The role of marinas and recreational boating in the occurrence and distribution of exotic caprellids (Crustacea: Amphipoda) in the Western Mediterranean: Mallorca Island as a case study

In the Mediterranean Sea, the number of alien marine crustacean species has increased over the past two decades. However, knowledge about small alien marine crustaceans, like caprellid amphipods, is still very scarce. To understand the role of marinas and recreational boating in the early step of the invasion process by non-indigenous caprellids, we studied the recreational boating pressure and the spatial distribution of caprellid species in Mallorca Island. We collected caprellids from 14 marinas and 9 exposed intertidal rocky shores between November 2011 and April 2012 and we analyzed the differences in habitat use of native and exotic caprellids. Eight caprellid species, six native and two exotic, were found. Alien caprellids were only present in marinas, reaching high densities of population. The analysis of recreational boating pressure reveals that Palma-Migjorn is the area that is subject to the highest potential risk of introduction of exotic species via ship fouling. In the secondary dispersal of alien caprellids, the study reflects that recreational boating seems effective as a secondary vector in the transport of exotic species from marinas to marinas but not from marinas to natural and exposed areas. An illustrated key of caprellids from Balearic Island is provided to differentiate native and non-indigenous species.