## 4 Discussion
Within the Mediterranean basin, the blue crab has been recently reported in several freshwater habitats, including the Ichkeul Lake in Tunisia [28], wetlands of the Moulouya mouth and Oued Kert mouth (Morocco) [31], the Tirso and Fenosu rivers in Western Sardinia [27], the mouth of the Basento River (Italy) [32], and Lake Volvi in Northern Greece [33]. As already stated by [10], the occurrence of _C. sapidus_ in freshwaters itself is not unusual (Figure 2): blue crab can exploit different salinity regimes for its life cycle, and low-salinity environments play a role in mating and nursery habitats [21]. Recent data collected in its native range in North America showed that blue crabs were distributed along a salinity gradient, depending on their sex and ontogenetic life stage [21, 34]. Urriver oligohaline waters were preferentially inhabited by male and juvenile crabs, whereas meso- and polyhaline portions of the rivers were selected mainly by adult females [34]. This pattern of distribution may explain why in our study only adult males were collected in portions of running waters characterized by freshwater salinity levels. The same results were also obtained by [27], who collected two male specimens in freshwater habitats of Sardinia, and by [31] in Morocco, where adult males where predominantly caught at river mouths. Unfortunately, in the records obtained by [28] in Tunisia, the gender of collected specimens was not reported. Although for invaded ranges the majority of data regards marine and estuarine environments (see [13]), the blue crab may also represent a dangerous threat to biodiversity for freshwater habitats (Figure 2). Due to its wide trophic niche, the blue crab can predate both vertebrates (i.e., fish fauna) and invertebrate species (i.e., bivalves) of conservation and commercial interest and also affect native riparian and aquatic vegetation. In our work, the blue crab posed a major concern for its sympatric recurrence with the European eel population of the Diaccia Botrona (see Supplementary Material displaying a video of eel predation by a blue crab inside a fishing cage). On the other hand, samples from Spergolaia were collected in one of the last remaining spawning areas of the Mediterranean shad _Alosa fallax_. It should be noted that the two native species are protected under the Regulation 1100/2007/EU and the Directive 43/1992/CEE, respectively. Similarly,predation on freshwater bivalves and herpetofauna, besides threatened fish species, has recently been described in the Iberian Peninsula [8, 35, 36]. Moreover, many freshwater ecosystems are nowadays suffering from the colonization and spread of multiple co-occurrent alien species [37]. In the specific case of euryhaline invaders, the co-occurrence of such benthic decapods as _C. sapidus_ and _Procambarus clarkii_ ([PERSON], 1852) at a specific local scale can enhance the impact on local biodiversity. Indeed, although they are following two different routes, _C. sapidus_ (from marine to inland waters) and _P. clarkii_ (from inland to marine waters) are deeply altering coastal waterbodies, occurring more and more frequently in both brackish and freshwater habitats [38]. This is why it is necessary to continue monitoring activities on the spread of _C. sapidus_ both along marine coastal habitats and in inland waters. Taking into account the present state of the art in Central Italy, more data on _C. sapidus_ in freshwaters are needed, considering the richness of endemic species in this part of the country. Involving local communities and tourists in the monitoring of \"unusual\" records is fundamental for the early detection of the blue crab [30]. In fact, recent approaches based on citizen science and LEK (Local Ecological Knowledge) may be particularly useful to update data on distribution and abundance of the species in freshwater habitats, as successfully demonstrated for marine and estuarine habitats [25, 26, 29, 30, 39].