δ13C and δ15N patterns of the blue crab, Callinectes sapidus, and its potential diets, were investigated in eleven localities within four invaded regions of the Spanish Mediterranean (Catalonia, Valencia, Alicante, and Murcia) subjected to contrasting salinities and degrees of connectivity with the open sea. These regions host blue crab fisheries of variable importance that might be regulated by availability of food resources and local environmental conditions. When present, large adults and immature, subadult sizes of each sex were captured to evaluate possible differences associated to age movement across habitats. SIBER and MixSIAR were used to assess patterns of isotopic niche and dietary contributions. Results showed significant effects for all factors and interactions (except for Sex and Sex x Size in δ15N). The effect size in Region (ηp2 = 0.82 and 0.70, respectively for δ13C and δ15N) and Locality (ηp2 = 0.53 and 0.46), was substantially greater than that of crab Size (ηp2 = 0.37 and 0.21) and Sex (ηp2 = 0.06 and 0.02), concurring with increasing levels of isotopic niche overlap (9% and 11% in Locality and Region, 32% in Size and 44% in Sex). MixSiar results indicated strongly variable contributions from food sources at each locality (TPs from 2.3 to 3.6), but with higher quantity of fish, algae, and crustaceans (27.7%, 18% and 15.1%). Dietary results showed little relation with isotopic patterns, whereas significant associations were found between local salinities and signatures in both crabs (R2 = 0.518 and 0.757, for δ13C and δ15N) and diets. Overall, our study suggests that blue crab habitat use in small Mediterranean estuaries might largely differ from native areas, with movements being mostly restricted to young individuals and/ or certain localities with higher connectivity with the open sea (e.g., the Ebro River). Salinity conditions emerge as a major variable shaping isotopic patterns of populations on a large scale., Funding: Funding was provided by the Spanish Government (Ministry of Science and Technology) under the ECESIS project (Subproject 1: PID2020-118476RR-C21 to P. Prado and S. Falco; and Subproject 2: PID2020-118476RR-C22 to E. Guijarro and C. Barberà). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript., Peer reviewed

This article belongs to the Special Issue Aquatic Environment and Ecosystems., The blue crab Callinectes sapidus Rathbun, 1896 is native to the western coast of the Atlantic Ocean. Although its arrival to the Mediterranean was probably due to ballast water, this species has several characteristics that have enabled it to successfully invade countless localities in the Mediterranean and the Black Sea. Little is known about its feeding habits and ecosystem impacts in the Mediterranean basin. This study aimed to provide information on the natural diet of C. sapidus by comparing the stomach contents of specimens caught in different seasons and habitats of the SE Iberian Peninsula (hypersaline waters in Mar Menor Lagoon and brackish waters in Guardamar Bay). This study also tested whether gender influences prey selection and if ovigerous females exhibit limited feeding activity. Regarding the frequency of occurrence, the results indicated that in Mar Menor Lagoon the most frequently consumed prey were Crustacea (60%), followed by fish (57%) and Mollusca (29%), whilst in Guardamar Bay, Mollusca (40%), sediment (32%), algae (24%) and Crustacea (24%) were dominant. It has been determined that this species predates heavily on Mediterranean shrimp Penaeus kerathurus, an economically important shrimp species in the lagoon area. Analysis using a generalised linear model indicated that sex, season and size class were factors that significantly influenced the stomach content weight. Furthermore, non-ovigerous females had significantly fuller stomachs than ovigerous individuals. Since the population of Callinectes sapidus tends to increase in the Mediterranean basin, monitoring of its feeding ecology is recommended to determine its impact on the ecosystem., The study framework is from the projects I+D+i ECESIS (PID2020-118476RR-C21) and PID2020-118476RR-C22, funded by the Spanish Ministry of Ciencia, Innovación y Universidades/AEI/10.13039/501100011033/. Additionally, TÜBİTAK BİDEB (2219 Program) supported the first author with a scholarship during post-doctoral study at Alicante University, Spain., Peer reviewed

