The white shells of the marsh periwinkle snail, Littoraria irrorata, are hard to miss on marsh sediments in winter and on the stems of cordgrass in summer. This snail is a close relative of the periwinkle snails of intertidal rocky shores; like the northern species, it lives on algae and fungi growing on surfaces. Periwinkle snails spend most of the summer rasping filamentous algae and fungal filaments from Spartina stems and leaves. But when the tide is out, the snails can’t stray too far from the damp mud of the marsh surface; like fiddler crabs, they need the mud's moisture to rewet their gills.
Photo: Two periwinkle snails on cordgrass leaves in a Georgia salt marsh. By E. Sherr.
In one square yard of Spartina marsh, there are usually about 50 to 300 periwinkle snails. The snails are tasty, and easy prey for hungry hunters. At high tide, the marsh snails crowd together on the tallest stems of marsh grass in order to escape predatory blue crabs and larger fish that invade the marsh with the incoming water. At low tide, clapper rails and terrapin turtles stalk the cordgrass for a meal of live escargot.
Periwinkle snails climb Spartina leaves to escape predators at high tide. "Marsh periwinkle 001" by Mary Hollinger, NESDIS/NODC biologist, NOAA. - NOAA Photo Library: line2637Uploaded by JoJan. Licensed under CC BY 2.0 via Wikimedia Commons -
But these snails have a darker side. In the late 1990's and early 2000's, a strange phenomenon struck southeastern salt marshes, from South Carolina to Louisiana. Swaths of Spartina plants died, turned brown, and then decayed and washed out of the marsh, leaving a bare mud plain.
Scientists weren't sure why this dieback happened; there were several theories which invoked excessivel heat and salt stress during a time of severe drought in the Southeastern US.
Brian Silliman, then a PhD student who had studied salt marshes for years, knew a lot about the interactions of marsh plants and animals. One startling thing he had found in his research was that the abundant marsh periwinkle snails "farmed" nutritious fungi they liked to eat. During high tide, the snails crowded on cordgrass stems to avoid predators swimming in from the creeks. They continued feeding by rasping the juicy outer layer of Spartina stems, making small wounds in the plant. Fungi present on the surface of the stems would grow in these rasped-out holes. The snails would repeatedly return to feed on the fungi and make the stem wounds larger. The Spartina plants could usually stay healthy and continue growing despite the fungal infection. But if the cordgrass plants were stressed, the fungal growth would overwhelm them and they would finally die.
Silliman and his colleagues discovered that this was the case: the fungi-farming snails played a role in the die-off of Spartina in at least some of the cases. In salt marshes in South Carolina, Georgia, and Louisiana, they often found huge periwinkle snail populations in marsh patches that exhibited browning, the first stage of the dieback. The researchers observed "snail fronts," hordes of hungry gastropods advancing from barren, grazed-over marsh into adjacent healthy cordgrass stands. In these outbreaks, instead of dozens or a few hundred snails, there were at times thousands per square yard of white shells crowding the grass stems.
Snail-front of thousands of marsh periwinkle snails over-grazing cordgrass during a salt marsh dieback event. Photo provided with permission by Dr. Brian Silliman.
When periwinkle snails were removed from fenced test plots, cordgrass plants started to sprout and grow back. Where the snails remained, the marsh was eventually reduced to bare mud. Even though the drought in the southeastern US had abated by the time of their study, Silliman and his coworkers (reference below) concluded that the snails had contributed to an overwhelming fungal infection of heat- and salt-stressed marsh plants during the dry years. The snail-farmed fungus grew luxuriously on the dying cordgrass plants, yielding a huge food bonanza for the snails.
After a resulting "snail boom," armies of snails swarmed over the marsh, continuing to mow down the marsh grass even after the drought was over. In some of the diebacks that Silliman studied, the grazing snail hordes extended the initial patches of barren marsh by over 200 percent. Although not all cases of marsh browning and dieback have been accompanied by snail outbreaks, the runaway consumption of cordgrass by massed snails has been observed in numerous salt marshes. If drought returns again, so might the snail booms.
Silliman, Brian R., Johan van de Koppel, Mark D. Bertness, Lee E. Stanton,and Irving A. Mendelssohn. "Drought, Snails, and Large-Scale Die-Off of Southern U.S. Salt Marshes." Science 310, no. 5755 (2005): 1803-6.