Establishment of the blue crab Callinectes sapidus (Rathbun, 1896) in a Mediterranean hypersaline basin: evidence of ecological plasticity and range expansion

281 Establishment of the blue crab Callinectes sapidus (Rathbun, 1896) in a Mediterranean hypersaline basin: evidence of ecological plasticity and range expansion Davide Cicala1, Cristina Andolina1,2,3 , Roberta Bardelli1, Giovanna Cilluffo1,2,3 , Laura Ciriminna1,2, Giorgio Mancinelli2,4 , Salvatrice Vizzini1,2,3 1 DipartimentodiScienzedellaTerraedelMare,UniversitàdegliStudidiPalermo,viaArchirafi18,90123,Palermo,Italy 2 CoNISMa,NationalInter-UniversityConsortiumforMarineSciences,00196Rome,Italy 3 NationalBiodiversityFutureCenter(NBFC),PiazzaMarina61,90133Palermo,Italy 4 DepartmentofBiologicalandEnvironmentalSciencesandTechnologies,UniversityofSalento,Prov.leLecce-Monteroni,EcotekneCentre,Lecce73100,Italy Correspondingauthor:CristinaAndolina([email protected]) Copyright: © Davide Cicala et al. This is an open access article distributed under terms of the Creative Commons Attribution License (Attribution 4.0 International – CC BY 4.0). Research Article Abstract A central aspect of ecology is the understanding of how environmental factors influence species distribution, population structure, and dynamics, ultimately highlighting species’ capacity to adapt to environmental shifts. This study investigates the population structure and environmental drivers influencing Callinectes sapidus (Rathbun, 1896) within a hypersaline basin (Stagnone di Marsala, Sicily, Italy), highlighting the species’ adaptability to Mediterranean hypersaline coastal ecosystems. The presence of both sexes at various developmental stages indicates the full establishment of the population within the Stagnone, confirming that ecological opportunism, high salinity tolerance, and thermal preference enable C. sapidus to invade areas encompassing a wide range of environments. Two sampling methods were adopted: baited traps and gillnets. The nets captured larger, more mobile males, while traps were effective for a broader size range and included more female individuals. Population structure varied across different sectors of the basin, with the Center–North area showing the highest abundance, likely due to favorable habitat conditions and limited water exchange with the sea. These results emphasize the ability of C. sapidus to complete its life cycle under hypersaline conditions and its preference for stable, sheltered environments. The findings have significant implications for managing C. sapidus populations and mitigating their ecological impact. Adaptive management strategies, such as targeted harvesting and monitoring of salinity and temperature, are key to controlling this invasive species and protecting Mediterranean coastal ecosystems. Key words: Coastal ecosystem, ecological opportunism, invasion success, invasive alien species, population dynamics, sampling methods Introduction Biological invasions are one of the major forces of global environmental change and are currently considered a threat to biodiversity and ecosystem services in terrestrial, freshwater, and marine ecosystems (Hejda et al. 2009; Strayer 2012; Havel et al. 2015; Walsh et al. 2016). The introduction of non-native species is one of the main drivers of biodiversity decline (Carlton 2002; Ricciardi et al. 2017; Haubrock et al. 2020, 2022) and, notwithstanding the growing interest in this Academic editor: Tammy Robinson-Smythe Received: 10 July 2025 Accepted: 7 October 2025 Published: 8 December 2025 Citation: Cicala D, Andolina C, Bardelli R, Cilluffo G, Ciriminna L, Mancinelli G, Vizzini S (2025) Establishment of the blue crab Callinectes sapidus (Rathbun, 1896) in a Mediterranean hypersaline basin: evidence of ecological plasticity and range expansion. NeoBiota 104: 281–300. https://doi.org/10.3897/ neobiota.104.160696 NeoBiota 104: 281–300 (2025) DOI: 10.3897/neobiota.104.160696 Advancing research on alien species and biological invasions A peer-reviewed open-access journal NeoBiota 282 NeoBiota 104: 281–300 (2025), DOI: 10.3897/neobiota.104.160696 Davide Cicala et al.: Range expansion of the blue crab in a Mediterranean hypersaline coastal basin issue, continues to accelerate (Seebens et al. 2017). In particular, during the last decade, the establishment of invasive alien species (IAS hereafter)—that is, non-indigenous species having an adverse effect on biological diversity, ecosystem functioning, socio-economic values, and/or human health in invaded regions (Olenin et al. 2011)—has rapidly become a central environmental concern in marine ecosystems (Occhipinti-Ambrogi 2007; Walther et al. 2009; Occhipinti-Ambrogi and Galil 2010; Zenetos 2010; Crocetta et al. 2015). Indeed, IAS can alter the food web structure of recipient ecosystems through predation and competition, ultimately