δ13C and δ15N patterns of the blue crab, Callinectes sapidus, and its potential diets, were investigated in eleven localities within four invaded regions of the Spanish Mediterranean (Catalonia, Valencia, Alicante, and Murcia) subjected to contrasting salinities and degrees of connectivity with the open sea. These regions host blue crab fisheries of variable importance that might be regulated by availability of food resources and local environmental conditions. When present, large adults and immature, subadult sizes of each sex were captured to evaluate possible differences associated to age movement across habitats. SIBER and MixSIAR were used to assess patterns of isotopic niche and dietary contributions. Results showed significant effects for all factors and interactions (except for Sex and Sex x Size in δ15N). The effect size in Region (ηp2 = 0.82 and 0.70, respectively for δ13C and δ15N) and Locality (ηp2 = 0.53 and 0.46), was substantially greater than that of crab Size (ηp2 = 0.37 and 0.21) and Sex (ηp2 = 0.06 and 0.02), concurring with increasing levels of isotopic niche overlap (9% and 11% in Locality and Region, 32% in Size and 44% in Sex). MixSiar results indicated strongly variable contributions from food sources at each locality (TPs from 2.3 to 3.6), but with higher quantity of fish, algae, and crustaceans (27.7%, 18% and 15.1%). Dietary results showed little relation with isotopic patterns, whereas significant associations were found between local salinities and signatures in both crabs (R2 = 0.518 and 0.757, for δ13C and δ15N) and diets. Overall, our study suggests that blue crab habitat use in small Mediterranean estuaries might largely differ from native areas, with movements being mostly restricted to young individuals and/ or certain localities with higher connectivity with the open sea (e.g., the Ebro River). Salinity conditions emerge as a major variable shaping isotopic patterns of populations on a large scale., Funding was provided by the Spanish Government (Ministry of Science and Technology) under the ECESIS project (Subproject 1: PID2020-118476RR-C21 to P. Prado and S. Falco; and Subproject 2: PID2020-118476RR-C22 to E. Guijarro and C. Barberà). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript., info:eu-repo/semantics/publishedVersion

The Ebro Delta has become a major blue crab (Callinectes sapidus) fishery area in the NW Mediterranean, but there is limited information on factors controlling the abundance of populations in open waters, a crucial habitat for ovigerous females. Here, we use a stable isotope approach (δ15N and δ13C), to assess blue crab trophic position, the potential consumption of food items using mixing models, and the isotopic niche overlap with local commercial species. For Octopus vulgaris, a potential blue crab key predator, a trophic enrichment experiment was also conducted to further assess predation control in wild populations. The blue crab showed 1.6 times higher trophic position than in other habitats of the Ebro Delta, and similar to that found in the harbor crab, Liocarcinus depurator, and several predatory fish. Additionally, the isotopic niche of blue crabs showed overlaps from 46.2 to 14.9% with native predators, and mixing models also suggest even dietary contributions throughout the food web. For O. vulgaris, field results showed a trophic position of 3.93, lower than that of blue crab, and lower δ15N signatures were also obtained in a captivity experiment drawing negative fractionation (−1.1‰). We conclude that high dietary contribution of animal prey might provide a high protein diet that could be crucial for allowing the maintenance of a large local population, but the overall functional trophic similarity could also disfavor local native species. The similarity between experimental fractionation and field differences between predator and prey (−1.6‰) suggests that predation of blue crab is possible, but further research is needed to clarify the metabolic routes involved in octopus δ15N fractionation., Financial support for fieldwork and SIA was provided by the Departament d’Agricultura, Ramaderia, Pesca i Alimentació through the BivalCat project (Ref. CAT 152CAT) to M. Ballesteros. P. Prado was contracted under the INIA-CCAA research program for postdoctoral incorporation from the Spanish National Institute for Agricultural and Food Research and Technology (INIA). Stabling of O. vulgaris was supported by the Spanish Government (Ministry of Science and Technology) under the ECESIS project (PID2020-118476RR-C21) to P. Prado and S. Falco. Authors are also grateful to C. Alcaraz for assessment with script preparation and interpretation of SIBER results, and to K.B. Andree, for a double comprehensive review and correction of English throughout the ms. We would also like to thank the detailed review conducted by two anonymous referees that has been of great help for improving the overall quality of the ms., info:eu-repo/semantics/acceptedVersion

