## 1 Introduction  
In the last decades, the introduction of non-indigenous species (NIS) in European waters has increased remarkably, both in marine and freshwater ecosystems [1]. There are currently more than 14,000 alien species recorded in Europe (EASIN Catalogue [2], [[https://easin.jrc.ec.eu](https://easin.jrc.ec.eu)]([https://easin.jrc.ec.eu](https://easin.jrc.ec.eu)) europa.eu/, accessed on 30 March 2022), with more than half originating from outside EU territories [2]. Their numbers are rapidly increasing, and in some cases so is their rate of spread [3]. The establishment success of NIS depends on the ecological characteristics of the invaded habitat as well as on the ecological plasticity of the species (with particular reference to reproductive capacity, response to environmental stress, trophic flexibility, and behavioral aspects) [4, 5]. Once introduced, NIS have the opportunity to establish self-sustaining populations, acting as sources from which to spread widely, and they become invasive by inflicting environmental and economic harm (invasive alien species, IAS hereafter [6]).  
This issue is particularly important in Italian rivers, where invasion by NIS has been listed among one of five main causes threatening freshwater biodiversity [7]. Interestingly, major attention has recently been dedicated to euryhaline taxa that can reach lotic habitats, migrating from transitional waters, due to their ecological and biological characteristics [8]. Climate changes are enhancing this migratory pattern due to persistent upstream shift of the salt wedge.  
One of the most widespread invasive species in coastal areas of the Mediterranean Sea is the blue crab _Callinectes sapidus_ [PERSON], 1896, belonging to the Portunidae family, a native of the Western Atlantic from Nova Scotia (Canada) to Argentina [9].  
_Callinectes sapidus_ has recently spread in Mediterranean coasts [10, 11, 12] following its introduction by ballast waters [9]. A comprehensive database on its distribution in its native and non-native ranges has recently been published [13], which verified 458 records in the Mediterranean Sea, stating the species can now be considered as \"virtually ubiquitous\" [13]. Indeed, despite the blue crab occurring mainly in the northern and eastern Mediterranean, the species is also expanding southward along the African coasts [13]. Therefore, the species is rapidly expanding in European waters and is currently considered invasive, as it represents a real threat to biodiversity at the local level as a predator and competitor of other benthic species [14, 15]. _Callinectes sapidus_ may play a major structuring and functional role in benthic systems, being a keystone consumer and feeding on fish, invertebrates, detritus, macrophytes, and algae [16]. It is therefore clear how the blue crab inserts itself on several levels of the trophic network and has a direct and indirect impact on benthic communities, both through direct predation and through the bioturbation of the sediment [17, 18, 19].  
It is noteworthy to mention that the species is euryhaline, and besides coastal waters, it is also commonly recorded in upper estuaries, lower reaches of rivers, and even under fully freshwater conditions in coastal lakes [13, 20]. The occurrence of the species in freshwaters is not exceptional and is related to its biological cycle. Specifically, mating occurs in the upper estuaries, after which females migrate to areas having higher salinities for spawning. After hatching, juveniles gradually migrate into shallower, less-saline upper estuaries and rivers, where they grow and mature until one to two years of age, before returning to both lower estuaries and sea coastal habitats [21].  
Considering this scenario, the main purposes of this study are to (i) update the data available on the records of _Callinectes sapidus_ in Italian inland waters and (ii) discuss whether its biological plasticity and invasive capability may represent, besides marine and coastal habitats, also a potential threat for inland waters.