causing negative impacts on native species (Simberloff et al. 2013; Cicala et al. 2020; Andolina et al. 2022a; De Santis et al. 2022; Katsanevakis et al. 2023; Cicala et al. 2023, 2024). Notably, alarm has been raised for the Mediterranean Sea, which is now recognized as one of the areas worldwide most severely affected by biological invasions, both in terms of the number of alien species detected and the rate of introduction (Occhipinti-Ambrogi and Galil 2010; Zenetos 2010; Davidson et al. 2018; Petrosyan et al. 2023). The study of invasive species has thus far concentrated on understanding the biological traits of potential invaders to predict invasion outcomes at local and larger scales (Whitney and Gabler 2008) and to assess the impacts of invasions on ecosystems (Kolar and Lodge 2002; Rahel 2002). In this scenario, increasing knowledge about IAS, their behavior, and the effects they cause in invaded habitats is of crucial importance. Understanding the patterns and population dynamics of invasions is therefore fundamental for predicting the timing and pathways of population expansion. Optimizing management actions that aim to eradicate or minimize the range expansion of these species is essential to controlling and mitigating their ecological impact (Arim et al. 2006; Davidson et al. 2018; Mancinelli et al. 2021). Accordingly, an essential aspect of invasion biology is understanding how environmental factors influence species distribution and population structure (MacIsaac 1996; Corrales et al. 2020) and how ecological alterations caused by invasive species affect native ecosystems (Nunes et al. 2014). Among other IAS, the last decade has seen increasing interest in the Atlantic blue crab Callinectes sapidus Rathbun, 1896 (hereafter blue crab). The species is an omnivorous portunid native to coastal environments of the western Atlantic Ocean from the Gulf of Maine to Argentina (Hines 2007), where it has long been recognized as a keystone species regulating trophic cascades and acting as a benthic–pelagic coupler (Silliman and Bertness 2002; Boudreau and Worm 2012). The blue crab was first recorded in Europe in 1901 along the Atlantic coasts of France, probably introduced via ballast water, and later appeared in the Mediterranean Sea in 1947 in the Aegean Sea (Nehring 2011). Subsequently, it showed progressive expansion in brackish systems throughout the northern coasts of the basin, from Spain to Greece (Nehring 2011; Mancinelli et al. 2017a), leading to its inclusion in the list of the 100 most invasive species in the Mediterranean (Streftaris and Zenetos 2006). The last decade has witnessed a further, almost ubiquitous expansion of the blue crab’s distribution (Mancinelli et al. 2021; Castriota et al. 2024), paralleled by growing evidence of its impacts on invaded systems, both at the single-species level (e.g., on the Manila clam Ruditapes philippinarum: Chiesa et al. 2025) and at the community scale (Clavero et al. 2022; Gavioli et al. 2025). The most recent records of the blue crab are from south-Mediterranean environments in Algeria, Malta, and Libya, which are characterized by limited and episodic freshwater inputs and generally high salinities (e.g., Benabdi and Belmahi 2020; 283 NeoBiota 104: 281–300 (2025), DOI: 10.3897/neobiota.104.160696 Davide Cicala et al.: Range expansion of the blue crab in a Mediterranean hypersaline coastal basin Kara and Chaoui 2021; Corsini-Foka et al. 2021; Deidun et al. 2022). The blue crab is euryhaline and eurythermal, showing a complex biphasic life cycle consisting of marine planktonic larvae followed by benthic postlarvae, with juveniles and adults residing in estuaries, lagoons, and other transitional systems, and only females migrating to marine waters for spawning (Lipcius et al. 2007). Thus, the recent records in south-Mediterranean environments may indicate an adaptation of the blue crab to local high-salinity conditions that allow it to complete its biological cycle. The structure and seasonal dynamics of these populations remain virtually unexplored, since available information is limited to studies conducted in invaded lagoons and estuaries characterized by strong salinity gradients (Kevrekidis et al. 2023; Mancinelli et al. 2024). The aim of this study was to monitor the population dynamics of the invasive blue crab (Callinectes sapidus) as it colonizes a hypersaline Mediterranean coastal ecosystem. In particular, we sought to (i) investigate the spatio-temporal distribution of C. sapidus in the study area, (ii) assess its population structure (e.g., size classes, sex ratio), and (iii) examine how species abundance correlates with key environmental variables, namely temperature and salinity. Methods Study area The Stagnone di Marsala (Fig. 1) is a semi-enclosed hypersaline basin located along the northwestern coast of