In the Ebro Delta (Catalonia, Spain), the abundance of burrowing bivalves has dramatically decreased, with the blue crab, Callinectes sapidus, being blamed by shellfish collectors. Trends from 2010 evidence a decrease in the capture of clams (Ruditapes spp.) before 2016 (start of blue crab fisheries), although a further decline in both clams and cockles (Cerastoderma glaucum) occurred in 2018. In contrast, captures of razor clams (Ensis siliqua) have increased by 3.6-fold since 2016. Predation risk for these taxa, with contrasting burrowing capacities (1.7 ± 0.3 cm, 0.4 ± 0.2 cm, and 26.3 ± 0.1 cm, respectively), was assessed using predation preference (N = 5 tanks; 5 individuals of each species) and no-choice experiments (N = 5 tanks; 15 individuals of the same taxa) in the absence and presence of sand. The results showed that, in the absence of sand, razor clams were fully preyed upon in 24 h, clams in 96 h, and cockles reached 60% after 144 h. Conversely, when sand was present, only 4% of razor clams were predated, while clams and cockles reached 60–100% in 120–144 h. The no-choice results featured similar patterns, depending on substrate availability. Overall, clams and cockles appear to be greatly vulnerable to blue crab predation, whereas razor clams may escape thanks to their deeper burrowing capacity., Predation experiments were supported by the Spanish Government (Ministry of Science and Technology) under the ECESIS project (PID2020-118476RR-C21) awarded to P. Prado and S. Falco., info:eu-repo/semantics/publishedVersion

[EN] The Ebro Delta has become a major blue crab (Callinectes sapidus) fishery area in the NW Mediterranean, but there is limited information on factors controlling the abundance of populations in open waters, a crucial habitat for ovigerous females. Here, we use a stable isotope approach (delta 15N and delta 13C), to assess blue crab trophic position, the potential consumption of food items using mixing models, and the isotopic niche overlap with local commercial species. For Octopus vulgaris, a potential blue crab key predator, a trophic enrichment experiment was also conducted to further assess predation control in wild populations. The blue crab showed 1.6 times higher trophic position than in other habitats of the Ebro Delta, and similar to that found in the harbor crab, Liocarcinus depurator, and several predatory fish. Additionally, the isotopic niche of blue crabs showed overlaps from 46.2 to 14.9% with native predators, and mixing models also suggest even dietary contributions throughout the food web. For O. vulgaris, field results showed a trophic position of 3.93, lower than that of blue crab, and lower delta 15N signatures were also obtained in a captivity experiment drawing negative fractionation (-1.1%o). We conclude that high dietary contribution of animal prey might provide a high protein diet that could be crucial for allowing the maintenance of a large local population, but the overall functional trophic similarity could also disfavor local native species. The similarity between experimental fractionation and field differences between predator and prey (-1.6%o) suggests that predation of blue crab is possible, but further research is needed to clarify the metabolic routes involved in octopus delta 15N fractionation., Financial support for fieldwork and SIA was provided by the Departament d'Agricultura, Ramaderia, Pesca i Alimentacio through the Bival Cat project (Ref. CAT 152CAT) to M. Ballesteros. P. Prado was contracted under the INIA-CCAA research program for postdoctoral incorporation from the Spanish National Institute for Agricultural and Food Research and Technology (INIA) . Stabling of O. vulgaris was supported by the Spanish Government (Ministry of Science and Technology) under the ECESIS project (PID2020-118476RR-C21) to P. Prado and S. Falco. Authors are also grateful to C. Alcaraz for assessment with script preparation and interpretation of SIBER results, and to K.B. Andree, for a double comprehensive review and correction of English throughout the ms. We would also like to thank the detailed review conducted by two anonymous referees that has been of great help for improving the overall quality of the ms.

