Five native S. balanoides (1) being encroached on by the invasive, alien Austrominius modestus, young ones of which (2) have settled on top of S. balanoides and are likely to overwhelm them as they grow.
The two species have similar apertures and tergoscutal flaps, but A. modestus has only four wall plates, and has grey marks on the opercular valves.
North Wirral coast, Cheshire, England. June 2011.