National Shell Museum collaborator Amy Tripp recently sent this great photo of a few Green Mussels, Perna viridis, on a dock piling at the south end of Marco Island. Young Green Mussels have a brilliant green color, and adult shells, reaching as much as 4 inches, are brownish with bright-green margins.
The Green Mussel originates from the Indo-West Pacific region, but was introduced, probably via ship hulls and/or ballast water, to the western Atlantic Ocean and other coastal waters of the world’s oceans. The first known occurrence of Green Mussels in the United States was in Tampa, in 1999. Since then, the species spread along the coast of the Gulf of Mexico and the eastern seaboard, apparently never approaching the high densities sometimes observed in its native areas, which can reach tens of thousands of individuals per square meter. (There are informal reports of the species first appearing in a given area, then disappearing after a couple of years, apparently due to low winter temperatures.)
Introduced species have the potential to compete with and negatively affect local species and, in the particular case of Green Mussels, to foul and clog marine equipment and to introduce parasites and diseases to other marine organisms. Please let us know in case you see Green Mussels in your local coastal areas!