[EN] delta 13C and delta 15N patterns of the blue crab, Callinectes sapidus, and its potential diets, were investigated in eleven localities within four invaded regions of the Spanish Mediterranean (Catalonia, Valencia, Alicante, and Murcia) subjected to contrasting salinities and degrees of connectivity with the open sea. These regions host blue crab fisheries of variable importance that might be regulated by availability of food resources and local environmental conditions. When present, large adults and immature, subadult sizes of each sex were captured to evaluate possible differences associated to age movement across habitats. SIBER and MixSIAR were used to assess patterns of isotopic niche and dietary contributions. Results showed significant effects for all factors and interactions (except for Sex and Sex x Size in delta 15N). The effect size in Region (eta p2 = 0.82 and 0.70, respectively for delta 13C and delta 15N) and Locality (eta p2 = 0.53 and 0.46), was substantially greater than that of crab Size (eta p2 = 0.37 and 0.21) and Sex (eta p2 = 0.06 and 0.02), concurring with increasing levels of isotopic niche overlap (9% and 11% in Locality and Region, 32% in Size and 44% in Sex). MixSiar results indicated strongly variable contributions from food sources at each locality (TPs from 2.3 to 3.6), but with higher quantity of fish, algae, and crustaceans (27.7%, 18% and 15.1%). Dietary results showed little relation with isotopic patterns, whereas significant associations were found between local salinities and signatures in both crabs (R2 = 0.518 and 0.757, for delta 13C and delta 15N) and diets. Overall, our study suggests that blue crab habitat use in small Mediterranean estuaries might largely differ from native areas, with movements being mostly restricted to young individuals and/ or certain localities with higher connectivity with the open sea (e.g., the Ebro River). Salinity conditions emerge as a major variable shaping isotopic patterns of populations on a large scale., Funding was provided by the Spanish Government (Ministry of Science and Technology) under the ECESIS project (Subproject 1: PID2020- 118476RR-C21 to P. Prado and S. Falco; and Subproject 2: PID2020-118476RR-C22 to E. Guijarro and C. Barbera). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

[EN] In the Ebro Delta (Catalonia, Spain), the abundance of burrowing bivalves has dramatically decreased, with the blue crab, Callinectes sapidus, being blamed by shellfish collectors. Trends from 2010 evidence a decrease in the capture of clams (Ruditapes spp.) before 2016 (start of blue crab fisheries), although a further decline in both clams and cockles (Cerastoderma glaucum) occurred in 2018. In contrast, captures of razor clams (Ensis siliqua) have increased by 3.6-fold since 2016. Predation risk for these taxa, with contrasting burrowing capacities (1.7 +/- 0.3 cm, 0.4 +/- 0.2 cm, and 26.3 +/- 0.1 cm, respectively), was assessed using predation preference (N = 5 tanks; 5 individuals of each species) and no-choice experiments (N = 5 tanks; 15 individuals of the same taxa) in the absence and presence of sand. The results showed that, in the absence of sand, razor clams were fully preyed upon in 24 h, clams in 96 h, and cockles reached 60% after 144 h. Conversely, when sand was present, only 4% of razor clams were predated, while clams and cockles reached 60-100% in 120-144 h. The no-choice results featured similar patterns, depending on substrate availability. Overall, clams and cockles appear to be greatly vulnerable to blue crab predation, whereas razor clams may escape thanks to their deeper burrowing capacity., Predation experiments were supported by the Spanish Government (Ministry of Science and Technology) under the ECESIS project (PID2020-118476RR-C21) awarded to P. Prado and S. Falco.

