Effect of urban isolation on the dynamics of river crabs

European river crabs usually disappear from areas affected by human activities. However, a Potamon fluviatile population was recently recorded within an archaeological excavation in the historical centre of Rome. Since adaptation of river-dwelling brachyurans to urban habitat is rare, we focused on some aspects of population dynamics which are considered to be strictly related to important biological processes and to the adaptation degree of a population to its habitat. To do so, length-frequency distributions of four Italian populations were obtained using the carapace length and the subsequent data was analyzed by the FiSAT program. The Roman population exhibited a lower growth rate (as demonstrated by ANCOVA) and higher asymptotic length and longevity than the other populations, suggesting that Roman crabs are affected by gigantism. Isolation was confirmed by the absence of crabs in adjacent water systems and in the urban tract of the River Tiber, and could be a result of the progressive expansion of the city of Rome. On the other hand, an alternative explanation could be represented by a historical transfaunation from western Greece. Excluding recent intentional introduction by man, the presence of P. fluviatile in the city of Rome is thus probably the result of this species' ecological preferences and physiological constraints coupled with its evolutionary history, human interference, and colonisation events. It is, therefore, necessary to devise a conservation plan to protect this relict population as its preservation should be considered as important as the preservation of the historical monuments.