10 June 2008

A friend went to Bar Harbor and all I got was Littorina littorea

Scale is in millimeters.

When Judy and her husband Bob returned from their vacation in Bar Harbor, Maine in September 2007, they brought back for me a couple of snails preserved in alcohol. Only recently did I have a chance to examine and identify the snails as Littorina littorea, a gastropod of the rocky intertidal.

This provided the impetus to read about an ongoing controversy in American malacology centered upon this species: is L. littorea native to the northeast coasts of North America or was it introduced by humans from Europe?

Live L. littorea was first found in North America in 1840 on the shore of the Gulf of St. Lawrence in Canada. By 1890, the snail had arrived at New Jersey coast about 1,300 km to the south. The rapid expansion of its range is usually considered to be a strong indication that it was brought intentionally from Europe as food (they are edible) by 19th century European settlers or even much earlier by the Vikings.

Widening of the range of L. littorea along the coast of northeast America. Map from Bertness, M.D. 2007. Atlantic Shorelines, Princeton U. Press; original from Carlton, J.T. 1982. Malacological Review, 15:146.

Recent papers arguing for the nativeness of L. littorea is Cunningham (2008) and for its exoticness are Chapman et al. (2007; 2008).

The main argument of Chapman et al. (2007), in summary, is the following:
No available genetic evidence demonstrates that North American L. littorea were isolated from Europe for millennia. The low allozyme and haplotype diversity of North American L. littorea is characteristic of a recent (past 200–1,000 years) introduction...
Whereas Cunningham (2008) claims that
Taken as a whole, the DNA sequence data supports the many sub-fossil reports of an American L. littorea population in the Canadian maritimes that preceded even the first visits by the Vikings.
The most definitive answer will perhaps be provided if fossil shells predating the Viking visits are ever located somewhere in Canada or the northest U.S.

John W. Chapman, James T. Carlton, M. Renee Bellinger & April M. H. Blakeslee. (2008). Premature refutation of a human-mediated marine species introduction: the case history of the marine snail Littorina littorea in the northwestern Atlantic. Biological Invasions 9:737-750. [Curiously, another copy of this paper with a different abstract was also published in Biological Invasions 9:995-1008.]

John W. Chapman, James T. Carlton, M. Renee Bellinger & April M. H. Blakeslee. (2008). Parsimony dictates a human introduction: on the use of genetic and other data to distinguish between the natural and human-mediated invasion of the European snail
Littorina littorea in North America. Biological Invasions 10:131-133.

Cunningham, C.W. (2008). How to Use Genetic Data to Distinguish Between Natural and Human-Mediated Introduction of
Littorina littorea to North America. Biological Invasions 10:1-6. pdf

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