[EN] The early life cycle of the Atlantic blue crab Callinectes sapidus includes seven zoeal stages that develop in coastal
waters, and a later megalopal stage that colonizes estuaries. Although these stages have been largely studied in
native ecosystems, very little is known about larval patterns in smaller-scale, invaded, estuarine environments
such as those available in the Mediterranean Sea. The general aim of this study is to assess the distribution and
abundance of early life stages of C. sapidus in the communication channels of the Albufera lagoon with the
Mediterranean Sea, and in adjacent coastal waters. Different sampling methods were employed to target each life
stage. (1) For zoeae, planktonic tows were conducted using a 300 ¿ m manta net along 200 m transects parallel to
the coastline at distances of 300, 1000 and 3000 m from the shore. (2) A small and manageable passive collector
model was used to capture megalopae and juveniles in estuarine areas. This system was first tested from 2020 to
2021 in a single channel, with a weekly sampling frequency, and two more channels were added in 2022.
Additionally, intensive daily sampling was conducted in one of the channels from September to November 2022
during the settlement peak. Results showed that zoeae of C. sapidus were more abundant in August 2021, at 1000
m from the coast, and appear to have a disjoint temporal distribution with zoeae of other crab species. Megalopae
and juveniles were more abundant from September to November at all sampling sites and times. In 2022, col-
lectors featured a dominance of megalopae at two of the sites (ca. 73%), while the site with the lowest salinity
showed a dominance of juveniles (84%). For megalopae, comparative results from the intensive and the regular
sampling campaigns showed that time of residence within collectors is just a single day. For juveniles, higher
numbers were detected with increased deployment time, suggesting a need for enhanced habitat complexity., We want to thank Professor Christoph D. Schubart for his support and training in molecular analysis to A. Gil-Fernandez, whose recent passing has left a big hole in the crab world. May his soul rest in peace.This project was funded by the Ministry of Science and Innovation (Grant PID 2020-118476RR-C21 by MCIN/AEI/10.13039/501100011033) and by the Universitat Politecnica de Valencia (PAID-01 -18, CRUE) , to which we are very thankful.

[EN] The blue crab is an invasive species that has rapidly spread along Mediterranean coasts, threatening marine ecosystems. Exploring natural ways to control its population could help reduce these impacts. In this study, we investigated the functional size preference and feeding rates of two native predators-the common octopus and the loggerhead sea turtle-when feeding on blue crabs. In aquarium experiments, predators were offered crabs of different sizes under controlled conditions to assess whether they preferred certain sizes and how much they could consume in a day. Both species consumed substantial amounts of blue crabs: octopuses fed only on soft tissues, whereas turtles ingested whole crabs, including the exoskeleton. Octopuses exhibited shorter handling times for small crabs, although neither predator showed a marked size preference. Our findings suggest that, particularly octopuses, thanks to their abundance and feeding efficiency, could contribute to limiting invasive crab populations and support ecosystem balance.

The rapid spread of the invasive blue crab (Callinectes sapidus) in Mediterranean coastal ecosystems poses ecological and socio-economic challenges, highlighting the need to explore natural control mechanisms. We conducted controlled aquarium experiments to assess the functional size preference and feeding rates of two native Mediterranean predators: the common octopus (Octopus vulgaris) and the loggerhead sea turtle (Caretta caretta). Two sets of trials were performed: (1) size preference experiments, in which individual predators were simultaneously offered crabs of varying sizes; and (2) ad libitum feeding experiments to estimate daily consumption rates. Octopuses consumed only the soft tissues biomass (STB, similar to 54% of total crab biomass), reaching maximum intake rates of 454 +/- 71 gWW.d(-1) in adult individuals. Sea turtles ingested whole crabs, including the exoskeleton, with total consumption rates up to 815 +/- 592 gWW.d(-1) of total biomass (TB) in adults. Subadult octopuses exhibited shorter handling times for small crabs, although neither predator showed a statistically significant size preference. Our findings indicate that both predators can consume substantial amounts of blue crab biomass, but O. vulgaris-due to its higher feeding efficiency and ecological abundance-might play a more prominent role as a natural biocontrol agent. These results provide valuable insights for integrated management strategies aimed at mitigating the ecological impact of C. sapidus in the Mediterranean., This research was funded by the Ministry of Science and Innovation (Grant PID 2020-118476RR-C21 by MCIN/AEI/10.13039/501100011033). P. Prado was contracted under the INIA-CCAA cooperative research program for postdoctoral incorporation from the Spanish National Institute for Agricultural and Food Research and Technology (INIA).

