The first European cave fish

Subterranean biodiversity in Europe is spectacularly rich, with the Western Balkans being home to about 400 cave species, representing the highest number of species per area worldwide [1]. Nonetheless, cave fishes, which are the most commonly found vertebrates in underground habitats [2], have not been described from Europe so far [3]. Here, we report the first European record of a cave fish population, a loach of the genus Barbatula (Figure 1), found in the Danube-Aach system, an underground karst water system in Southern Germany [4]. The fish exhibit traits typically observed in organisms adapted to subterranean life including reduced eyes and pale body coloration [5]. The newly discovered population also represents globally the northernmost cave fish found so far. The geological history of the region implies that the Danube-Aach system was colonized post-glacially. A recent origin of the cave fish is supported by genetic analyses, because the subterranean population shares COI gene haplotypes with adjacent surface stone loach (Barbatula barbatula) populations (Figure 1D). Nonetheless, population genetic analyses based on microsatellites indicated that cave fish are genetically isolated from populations in surface habitats (Figure 1E) and exhibit reduced genetic variability. Hence, the newly discovered European cave loaches do not represent individuals displaced from surface populations, but they follow a unique evolutionary trajectory towards cave life.