Disjunct distribution of the freshwater crab Potamon fluviatile – natural expansion or human introduction?

Human mediated biological invasions are seen as an increasing danger for biodiversity. On the other hand, range expansions are natural processes. It is often practically not possible to tell these processes apart, like in the case of the freshwater crab Potamon fluviatile. This species has a disjunct distribution on the Balkan Peninsula, Italy, Sicily and Malta. An innovative analysis framework involving phylogeographic model selection and temporal coalescentanalyses on a mitochondrial dataset (COI and NADH1) could clarify that the origin of the specie was on the Balkans and the colonisation of Italy proceeded from the northern Balkans via southern Italy in the Otranto Strait region. The population expansion associated with this invasion was estimated to have taken place 15,000 years before present (95% c.f. 10,000–24,000 years BP). An anthropogenic introduction is therefore implausible and a natural expansion likely. We argue that the species should thus be included in the national conservation management in Italy.