Geometric morphometrics throws light on evolution of the subterranean catfish Rhamdiopsis krugi (Teleostei: Siluriformes: Heptapteridae) in eastern Brazil

Rhamdiopsis krugi is a highly specialized troglobitic (exclusively subterranean) catfish from phreatic water bodies\ud of caves located within two separated metasedimentary basins in the region of Chapada Diamantina, Bahia state,\ud Brazil. In order to test the hypothesis of isolation with differentiation of the groups from the Una-Utinga and Irecê\ud metasedimentary basins, we compared five populations among themselves and with an epigean species of\ud Rhamdiopsis. This was accomplished using geometric morphometrics, a powerful tool for detecting differences in\ud body shape at population and species levels. All studied samples differed significantly from each other, the epigean\ud sample being the most distinct and the Una Basin populations clustering together. Geological and hydrological\ud barriers explain the differences among the subterranean populations. We discuss our results together with the\ud autapomorphies found in R. krugi, which validate its monophyly. These results imply an old age for the R. krugi\ud clade, more than 10 Myr; alternative hypotheses are also presented. We propose a two-step vertical colonization\ud model of the subterranean habitat through the hyporheic zone by an epigean ancestral, with a progressive\ud acquisition of the autapomorphies characterizing R. krugi. For conservation purposes, the two differentiated sets\ud of populations should be considered and referred to as R. krugi ‘Una morphotype’ and R. krugi ‘Irecê\ud morphotype’.We are grateful to those who have helped us during field and laboratory work: S. V. Nascimento and R. J. Silva, for support on Itaetê and Palmeiras Caves; D. R. Pedroso, F. D. Passos, J. E. Gallão, R. Motta, R. Bessi, R. Borowsky and T. L. C. Scatolini for help in the fieldwork; P. P. Rizzato for the photographs of epigean Rhamdiopsis, clear and staining preparation of R. krugi, and the drawing of Figure 3; A. Gambarini for photographies of Figure 7; D. Monteiro von Schimonsky for map confection (Fig. 1); F. Machado for running analyses on R software and comments on the manuscript; the geologists F. Laureano, M. Berbert-Born and R. F. Pereira for valuable geological discussions and contributions; M. de Carvalho, for critical reading of the manuscript, and F. A. Bockmann for critical reading of the manuscript and sharing unpublished data. Mary Andriani, native speaker, revised the English style. This work constitutes part of B. Rantin's graduation project, and also part of a research project of M. E. Bichuette. Both were funded by the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP, 07/56337-2 and 03/00794-5, respectively). E. Trajano and M. E. Bichuette are partially supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico – CNPq (fellowships 302956/2010-7 and 303715/2011-1, respectively). Permission for collecting was granted by the Instituto Chico Mendes de Biodiversidade (ICMBIO, 20165-1). Two anonymous reviewers provided valuable suggestions for improvement of the work