Eight years ago, zebra mussels from Russia infested the waters of Lake Erie, glomming onto native clams and suffocating them. Now, scientists report that some clams have survived by burrowing into soft mud that suffocates the zebra mussels attached to their shells. The findings, reported in the current issue of Nature, suggest that some species threatened by the rapacious zebra mussel might be able to avoid local extinction after all.
Researchers stumbled across the stalwart clams in a project to restore Metzger marsh on the western shore of Lake Erie. During an ecological survey before construction of a dike, Jerrine Nichols, a benthic ecologist at the U.S. Geological Survey in Ann Arbor, Michigan, says she found five clams in a protective sandbar. But after workers began draining the wetlands, her group unearthed some 7000 clams of 21 species buried in the muck. Nichols says this unexpected cache represents the range of Lake Erie's normal diversity before zebra mussels invaded after crossing the Atlantic as stowaways in ship ballast water.
The warm waters in the marsh and its mucky, "puddinglike" bottom appear to have conspired to save the clams. Water in most parts of Lake Erie in summertime hovers around 22 degrees Celsius, while that in the wetland is about 5 degrees warmer. The clams, preferring cooler temperatures, burrow a few meters into the muck. Low levels of oxygen at these depths suit the clams fine--but kill zebra mussels. In the lab, Nichols says, clams in 22-degree water are less inclined to burrow.
The finding is "a ray of hope," says Fred Snyder, a fisheries biologist at Ohio State University in Columbus, who says biologists have worried that the zebra mussel might entirely wipe out native species. Snyder says that it might be possible to manage wetlands in the Great Lakes region to ensure the protection of isolated pockets of native clams, although he doubts the native species will ever wrest back the Great Lakes from zebra mussels entirely. Nichols's team, meanwhile, is boarding the clams in laboratory tanks, and will return them to the wetland after the restoration project finishes in a few years.