Sicily (37.872580°N, 12.464837°E). This shallow coastal marine area has a surface area of approximately 21 km2 and an average depth of 1.5 m (Andolina et al. 2022b). Due to limited openings and the absence of freshwater inputs (Sarà et al. 1999; Vizzini et al. 2002), the basin is hypersaline and characterized by salinities and temperatures ranging seasonally between 32.8 and 47.1 PSU and 11.2 and 29.1 °C, respectively (Tomasello et al. 2009; Mazzola et al. 2010). The basin is separated from the sea by a calcarenite platform (Isola Longa) and divided into two main sub-basins (Vizzini et al. 2002). The northern basin is connected to the sea by a narrow and shallow opening named Bocca San Teodoro. This sub-basin covers an area of approximately 400 m2 and has a depth of 0.3–0.4 m (Vizzini et al. 2013). Conversely, the southern sub-basin is connected to the sea by a larger opening, called Bocca Grande, which is about 1,200 m2 wide and 1–2 m deep (Vizzini et al. 2013). These different degrees of connection with the sea result in limited water exchange in the central–northern part of the basin, while the southern part is more influenced by seawater influxes in terms of temperature and salinity (La Loggia et al. 2004; Mazzola et al. 2010). The basin is characterized by sandy–muddy bottoms with few rocky patches, mainly emerging in the southern sector. The seabed is mostly colonized by the seagrass Cymodocea nodosa (Ucria) Asherson, 1870, often associated with Caulerpa prolifera (Forsskål) Lamouroux, 1809, which are unevenly distributed throughout the basin. Posidonia oceanica (Linnaeus) Delile, 1813, is also present in the central and southern parts of the northern sub-basin, although in regression (Tomasello et al. 2009). Sample collection and processing For the purpose of this study, and considering the hydrodynamic features (La Loggia et al. 2004), four sampling zones were identified with- 284 NeoBiota 104: 281–300 (2025), DOI: 10.3897/neobiota.104.160696 Davide Cicala et al.: Range expansion of the blue crab in a Mediterranean hypersaline coastal basin in the study area: North (37.901727°N, 12.458296°E), Center–North (37.871882°N, 12.465895°E), Center–South (37.845351°N, 12.457651°E), and South (37.820750°N, 12.454201°E). Within each zone, the number of sites was determined according to habitat heterogeneity. The selected sites were one in the North zone (site A), three in the Center–North (sites B, C, D) and Center–South (sites E, F, G) zones, and two in the South zone (sites H, I) (Fig. 1). Given the homogeneous habitat features of the North zone, only one site (Site A) was selected, located close to the northern narrow opening. This zone is characterized by a sandy bottom and a depth of approximately 30 cm. Sites B, C, and D in the Center–North zone are the most remote from the open sea and are characterized by muddy bottoms, a depth of about 30 cm, and abundant macrophyte coverage. Especially in summer, the seagrass C. nodosa and the red macroalga Laurencia sp. are very dense in sites B and C. The southern zones, Center–South and South, present a more heterogeneous bottom. At sites E, F, and G in the Center–South zone, depth is about 45 cm. Macrophytes, mainly represented by C. prolifera and C. nodosa, are abundant in summer but scarce during other periods of the year. Sites H and I, in the South zone, have sandy bottoms with a depth of approximately 60 cm and a high abundance of C. prolifera. Sampling of the blue crab was carried out from June 2021 to September 2023 using traps (60 × 30 cm, nylon mesh size 15 mm). Sampling frequency was Figure 1. Sampling sites grouped by zones in the study area of the Stagnone di Marsala. 285 NeoBiota 104: 281–300 (2025), DOI: 10.3897/neobiota.104.160696 Davide Cicala et al.: Range expansion of the blue crab in a Mediterranean hypersaline coastal basin monthly from June 2021 to March 2023 (except in March and September 2022 and January 2023, when sampling was not conducted due to logistical constraints) and then every other month from May to September 2023 (see Suppl. material 1: table S1 for details on the total number of traps used in each season). Each trap was baited with sardines and chicken discards. Four traps were deployed late in the afternoon at each site and sampling occasion, approximately 70 m apart, left overnight, and retrieved the following morning. Additionally, to increase the chance of capture, from April 2022 to September 2023, crabs were collected in two additional sites in the Center–North zone (sites C and D) and two in the Center–South zone (sites F and G) (Fig. 1). In these additional sites, besides the four traps described above, one gillnet (hereafter net; 50 m long, mesh size 30 mm) was also deployed late in the afternoon and retrieved the following morning. Environmental parameters such as water temperature and