The blue crab Callinectes sapidus was first observed in the Ebro Delta in 2012 and since then captures have increased exponentially up to over 2 t per day, while its presence remains low in other Catalonian estuarine areas. Here, we use a stable isotope approach (δ15N and δ13C) to explore the dietary habits of adult blue crab in four different invaded habitats—bays, coastal lagoons, rice field drainage channels, and the Ebro River—in order to assess the strength of bottom-up forces and identify risks for native and aquaculture species, as well as patterns of site fidelity (male individuals). Mixing models showed average contributions of 35.89% from organic matter in sediments, 34.25% from animal resources (fish, crustaceans, gastropods, and bivalves), and 23.84% from vegetal resources (aquatic plants and algae), although there were important differences across habitat sites. In sites where bivalves were available, they can represent up to ca. 75% of the diet, thus threatening natural banks and local oyster and mussel farms. The average estimated trophic position of blue crabs in those sites was only 2.8, which confirms an omnivorous behavior but also can be attributed to the fact that mollusks were rare in the majority of the areas sampled in the Ebro Delta. Crabs from the same habitat site exhibited very little isotopic variability, suggesting that they remain in those environments long enough to reflect local salinity conditions. Overall, our results suggest that blue crabs are likely using all locally available resources and remain in certain sites, even when preferred animal preys are scarce and low-quality items are the main dietary option., info:eu-repo/semantics/acceptedVersion

In the Ebro Delta (Catalonia, Spain), the abundance of burrowing bivalves has dramatically decreased, with the blue crab, Callinectes sapidus, being blamed by shellfish collectors. Trends from 2010 evidence a decrease in the capture of clams (Ruditapes spp.) before 2016 (start of blue crab fisheries), although a further decline in both clams and cockles (Cerastoderma glaucum) occurred in 2018. In contrast, captures of razor clams (Ensis siliqua) have increased by 3.6-fold since 2016. Predation risk for these taxa, with contrasting burrowing capacities (1.7 ± 0.3 cm, 0.4 ± 0.2 cm, and 26.3 ± 0.1 cm, respectively), was assessed using predation preference (N = 5 tanks; 5 individuals of each species) and no-choice experiments (N = 5 tanks; 15 individuals of the same taxa) in the absence and presence of sand. The results showed that, in the absence of sand, razor clams were fully preyed upon in 24 h, clams in 96 h, and cockles reached 60% after 144 h. Conversely, when sand was present, only 4% of razor clams were predated, while clams and cockles reached 60–100% in 120–144 h. The no-choice results featured similar patterns, depending on substrate availability. Overall, clams and cockles appear to be greatly vulnerable to blue crab predation, whereas razor clams may escape thanks to their deeper burrowing capacity., Predation experiments were supported by the Spanish Government (Ministry of Science and Technology) under the ECESIS project (PID2020-118476RR-C21) awarded to P. Prado and S. Falco., info:eu-repo/semantics/publishedVersion

