"An European species of whelk or sea snail established in North America on both East and West Coast (in eastern North America
from Nova Scotia to Virginia). Whether it is native to eastern North America has been debated since the late 1800s, but recent
work makes a compelling case: Littorina littorea invaded North America in the late 1700s or early 1800s, traveling from the United Kingdom or Ireland
to Nova Scotia, probably in rocks used as ballast (Blakeslee and Byers 2008; Brawley et al. 2009)."--
From: A.N.Cohen (2011): The Exotics Guide: Non-native Marine Species of the North American Pacific Coast. Center for Research
on Aquatic Bioinvasions, Richmond, CA, and San Francisco Estuary Institute, Oakland, CA.