LAUSR

We address the diversity of two species groups of the cichlid genus Gymnogeophagus in the Middle Paraná basin using molecular phylogeography and traditional morphological characters and conclude that they are composed of a higher diversity of endemic species than previously thought. The nominal Gymnogeophagus setequedas is found non-monophyletic with each phylogeographic lineage endemic to a single tributary of the Middle Paraná (Rio Acaray, Rio Monday, the Itaipu area). Prime candidates for the origin of the species in the G. setequedas group are waterfalls that separate most of the tributaries from the Middle Paraná River. The postulated waterfall- and rapids-driven fragmentation of Gymnogeophagus in the studied area falls into a narrow time window coinciding with the Pleistocene epoch. We further demonstrate that the endemism in Gymnogeophagus shows a high degree of biogeographical congruence with other sympatric cichlid genera. The Middle Paraná basin has up to 27 globaly endemic species of cichlid fishes in three genera and 21 of these species are endemics of single tributaries of the Middle Paraná. As a final consideration we address threats to the long term survival of the still poorly known but clearly highly endemic and endangered fish fauna of the Middle Paraná basin.