salinity were measured with a multiparameter probe (Hanna® HI98194) at each site and sampling occasion. Captured crabs were placed in individually labeled plastic bags and transferred to the laboratory in refrigerated cool boxes. Once in the laboratory, identification of C. sapidus was based on the presence of two large, obtuse teeth on the frontal margin between the inner orbital teeth (Castriota et al. 2024). All individuals collected had intact lateral spines. Specimens were weighed to the nearest 0.1 g, and carapace length (CL) and carapace width (CW) were measured to the nearest millimeter using a caliper. CW was determined as the distance between the two lateral spines, while CL was measured from the teeth on the frontal margin to the posterior margin of the dorsal carapace. For each specimen, sex was determined by inspecting the shape of the abdomen. Data analysis The relative abundance of C. sapidus was estimated as catch per unit effort (CPUE) by dividing the number of crabs collected with each fishing gear by the number of fishing gears deployed at each site. CPUE was calculated separately by catch method, hereafter referred to as CPUE by trap and CPUE by net. The monthly CPUE data were averaged by season for each sampling year and zone to summarize the large amount of data and highlight any seasonal trends that could be compared with those reported in the published literature. To assess the spatial distribution of blue crab abundance across the sampling period, distribution maps of CPUE by trap and by net were created using the ggplot2 package in R v. 3.6.1 (R Core Team 2022). A univariate permutational analysis of variance (PERMANOVA; PRIMER 6 v.6.1.10 and PERMANOVA+ β20; Anderson et al. 2008) was used to test for differences in blue crab abundance (CPUE) and size (CW) according to the factors Year (2021–2023), Season (summer–spring), Zone (North, Center–North, Center–South, and South), and Sex (female and male), separately for the two catch methods (traps and nets). The PERMANOVA (9,999 permutations) was performed on distance matrices constructed using the Bray–Curtis similarity index applied to log-transformed CPUE data and the Euclidean distance of non-transformed CW data. Spearman’s correlation coefficients were used to investigate relationships between CPUE by trap and by net and the environmental variables (temperature and salinity). 286 NeoBiota 104: 281–300 (2025), DOI: 10.3897/neobiota.104.160696 Davide Cicala et al.: Range expansion of the blue crab in a Mediterranean hypersaline coastal basin Results Spatio-temporal distribution of C. sapidus abundance Overall, 495 individuals of C. sapidus were captured (Suppl. material 1: table S1). A total of 327 specimens were males (66%), and 168 were females (34%), with an overall sex ratio of 1.94:1 (♂:♀). Distribution maps clearly highlight an increasing trend in blue crab abundance over time across all sampling zones (Fig. 2). The peak of catches was recorded during the last sampling period (summer 2023) for both capture methods, with a value of 3.3 CPUE by trap at site B (Center–North zone; Fig. 3a) and a mean value of 10 ± 2.8 CPUE (± SD) by net at site C (Center–North zone; Fig. 3b). CPUE by trap ranged between 0 and 3.3 ± 0.7 for females, with the highest abundance recorded in summer 2022 in the North zone, while for males, mean CPUE ranged between 0 and 2.4 ± 1.0, with the highest abundance recorded in summer 2023 in the Center–North zone (Fig. 3a). PERMANOVA results based on CPUE by trap indicated a significantly higher abundance of females in the North zone than in all other zones during spring and summer, and in the latter season, a significantly higher abundance of males in the Center–North zone compared to all other zones (p < 0.05; Suppl. material 1: table S2c). Furthermore, an increasing trend emerged in overall crab abundance, with CPUE being significantly higher in the last sampling year compared to the first (2023 vs 2021; p < 0.05; Suppl. material 1: table S2e). A clear temporal growth pattern also emerged when comparing CPUE in 2022 and 2023 (p < 0.05; Suppl. material 1: table S3d). Overall, nets yielded higher crab catches than traps, with mean CPUE values ranging between 0 and 1 ± 1.4 for females and between 0 and 6.5 ± 7.7 for males, whose peak was recorded in summer 2023 in both central zones (Fig. 3b). The CPUE of males increased significantly in spring and summer compared to winter and was significantly higher than that of females during summer (p < 0.05 in all cases; Suppl. material 1: table S3b). Population structure The population structure of C. sapidus was largely heterogeneous across the sampling zones and seasons (Suppl. material 1: table S1), with carapace width (CW) ranging between 22 and 186 mm (Fig. 4, Suppl. material 1: fig S1). In general, nets captured larger specimens than