G eometric morphometrics throws light on evolution of the subterranean catfish R hamdiopsis krugi ( T eleostei: S iluriformes: H eptapteridae) in eastern B razil

Rhamdiopsis krugi is a highly specialized troglobitic (exclusively subterranean) catfish from phreatic water bodies of caves located within two separated metasedimentary basins in the region of C hapada D iamantina, B ahia state, B razil. In order to test the hypothesis of isolation with differentiation of the groups from the U na‐ U tinga and Irecê metasedimentary basins, we compared five populations among themselves and with an epigean species of Rhamdiopsis . This was accomplished using geometric morphometrics, a powerful tool for detecting differences in body shape at population and species levels. All studied samples differed significantly from each other, the epigean sample being the most distinct and the U na B asin populations clustering together. Geological and hydrological barriers explain the differences among the subterranean populations. We discuss our results together with the autapomorphies found in R. krugi , which validate its monophyly. These results imply an old age for the R. krugi clade, more than 10 Myr; alternative hypotheses are also presented. We propose a two‐step vertical colonization model of the subterranean habitat through the hyporheic zone by an epigean ancestral, with a progressive acquisition of the autapomorphies characterizing R. krugi . For conservation purposes, the two differentiated sets of populations should be considered and referred to as R. krugi ‘Una morphotype’ and R. krugi ‘Irecê morphotype’. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society , 2014, 114 , 136–151.