Areas of Endemism Going Underground: Obligate Groundwater-Dwelling Copepods as a Model Group for Assessing Spatial Features of Subterranean Endemics

ABSTRACTAim: Groundwater ecosystems host a highly diverse and specialized biodiversity; however, current knowledge remains limited,partly due to the low accessibility of subterranean environments. We aimed to delineate Areas of Endemism (AoEs) for Europeangroundwater copepods, concurrently developing a reproducible, conservation-oriented workflow to individuate areas of highendemicity under sparse data availability.Location: Europe.Taxon: Crustacea: Copepoda.Methods: We took advantage of the EGCop dataset, retaining 5709 occurrences across 467 obligate groundwater-dwelling spe-cies after data filtering for taxonomic uncertainty. AoEs were delineated using a grid-free, density-based clustering approach(CLUENDA, DBSCAN-based) across 60 parameter combinations. A consensus procedure was applied to identify robust AoEs,namely those emerging from ≥ 95% of parameter combinations. We also developed a species-level Endemicity Score (ES) as afunction of spatial dispersion of occurrence records. Based on ES, we derived three AoE-level metrics, respectively representingraw, richness-weighted and density-weighted endemicity.Results: We identified 17 robust AoEs across Europe (i.e., areas consistently supported across different parameter combina-tions), largely associated with karst regions and historically stable hydrogeological systems. These AoEs varied widely in extent,species richness and endemicity. A total of 91 point endemics (i.e., species known only from a single locality) were recordedwithin the identified AoEs. Endemicity patterns differed among metrics, with some areas maximizing total endemicity andothers showing higher richness-weighted or density-weighted scores. Nonetheless, narrow- and point-endemic taxa consistentlyshaped the observed spatial patterns of endemicity.Main Conclusions: European groundwater ecosystems are structured into distinct centres of endemism driven by hydroge-ological compartmentalization, long-term isolation and limited dispersal. The high frequency of point endemics implies thatlocal habitat loss may cause irreversible loss of unique lineages. By integrating grid-free clustering, endemicity weighting andsensitivity analysis, our framework provides a reproducible and conservation-relevant tool for identifying priority areas underuncertainty, with potential applications across other sparsely sampled subterranean taxa.