The spread of the Atlantic blue crab Callinectes sapidus Rathbun 1896 in the Mediterranean Sea: analysis of environmental and trophic niche and metal levels at different spatial scales

Biological invasions are globally acknowledged as one of the major causes of biodiversity loss. Considering the urgency of understanding what are the effects of biological invasions on recipient ecosystems, this research focuses on the Atlantic blue crab Callinectes sapidus, one of the most successful invaders of Mediterranean coastal ecosystems. First, a general assessment of the overlap of the Grinnellian niche of the species in native and invaded ranges was carried out to verify whether the blue crab maintained the characteristics of its climatic niche when establishing in invaded areas (niche conservatism hypothesis) or, alternatively, it adapted to the specific abiotic characteristics of recipient environments (niche shift hypothesis). The results showed a low degree of niche overlap between native and invaded ranges, highlighting that a niche shift occurs and indicating that the species not only colonized the areas where the oceanographic conditions reproduced those occurring in native ranges, but was also able to adapt to novel environmental contexts occurring in the Mediterranean Sea. Subsequently, a study of a newly recorded population within the Stagnone di Marsala, a marine coastal area in north-west Sicily (Italy), was performed. At first, an investigation of the structure and seasonal dynamics of the population within this basin was performed. Thereafter, an analysis of the isotopic niche and trophic position was conducted comparing C. sapidus with two autochthonous brachyurans, Carcinus aestuarii and Eriphia verrucosa in order to clarify the trophic role of the blue crab and potential interaction with other crabs. Results showed that in this marine coastal area the blue crab is much abundant in the northern part of the basin during summer, and males and females are not uniformly distributed. Moreover, no relationship among abundance of the species, and abiotic (salinity and temperature) and biotic (macrophyte biomass) factors investigated was found. The study revealed that, even if is not as abundant as in other Mediterranean area, this species is largely capable of establish and reproduce even in a habitat where environmental characteristics (e.g., salinity and temperature) are remarkably different from those it experiences in its native environment as well in other areas to date invaded. Furthermore, the results of the analysis performed on the isotopic niche and trophic position of the three brachyurans species suggest that they exploit different food sources and have considerably diverse trophic habits. Specifically, the different position of the niches in the isotopic space, the negligible overlap and the relatively different trophic positions of the three species, suggest a repartition of the niche. Moreover, a mixing model performed to elucidate which sources are the most consumed by C. sapidus, highlighted a general preference for animal prey, although characterized by a considerable spatial and seasonal variation likely influenced by the temporal and local availability of animal and vegetal trophic resources. Eventually, an assessment of Callinectes sapidus as a biomonitor species was carried out, through the evaluation of metal concentration in blue crabs sampled in five locations across the Mediterranean Sea. Subsequently, a comparison with bivalves, the most used marine organisms for biomonitoring purposes, was performed to check whether the blue crab is able to provide reliable information on metal contamination in the environment. Results indicated that its omnivorous trophic habits may represent an important factor affecting trace metal content and, ultimately, its ability to provide information consistent with those that can be obtained from bivalves. The results emphasized that the trophic plasticity of blue crab and other invertebrate taxa proposed for biomonitoring purposes may represent an open issue in environmental toxicology. Accordingly, an advanced analysis of trophic habits through stable isotope technique may help to understand the observed patterns of variation in contaminant levels. Overall, the findings of the present thesis provided an advanced perspective on the invasion biology and ecology of Callinectes sapidus, in particular addressing previously unexplored knowledge voids regarding the variation and expansion of the species, its population structure and dynamics as well its trophic role in a recently invaded marine coastal area, and the potential use of the blue crab as a bioindicator. These results are an additional step towards a deeper knowledge of this invasive species, an essential aspect in the implementation of successful management strategies for the control and mitigation of its impact on Mediterranean Sea ecosystems.