ten bores, measured key physicochemical parameters, and identified crustaceans collected through on-site filtration. Principal Component Analysis and Canonical Analysis of Principal coordinates highlighted a clear spatial pattern of crustacean assemblages along a salinity gradient: stygobitic species occurred only in low- and intermediate-salinity bores while non-stygobitic taxa dominated sites with elevated electrical conductivity and high Na+ and Cl− concentrations. The most saline bore contained no stygobitic species. Insights from this study point to salinity as a key driver of groundwater biodiversity on small Mediterranean islands and highlight the vulnerability of stygobitic fauna to ongoing salinization.