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The Atlantic blue crab Callinectes sapidus is to date widely distributed in the Mediterranean Sea, where is recognized as an Invasive Alien Species. Noticeably, while records of its occurrence are increasing, quantitative information on its functional role in the benthic food webs of invaded coastal habitats are scarce.Here, carbon and nitrogen isotopic signatures were measured in C. sapidus and other representative taxa of the benthic flora and fauna sampled in summer in the Parila Lagoon, a coastal basin located in the Neretva River estuary area (Croatia). d15N values of individual blue crab specimens were used to calculate their trophic position (TP) , using the filter feeder mussel Mytilus galloprovincialis as the isotopic baseline. In general, the isotopic signature of C. sapidus was comparable with those characterizing invertebrate and fish predators (e.g., Hexaplex trunculus, Gobius niger). An average  trophic position of 3.73 was estimated for the blue crab population. However, a considerable inter-individual variation was observed in TP values, significantly related with the size of the specimens. Interestingly, a hump-shaped quadratic polynomial resulted the best fitting model for the data, characterized for small-size individuals by a positive body size-TP relationship, leveling or even turning negative for larger adult crabs. The interplay between environmental factors and ontogenetic diet shifts in determining the trophic position of the blue crab and, ultimately, its ecological impact on the benthic community of the Parila Lagoon is discussed.

The blue crab Callinectes sapidus, native to the western coasts of the Atlantic Ocean, has been introduced in Mediterranean waters where it is currently considered an invasive species. Here, we verified the occurrence of an established population of C. sapidus in the Torre Colimena basin, a Mediterranean habitat located in SE Italy, and provided a first assessment of its functional role in a Mediterranean coastal ecosystem. Crab traps were used to estimate the species abundance at a seasonal frequency; in addition, its trophic position was estimated in summer using nitrogen stable isotopes. Estimations were performed using both a generally adopted value of 3.4 % for the nitrogen trophic level fractionation factor D15N, and species-specific\ud estimations obtained from the literature. C. sapidus was sampled throughout the year, reaching peak abundances in summer. This result, taken together with the observed patterns of variation in the average body size and sex ratio of captured specimens, provided evidence of a fully established population in the Torre Colimena basin. The estimated trophic position of the crab was consistent with the literature information on western Atlantic populations,\ud and resulted, irrespective of the fractionation factor used, significantly provided novel information on the occurrence and potential functional impact of this non-indigenous species in Mediterranean coastal habitats, highlighting current knowledge lacunae and identifying future research lines on marine\ud alien species. than those characterizing an autochthonous brachyuran (Pachygrapsus marmoratus) and a fish predator (Sparus aurata). The present study provided novel information on the occurrence and potential functional impact of this non-indigenous species in Mediterranean coastal habitats, highlighting current knowledge lacunae and identifying future research lines on marine alien species

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