The common periwinkle was first recorded in Nova Scotia in the 1840s and reached Long Island Sound by the 1880s, an unusually fast and well-documented spread. Its arrival changed the shoreline it colonised: heavy grazing on ephemeral green algae shifted the balance in favour of bare rock and perennial seaweeds, and altered the sediment and species composition of salt marsh edges.
The origin story is where the record turns awkward. The mainstream reading is a nineteenth-century introduction from Europe, in ship ballast or with fishing traffic. Against that, some shell material from earlier archaeological and geological contexts in Atlantic Canada has been advanced as evidence of a much older presence, and genetic studies have been read both ways.
The honest summary is that a European origin for the modern population is well supported, while a small pre-contact presence is not fully excluded. That the species is now a dominant grazer over hundreds of kilometres of shoreline is not in question, and it remains one of the most-cited cases of an invertebrate invasion restructuring a coastline.