Taxonomic invisibility and knowledge shortfalls in terrestrial molluscs of the Caatinga Dominion, a seasonally dry tropical region

The terrestrial molluscan fauna of the Caatinga Dominion, a seasonally dry tropical region in north-eastern Brazil, remains one of the least-documented components of its biota, thereby limiting its inclusion in ecological, biogeographical and conservation assessments. Here, we present a diagnosis of Caatinga terrestrial molluscs by addressing three questions: how much diversity is currently known, how strongly available records are spatially biased, and how sensitive species visibility is to minimum-occurrence thresholds commonly applied in spatial analyses. We compiled 944 records representing 155 species, 62 genera and 25 families. Stylommatophora accounted for 141 species (91%), highlighting the great taxonomic concentration of the known fauna. Despite this apparent richness, records were sparse and unevenly distributed. Only 221 of 8,761 grid cells (2.5%) contained records, whereas 8,443 cells (97.4%) lay within the expected occupancy area but lacked any record. Mean sampling completeness per cell, measured as the ratio between observed and expected richness, was 0.003, and observed-expected overlap was low (Jaccard = 0.0255). Species retained under increasing occurrence thresholds declined from 88 at t = 3 to one at t = 38, revealing pronounced taxonomic invisibility. These patterns may directly affect conservation planning and the reliability of spatial analyses that depend on minimum occurrence thresholds.