Testing for inter-drainage connections on the basis of the distribution pattern of endemic freshwater fishes

We classified the secondary rivers of the Duero and Tagus drainages on the basis of the presence or absence of native freshwater fishes in order to an alyse hypothetical interconnections between them and to survey intra-draina ge biogeographical complexity. Except for a few rivers with low diversity, the tributaries were classified into two main biogeographical areas (Duero and Tagus basins), which were subdivided into different biotic regions deli mited by significant boundaries. In the Duero basin, in particular, three r egions were identified. One corresponds to a large and homogeneous group of tributaries, containing species not shared with the 'Tagus basin. Two othe r small regions, at the south-western border of the Duero basin, were chara cterized by the presence of a local endemic species and by a fauna related to that of the Tagus basin. Similarly, three biotic regions were found in t he Tagus basin. The middle-upper basin plus the Alagon River was defined by the presence of 5. trutta and Ch. arcasii. The other two regions, located in the lower basin, were both characterized by the presence of Ch. lemmingi i; but were differentiated on the basis of the distribution patterns of the ir lentic species. The internal biogeographical complexity detected in the Duero and Tagus drainages showed both geographical and historical component s. Our data support the existence of interconnections between the basins du ring the Cenozoic period, and we postulate the existence of an ancient endo rheic basin connecting the south-western part of the Duero and the north-we st of the: Tagus basin, as a historical explanation for translocation of fa una.