The processes of biological invasions have gained significant attention in the field of ecology and environmental sciences, due to their marked and detrimental impacts on both native species and ecosystem functioning [
1]. In an increasingly interconnected world, biological invasions have become more frequent and widespread. Furthermore, factors like trade and climate change have facilitated the spread of non-indigenous species to new areas far from their native distribution range [
2]. The introduction of invasive species poses a major threat to ecosystems structure and integrity, leading to species extinction and biodiversity loss worldwide [
3,
4]. In fact, albeit the magnitude of impacts can strongly vary across different taxa [
5], invasive species have been implicated in the extinction of enormous numbers of animal and plant species [
6]. In addition, invasive alien species (IAS) can have a negative impact on the economy and even on human health [
7].
Biological invasions involve a multi-step process. Initially, introduced species must successfully arrive, survive, and establish themselves in a new area. Subsequently, these alien species can become invasive, spreading and impacting native environments and species [
8,
9]. Throughout these stages, complex ecological processes unfold in the invaded habitats, including changes in the biological community, characteristics of the invaders, abiotic environment, and ecosystem component interactions. The temporal dimension is a critical factor when analyzing the impacts of IAS [
10]. Research focusing on later invasions phases often reveals extensively altered communities, whereas studying the early stages provides insights into how ongoing processes modify the invaded communities. Within the freshwater environments the emerging scenario poses critical challenges. In fact, due to intense human modifications and to the numerous routes of introduction, freshwater environments are particularly subject to biological invasions [
11]. Together with the numerous other anthropogenic stressors (e.g. pollution, climate change, hydropower, and habitat degradation), the introduction of alien species jeopardizes the biodiversity conservation, that in freshwater ecosystems is much more vulnerable than elsewhere [
12]. For example, freshwater environments are thought to be more sensitive to the introduction of predators than their marine or terrestrial counterparts. This phenomenon is potentially attributable to the high heterogeneity in predation regimes between the different freshwater systems, that promotes naiveté in native preys [
13]. In this context, the worst scenario is represented by extinction of native species by invasive ones [
14]. For this reason, an early assessment is fundamental to monitoring and predict the impacts of IAS from damaging environmental integrity and native species [
9].
In Europe, alien crayfishes now outnumber native crayfish species, and represent the first threat for their conservation. In fact, at least 9 alien crayfish species have formed established populations in European freshwater environments, coming from both the Americas and Australia. For example, in North America, non-indigenous crayfishes have displaced indigenous ones through direct competition [
15]. In Europe, a similar situation has occurred, but the presence of the crayfish plague has exacerbated the devastation among the limited number of native species to a far greater extent. The success of non-indigenous crayfishes in Europe can be attributed primarily to their resilience, rapid individual and population growth, high reproductive capacity, and invasive characteristics, as noted by Lindqvist and Huner [
16]. Tricarico et al. [
17] introduced the “Freshwater Invertebrate Invasiveness Scoring Kit (FI-ISK)” to evaluate the invasive potential of various crayfish species, including those from the Astacidae, Cambaridae, and Parastacidae families, in the Italian freshwater ecosystem. Their findings attributed high risk scores for the red swamp crayfish
Procambarus clarkii (Girard, 1852), in contrast to other species that were classified as having a moderate risk. This assessment highlights the utility of FI-ISK as a valuable tool for aiding decision-makers and policymakers in categorizing freshwater invertebrates based on their potential invasiveness. Importantly, this information is relevant to compliance with the 2007 European Union regulation (Council Regulation No. 708/2007) concerning the introduction of alien species for aquaculture and related activities.
The genus
Procambarus encompasses several IAS in Europe, and even other species often subject to aquarium trade [
18]. Among them,
P. clarkii, native to the United States and northern Mexico, has been introduced in many countries across the globe for commercial purposes [
19]. This species has displayed a remarkable capacity to adapt to and thrive in diverse natural and artificial habitats, leading to significant consequences for local ecosystems and their prey-predator dynamics [
19,
20]. Among its various ecological impacts,
P. clarkii has been observed to significantly reduce the abundance, diversity, or biomass of many native species, feeding on amphibians [
20,
21], insects [
14,
20], snails [
22], and macrophites [
22,
23]. Moreover, this species serves as a vector for the oomycete
Aphanomyces astaci Schikora, which is responsible for the crayfish plague, a disease to which the European crayfish species are highly susceptible [
18]. These combined factors underscore the considerable ecological and economic challenges posed by the widespread introduction ad proliferation of
P. clarkii in various regions. In Italy,
P. clarkii was observed for the first time in the wild in the Turin province (northern Italy) in 1989 [
24]. However, albeit the presence of the species in the country was previously considered nearly limited to the northern and central regions, with just few records occurring in the south [
25], over recent decades, it rapidly spread across the nation [
26]. In fact, recent studies suggest that
P. clarkii is constantly increasing its presence in southern regions, as well as in the two main Italian islands (i.e. Sicily and Sardinia) [
27]. This poses a serious threat to the native crayfish communities, which are particularly rich and complex in Italy [
18]. Its invasiveness is attributed to a confluence of many different factors. First of all, the species is r-selected, exhibiting high early maturity, rapid growth rates and large numbers of offspring production [
19]. Its burrowing behaviour further enhances its adaptability, enabling
P. clarkii to also live in temporary streams [
18,
19,
28]. Besides, it has great dispersal abilities and a generalist diet that can change among the different habitats, further enhancing its invasiveness [
19]. It can also tolerate highly polluted environments. Lastly,
P. clarkii can withstand broad ranges of water quality parameters: for example, it tolerates a wide range of salinity. In fact, it has been proposed that the species could be able to descend some rivers up to the sea, and thus migrate from coastal waters to new, upstream, freshwater environments [
29]. A similar invasion route has also been proposed for other freshwater species, like the topmouth gudgeon
Pseudorasbora parva, a small cyprinid native to East Asia, apparently able to withstand coastal salinities in the proximity of streams [
30]. Due to its remarkable biological adaptability, behavioral flexibility, and dispersal capabilities,
P. clarkii has the capacity to establish itself in a broad spectrum of environments, including lagoons and brackish ecosystems [
29]. For all these reasons, this species is ranked by the European Union in the top places among the 100 most harmful IAS (Regulation (EU) 1143/2014, see [
31]). Evolutionary histories in harsh environments with changing environmental conditions can lead to the development of phenotypic plasticity [
32]. For this reason, species originating from brackish and transitional waters are often predisposed to invasion, as they often show broad plasticity in terms of resistance to varying levels of salinity and other environmental variables [
33]. This makes salinity tolerance one of the most studied “predictors of invasion”. Moreover, given euryhaline species are often able to colonize multiple environments, they can also take advantage of multiple invasion routes. For example, it has been proposed that increased salinity tolerance can promote invaders’ ability to survive in ships ballast waters [
34]. Despite the broad salinity tolerance of
P. clarkii, however, no consistent observations of the species in the marine environment have been reported until now, nor have interactions with marine fishing activities been documented. In this study, on the basis of recent records coming from citizen scientists, we provide the first evidence of the species presence within marine ecosystems, and its interaction with artisanal fishery.