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.

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.