Successful invasion must be viewed as the result of a unique sequence of events, with the established species overcoming a number of previously prohibitive obstacles, for example lack of dispersal vectors, habitat characteristics and environmental conditions of the new area, and the ability to persist in interspecific interactions in the new community. The Japanese skeleton shrimp, Caprella mutica, is proving to be a highly successful non-native crustacean in coastal waters outside its native range having overcome these obstacles. In the past 40 years, C. mutica has spread from its native sub-boreal waters of north-east Asia to numerous locations in both the northern and southern hemisphere, where it has successfully established self-sustaining and thriving populations. After its first European record from the Netherlands in 1995, C. mutica spread rapidly within the North Sea and later to the west coast of Scotland and to Ireland in less than 15 years. Caprella mutica is generally associated with man-made structures and can be found in abundance on boat hulls, floating pontoons and aquaculture infrastructure clinging to fouling organisms.
Species-specific traits which enable C. mutica to perform superiorly in its introduced ranges include: rapid growth, short maturation time, high reproductive activity, broad tolerance towards prevailing environmental conditions, omnivorous feeding behaviour and efficient dispersal and distribution. These traits provide excellent prerequisites for any species to colonise a variety of different microhabitats. The availability of suitable structures, however, is of paramount importance to the invasion success of C. mutica in European coastal waters and most likely elsewhere. On artificial structures directed away from the seabed, C. mutica is able to avoid benthic predation pressure and thus, to develop massive population densities. These high abundances in disturbed habitats may also reflect the species’ ability to inhabit fluctuating systems better than other species. Caprella mutica was found to be competitively superior over native congeners and to show aggressive behaviour towards conspecifics. The characteristics described here are surely not unique to C. mutica and, therefore, cannot explain its success relative to other potential non-native species. They do, however, provide insight into why C. mutica has been so successful and consequently in determining whether another species may also be successful. When making these assessments, it is of particular importance to consider potential risks of the species and how important prevention or eradication may be. Although C. mutica can be assigned a potentially harmful invader, detailed knowledge on community or ecosystem level impacts are still lacking. Recommendations on management and control actions, therefore, remain insufficient.