Recent expansion of two invasive crabs species Hemigrapsus sanguineus (de Haan, 1835) and H. takanoi Asakura and Watanabe 2005 along the Opal Coast, France.

Nowadays, invasions of invertebrate species in coastal ecosystems have become an ineluctable and irreversible phenomenon. The recent introduction of two western pacific crustacean decapods—the Asian shore crab, Hemigrapsus sanguineus, and the brushclawed shore crab, Hemigrapsus takanoi—along the French coast has revealed the problematic effects that invasive species can have on biodiversity and also the competition that invasive species represent for native crab species. This study describes the distribution and abundance of both Hemigrapsus species along the Opal coast on the French side of the Dover Strait in spring 2008. Both species occupy habitats similar to that of the green crab, Carcinus maenas. However, the habitats colonised by the two species are clearly segregated: low hydrodynamic muddy habitats for H. takanoi and high hydrodynamic habitats with fine and medium sands for H. sanguineus. Both species can live in sympatry in harbours. In spring 2008, the maximum density outside Dunkirk harbour was 12 ind.m for H. sanguineus, and inside Dunkirk harbour, the maximum density was 60 ind.m for H. takanoi. In this location, H. takanoi dominated C. maenas significantly. No ovigerous females of either invasive species were found during the spring. Both species have high colonisation potential, ranging from south of the Bay of Biscay to Germany for H. takanoi and from the western part of the English Channel to Germany for H. sanguineus.