Abstract Evidence available for most inland water and terrestrial organisms highlights the significant role played by southern Italy, Sicily and the Maltese islands as refuges during Pleistocene climatic fluctuations. However, to date, the hypothesis that these areas may have acted as Pleistocene refugia for the freshwater crab Potamon fluviatile has not been explicitly tested, and a recent origin of local P. fluviatile populations was proposed on the basis of a small set of analysed molecular data. We have thus expanded the currently available data set on the population genetic structure of P. fluviatile through dedicated samplings in Sicily (Italy, 18 specimens), the Maltese Islands (Malta, 15 specimens) and the island of Corfu (Greece, seven specimens), with the explicit aim of testing the role of southern Italy, Sicily and the Maltese archipelago as refugia where early Pleistocene or pre-Quaternary populations of the freshwater crab could have persisted to date. Based on the analysis of both novel and published cytochrome c oxidase subunit I (COI) sequence data, we gathered evidence supporting a recent origin of the southern Italian, Sicilian and Maltese populations of Potamon fluviatile from the Balkans, thus rejecting the hypothesised Pleistocene persistence in the area of pre-existing local populations of the same species.

Ocypode cursor (Linnaeus, 1758) is the only Ocypode species present in the Mediterranean Sea, and one of the nine marine crustacean species protected in the basin. It is widely distributed in the eastern Mediterranean, but knowledge about its presence in the central Mediterranean is very limited so far. We hereby first document the established presence of O. cursor in the central Mediterranean (Sicily and Malta), backdate the known presence of this taxon in Italy, and offer preliminary observations on the main known Maltese population. In Sicily, O. cursor is distributed along most of the south-western coast of the island of Sicily, whilst at least three beaches in the Maltese Islands are known to support populations of this species. The main Maltese population exhibits numerous similarities (e.g. burrow width, zonation along the beach) to another Mediterranean population studied in northern Cyprus, although occurring at lower densities. We conclude that the species has been probably present within the study area for a long period, but went undetected in view of the low population densities at which it previously occurred, the lack of a comprehensive census for the species within the same study area, and its nocturnal habits. The presence of this species in the central Mediterranean seems to be attributable to secondary natural spreading.

Evidence available for most inland water and terrestrial organisms highlights the significant role played by southern Italy, Sicily and the Maltese islands as refuges during Pleistocene climatic fluctuations. However, to date, the hypothesis that these areas may have acted as Pleistocene refugia for the freshwater crab Potamon fluviatile has not been explicitly tested, and a recent origin of local P. fluviatile populations was proposed on the basis of a small set of analysed molecular data. We have thus expanded the currently available data set on the population genetic structure of P. fluviatile through dedicated samplings in Sicily (Italy, 18 specimens), the Maltese Islands (Malta, 15 specimens) and the island of Corfu (Greece, seven specimens), with the explicit aim of testing the role of southern Italy, Sicily and the Maltese archipelago as refugia where early Pleistocene or pre-Quaternary populations of the freshwater crab could have persisted to date. Based on the analysis of both novel and published cytochrome c oxidase subunit I (COI) sequence data, we gathered evidence supporting a recent origin of the southern Italian, Sicilian and Maltese populations of Potamon fluviatile from the Balkans, thus rejecting the hypothesised Pleistocene persistence in the area of pre-existing local populations of the same species.