Is the Cantabrian region of northern Spain a biodiversity hotspot for obligate groundwater fauna? The case of oligochaetes (Annelida, Clitellata)

This study aimed to (i) evaluate species richness and endemicity for the stygobiont oligochaete taxa (Annelida, Clitellata) in the Cantabrian region of northern Spain; and (ii) identify the environmental factors that determine the stygobiont assemblages at the regional scale. We recorded and examined more than 7,000 specimens of groundwater oligochaetes from 58 sampling sites. Of 62 identified species, 15 were classified as stygobionts, raising the known stygobiont oligochaete richness in the Cantabrian region to 21 species. As a result, the region is the second richest area for stygobiont oligochaete fauna in Europe after Slovenia with 28 species. Stygobiont oligochaetes exhibited a high degree of endemicity: 9 stygobionts were endemic to their type localities and 8 were restricted to geographic areas of less than 240 km in length, which underlines the narrow geographic ranges for subterranean fauna. Differences among the karsts were observed but the measured environmental factors did not explain the structure of the stygobiont assemblages in the Cantabrian region. Electrical conductivity, latitude and longitude influenced stygobiont species distributions but correlation values were low. Other ecological and historical factors should be evaluated in future; in particular, marine transgressions-regressions may have played a major role as evidenced by the large number of stygobionts of marine origin (8 species) found in both coastal and inland karsts.