Biological invasions can modify interactions among species and food web structure. A key question in invasion ecology is whether invasive species succeed by maintaining their trophic niche across geographic ranges (niche conservatism) or by exploiting novel resources and expanding their niche in recipient ecosystems (niche expansion). The blue crab Callinectes sapidus, native to the western Atlantic Ocean, has quickly spread in the Mediterranean Sea in recent decades, representing an ecological and socio-economic threat due to its predatory pressure.

This study compared the isotopic niche (δ¹³C, δ¹⁵N, δ³⁴S) - as a proxy for the trophic niche - between C. sapidus populations in native (three sites along the western Atlantic coast) and invaded areas (two sites in the Mediterranean Sea) to assess whether its invasion success reflects niche conservatism or expansion. Furthermore, since trophic niche dynamics are also shaped by interspecific interactions, the isotopic niche of C. sapidus was compared with that of sympatric crab species, to evaluate potential trophic overlap and competition. Sympatric crab species were Carcinus maenas, invasive within the native area of C. sapidus and C. aestuarii, native to the Mediterranean.

The blue crab showed a significantly broader isotopic niche in invaded areas, as indicated by higher values of the Bayesian standard ellipse area and Layman’s metrics, supporting niche expansion. Also, C. sapidus occupied a broader niche than both Carcinus spp., reflecting a higher degree of trophic generalism, and showed no significant niche overlap in either area, indicating resource partitioning between the species

Overall, the observed isotopic niche expansion highlights the high trophic adaptability of C. sapidus, which seems to limit direct trophic competition with sympatric Carcinus spp. by exploiting different trophic sources. Such high adaptability represents a key factor underlying the invasion success of the blue crab, potentially leading to the restructuring of food webs in recipient ecosystems.