The Ebro Delta has become a major blue crab (Callinectes sapidus) fishery area in the NW Mediterranean, but there is limited information on factors controlling the abundance of populations in open waters, a crucial habitat for ovigerous females. Here, we use a stable isotope approach (δ15N and δ13C), to assess blue crab trophic position, the potential consumption of food items using mixing models, and the isotopic niche overlap with local commercial species. For Octopus vulgaris, a potential blue crab key predator, a trophic enrichment experiment was also conducted to further assess predation control in wild populations. The blue crab showed 1.6 times higher trophic position than in other habitats of the Ebro Delta, and similar to that found in the harbor crab, Liocarcinus depurator, and several predatory fish. Additionally, the isotopic niche of blue crabs showed overlaps from 46.2 to 14.9% with native predators, and mixing models also suggest even dietary contributions throughout the food web. For O. vulgaris, field results showed a trophic position of 3.93, lower than that of blue crab, and lower δ15N signatures were also obtained in a captivity experiment drawing negative fractionation (−1.1‰). We conclude that high dietary contribution of animal prey might provide a high protein diet that could be crucial for allowing the maintenance of a large local population, but the overall functional trophic similarity could also disfavor local native species. The similarity between experimental fractionation and field differences between predator and prey (−1.6‰) suggests that predation of blue crab is possible, but further research is needed to clarify the metabolic routes involved in octopus δ15N fractionation., Financial support for fieldwork and SIA was provided by the Departament d’Agricultura, Ramaderia, Pesca i Alimentació through the BivalCat project (Ref. CAT 152CAT) to M. Ballesteros. P. Prado was contracted under the INIA-CCAA research program for postdoctoral incorporation from the Spanish National Institute for Agricultural and Food Research and Technology (INIA). Stabling of O. vulgaris was supported by the Spanish Government (Ministry of Science and Technology) under the ECESIS project (PID2020-118476RR-C21) to P. Prado and S. Falco. Authors are also grateful to C. Alcaraz for assessment with script preparation and interpretation of SIBER results, and to K.B. Andree, for a double comprehensive review and correction of English throughout the ms. We would also like to thank the detailed review conducted by two anonymous referees that has been of great help for improving the overall quality of the ms., info:eu-repo/semantics/acceptedVersion

The early life cycle of the Atlantic blue crab Callinectes sapidus includes seven zoeal stages that develop in coastal waters, and a later megalopal stage that colonizes estuaries. Although these stages have been largely studied in native ecosystems, very little is known about larval patterns in smaller-scale, invaded, estuarine environments such as those available in the Mediterranean Sea. The general aim of this study is to assess the distribution and abundance of early life stages of C. sapidus in the communication channels of the Albufera lagoon with the Mediterranean Sea, and in adjacent coastal waters. Different sampling methods were employed to target each life stage. (1) For zoeae, planktonic tows were conducted using a 300 μm manta net along 200 m transects parallel to the coastline at distances of 300, 1000 and 3000 m from the shore. (2) A small and manageable passive collector model was used to capture megalopae and juveniles in estuarine areas. This system was first tested from 2020 to 2021 in a single channel, with a weekly sampling frequency, and two more channels were added in 2022. Additionally, intensive daily sampling was conducted in one of the channels from September to November 2022 during the settlement peak. Results showed that zoeae of C. sapidus were more abundant in August 2021, at 1000 m from the coast, and appear to have a disjoint temporal distribution with zoeae of other crab species. Megalopae and juveniles were more abundant from September to November at all sampling sites and times. In 2022, collectors featured a dominance of megalopae at two of the sites (ca. 73%), while the site with the lowest salinity showed a dominance of juveniles (84%). For megalopae, comparative results from the intensive and the regular sampling campaigns showed that time of residence within collectors is just a single day. For juveniles, higher numbers were detected with increased deployment time, suggesting a need for enhanced habitat complexity., This project was funded by the Ministry of Science and Innovation (Grant PID 2020-118476RR-C21 by MCIN/AEI/10.13039/501100011033) and by the Universitat Politècnica de València (PAID-01-18), to which we are very thankful., info:eu-repo/semantics/acceptedVersion

