BioEd Online

Researchers have renewed a century-old debate over whether the common periwinkle, a sea snail that has reshaped the ecology of much of the east coast of North America, was imported from Europe by humans.

Many considered the matter settled five years ago, when a team of ecologists argued that the genetic differences between European and American periwinkles showed that they had been in America for at least 8,000 years, well before any Europeans crossed the Atlantic.

But a reanalysis of these data challenges that conclusion. Some ecologists have changed their position; others are sticking to their guns.

Littorina littorea was first spotted in North America in Nova Scotia in the 1850s. Within 30 years, the winkle, as it's known in the United Kingdom, had swept south as far as New Jersey.

The snail grazes algae from rocks, keeping its preferred species in check and letting unappetizing species run rampant. And winkle shells are the preferred real estate of hermit crabs. "There are billions of these snails — they are major ecological engineers," says James Carlton, a co-author on the reanalysis¹ and a marine ecologist at Williams College in Mystic, Connecticut.

The rapid expansion suggested a newly introduced, invasive species. But in the 1960s, archaeologists excavating Viking camps established in the Americas more than 1,000 years ago found two winkle shells in the strata beneath the camps. Many decided that the snail was an old American native that had gone unnoticed.

Missing evidence

In 2002, to date the winkle's American landfall, marine ecologist Clifford Cunningham at Duke University in Durham, North Carolina, and his colleagues sequenced DNA from European and North American winkles².

If the North American winkles had a recent European ancestor, many of the sequences should have matched. But they were too different to support this hypothesis.

Now, Carlton's team has picked back through the previous evidence. The age of the two apparently pre-Viking shells was originally inferred from where they were found. But when the researchers asked to see the shells again, they were told they were missing.

"The museum couldn't find the specimens," says team member John Chapman at Oregon State University in Newport. Without direct dating of the shells, Chapman argues that the archaeological evidence is weak. He believes the Vikings brought the snails with them.

Carlton's team also reworked the genetic analysis. Writing in Biological Invasions, they calculate that the European samples used in the original study represented only half of the genetic diversity of European winkles. It's possible, says Carlton, that the North American snails have an as-yet-undiscovered European ancestor. He thinks the later European colonists brought L. littorea to the East Coast.

John Wares, first author on the original genetic analysis and now a marine ecologist at the University of Georgia in Atlanta, agrees with the re-evaluation. "At this point, I'd prefer to work with the simplest explanation until it is proven otherwise," he says. "L. littorea was probably accidentally introduced sometime in the early nineteenth century."

But Cunningham, while recognizing some limitations of the earlier paper, stands by the previous findings. "Pending the collection of more genetic data, the evidence continues to support the hypothesis that extant American populations of L. littorea were established long before European exploration," he writes³, citing the fossil evidence as one line of support.