traps (PERMANOVA, p < 0.05). Specifically, the CW of crabs captured by traps ranged between 49 and 153 mm for females and between 34 and 166 mm for males (Fig. 4a; Suppl. material 1: fig S1a). Conversely, the size of crabs captured by nets ranged between 79 and 154 mm CW for females and between 78 and 186 mm CW for males (Fig. 4b; Suppl. material 1: fig S1b). Larger individuals were captured in spring and summer compared to winter (Fig. 4b; Suppl. material 1: table S5b), while sex-based differences were less pronounced. Spatial variations also occurred among zones, with larger individuals recorded in the Center–North zones (Fig. 4a; Suppl. material 1: table S4b). As observed with nets, size followed seasonal patterns, with generally higher CW values during warmer seasons. Specifically, during spring and summer, a significant increase in CW was observed compared to winter for both females and males, the latter being larger than the former (Suppl. material 1: table S4c, d). 287 NeoBiota 104: 281–300 (2025), DOI: 10.3897/neobiota.104.160696 Davide Cicala et al.: Range expansion of the blue crab in a Mediterranean hypersaline coastal basin Figure 2. Spatial distribution of mean C. sapidus CPUE values by trap (a) and by net (b) across sampling sites and seasons in the Stagnone di Marsala. CPUE values were averaged by sampling months within each season. Environmental variables and relationship with C. sapidus abundance Water temperature and salinity showed a clear seasonal pattern (Suppl. material 1: fig S2). The temperature trend was comparable among zones, ranging from 9.1 °C in winter 2023 in the Center–South zone to 35.5 °C in summer 2022 in the Center–North zone (Suppl. material 1: fig S2a). Conversely, salinity was more variable among zones, with the Center–North zone showing the highest values. 288 NeoBiota 104: 281–300 (2025), DOI: 10.3897/neobiota.104.160696 Davide Cicala et al.: Range expansion of the blue crab in a Mediterranean hypersaline coastal basin Salinity ranged from 29.3 PSU in winter 2022 in the South zone to 52.2 PSU in the Center–North zone in summer 2021 (Suppl. material 1: fig S2b). Overall, blue crab abundance was slightly correlated with both temperature and salinity (Fig. 5), although the relationship was significant only for CPUE by trap and salinity (Spearman’s correlation coefficient, p < 0.05; Fig. 5b). Figure 3. Mean CPUE (± SE) of female (coral bars) and male (cyan bars) specimens of C. sapidus caught by trap (a) and by net (b) in all sampling zones, seasons, and years in the Stagnone di Marsala. Figure 4. Mean (± SE) carapace width (mm) values of blue crabs caught through traps (a) and nets (b) in the Stagnone di Marsala. Coral and cyan bars represent females and males, respectively. 289 NeoBiota 104: 281–300 (2025), DOI: 10.3897/neobiota.104.160696 Davide Cicala et al.: Range expansion of the blue crab in a Mediterranean hypersaline coastal basin Discussion The observed occurrence, abundance, and spatial and temporal dynamics of the population of Callinectes sapidus within the Stagnone di Marsala provide evidence of the invasion success of this non-indigenous species in a Mediterranean hypersaline coastal marine ecosystem. The present study indicates that C. sapidus is able to establish in hypersaline waters, confirming its ability to spread in a wide variety of environments (Mancinelli et al. 2013, 2017a, 2024) and showing seasonal and spatial dynamics of its population at a local scale (Hines 2007; Kevrekidis and Antoniadou 2018). The invasion success of C. sapidus in Mediterranean coastal ecosystems can be attributed to a combination of biological and ecological traits. Its ecological opportunism, trophic adaptability, strong swimming capacity, and aggressive behavior (Williams 1974; Dulčić et al. 2011) allow it to exploit a wide range of habitats and resources. Most notably, its exceptional tolerance to extreme salinity and temperature conditions enables it to thrive in such environments, thereby facilitating its rapid establishment and expansion. 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Biological Invasions 12(9): 3379– 3381. https://doi.org/10.1007/s10530-009-9679-x Supplementary material 1 Range expansion of the blue crab in a Mediterranean hypersaline coastal basin Authors: Davide Cicala, Cristina Andolina, Roberta Bardelli, Giovanna Cilluffo, Laura Ciriminna, Giorgio Mancinelli, Salvatrice Vizzini Data type: docx Copyright notice: This dataset is made available under the Open Database License (http://opendatacommons.org/licenses/odbl/1.0/). The Open Database License (ODbL) is a license agreement intended to allow users to freely share, modify, and use this Dataset while maintaining this same freedom for others, provided that the original source and author(s) are credited. Link: https://doi.org/10.3897/neobiota.104.160696.suppl1