The rapid spread of the invasive blue crab (Callinectes sapidus) in Mediterranean coastal ecosystems poses ecological and socio-economic challenges, highlighting the need to explore natural control mechanisms. We conducted controlled aquarium experiments to assess the functional size preference and feeding rates of two native Mediterranean predators: the common octopus (Octopus vulgaris) and the loggerhead sea turtle (Caretta caretta). Two sets of trials were performed: (1) size preference experiments, in which individual predators were simultaneously offered crabs of varying sizes; and (2) ad libitum feeding experiments to estimate daily consumption rates. Octopuses consumed only the soft tissues biomass (STB, ~54% of total crab biomass), reaching maximum intake rates of 454 ± 71 gWW·d−1 in adult individuals. Sea turtles ingested whole crabs, including the exoskeleton, with total consumption rates up to 815 ± 592 gWW·d−1 of total biomass (TB) in adults. Subadult octopuses exhibited shorter handling times for small crabs, although neither predator showed a statistically significant size preference. Our findings indicate that both predators can consume substantial amounts of blue crab biomass, but O. vulgaris—due to its higher feeding efficiency and ecological abundance—might play a more prominent role as a natural biocontrol agent. These results provide valuable insights for integrated management strategies aimed at mitigating the ecological impact of C. sapidus in the Mediterranean., This research was funded by the Ministry of Science and Innovation (Grant PID 2020-118476RR-C21 by MCIN/AEI/10.13039/501100011033). P. Prado was contracted under the INIA-CCAA cooperative research program for postdoctoral incorporation from the Spanish National Institute for Agricultural and Food Research and Technology (INIA)., info:eu-repo/semantics/publishedVersion

δ13C and δ15N patterns of the blue crab, Callinectes sapidus, and its potential diets, were investigated in eleven localities within four invaded regions of the Spanish Mediterranean (Catalonia, Valencia, Alicante, and Murcia) subjected to contrasting salinities and degrees of connectivity with the open sea. These regions host blue crab fisheries of variable importance that might be regulated by availability of food resources and local environmental conditions. When present, large adults and immature, subadult sizes of each sex were captured to evaluate possible differences associated to age movement across habitats. SIBER and MixSIAR were used to assess patterns of isotopic niche and dietary contributions. Results showed significant effects for all factors and interactions (except for Sex and Sex x Size in δ15N). The effect size in Region (ηp2 = 0.82 and 0.70, respectively for δ13C and δ15N) and Locality (ηp2 = 0.53 and 0.46), was substantially greater than that of crab Size (ηp2 = 0.37 and 0.21) and Sex (ηp2 = 0.06 and 0.02), concurring with increasing levels of isotopic niche overlap (9% and 11% in Locality and Region, 32% in Size and 44% in Sex). MixSiar results indicated strongly variable contributions from food sources at each locality (TPs from 2.3 to 3.6), but with higher quantity of fish, algae, and crustaceans (27.7%, 18% and 15.1%). Dietary results showed little relation with isotopic patterns, whereas significant associations were found between local salinities and signatures in both crabs (R2 = 0.518 and 0.757, for δ13C and δ15N) and diets. Overall, our study suggests that blue crab habitat use in small Mediterranean estuaries might largely differ from native areas, with movements being mostly restricted to young individuals and/ or certain localities with higher connectivity with the open sea (e.g., the Ebro River). Salinity conditions emerge as a major variable shaping isotopic patterns of populations on a large scale., Funding was provided by the Spanish Government (Ministry of Science and Technology) under the ECESIS project (Subproject 1: PID2020-118476RR-C21 to P. Prado and S. Falco; and Subproject 2: PID2020-118476RR-C22 to E. Guijarro and C. Barberà). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript., info:eu-repo/semantics/publishedVersion