Two Northwest Pacific crabs have almost simultaneously invaded Northeast Atlantic shores. In the Wadden Sea, Hemigrapsus sanguineus and H. takanoi have become established on artificial-boulder shorelines. Peak densities of >100 crabs (>5 mm carapace width) per m2 were attained within 2-3 years of arrival. The invaders segregated by microhabitat, with H. sanguineus dominant on exposed shores and H. takanoi dominant on sheltered shores as well as colonizing beds of mussels and oysters on mudflats. A field experiment indicated H. sanguineus displaced juvenile native shore crabs Carcinus maenas from beneath-boulder substrate. However, this displacement appeared restricted to armored shorelines, with little to no effect on large populations of C. maenas that occur widely in shallow, soft bottom, habitats of the coastal zone of the North Sea. © 2013 The Author(s).