Freshwater crabs from Crete: a unique key for the reconstruction of the biogeographical history of Potamon Savigny, 1816 (Brachyura, Potamidae) in the Aegean region

Abstract The Aegean region is biogeographically complex for freshwater crabs of the genus Potamon Savigny, 1816. Especially Potamon kretaion Giavarini, 1943 has a remarkably different distribution pattern compared to its congeners: With its exclusive occurrence on the island of Crete, it is the only Potamon species that is endemic to an Aegean island, and its range is geographically disjunct from its sister group, including Potamon ibericum (Bieberstein, 1809) and Potamon bilobatum Brandis, Storch and Türkay, 2000, with a Ponto-Caspian distribution. In addition, the morphology of the first gonopods differs substantially from that of closely related species. Therefore, the aim of this study was to (1) develop a new plausible hypothesis for the dispersal history of the genus Potamon into the Aegean area based on a re-analysis of gonopodal characters and existing phylogenetic data, and under consideration of recent palaeogeographical hypotheses, and (2) test whether the first gonopods of P. kretaion — being substantially different from its closest relatives — represent the ancestral or derived character state within Aegean Potamon . The results revealed a close link between the distribution of freshwater crabs and the biogeographical history of the Aegean region and that P. kretaion shows plesiomorphic characteristics of the first gonopods. These findings indicate that the original population of Pontipotamon (including P. kretaion ) was isolated on Crete about 9 Mya, spread northwards during the Messinian salinity crisis and was separated in the original populations of P. kretaion and P. ibericum by the flooding of the Mediterranean Sea 5.33 Mya.