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Recent Dartmouth graduate is lead author on research paper concluding that parasites' impact extends beyond their hosts

Dartmouth College Office of Public Affairs • Press Release
Posted 05/17/07
Rebecca Bailey • (603) 646-3661

A parasite can do more than make life miserable for its host; it can also change the host's environment, according to an article by a team that included Dartmouth researchers.

The article, "Parasites alter community structure," published during the week of May 14 in the Early Edition of Proceedings of the National Academy of Sciences, was based on research conducted by its lead author, Chelsea Wood, Dartmouth Class of 2006, during the summer before her senior year.

Wood, whose undergraduate degree is in environmental and evolutionary biology, performed the research at Shoals Marine Laboratory as part of the lab's highly competitive undergraduate research program. She was co-advised by Kathryn Cottingham, associate professor of biological sciences at Dartmouth College, and James Byers, associate professor of zoology at the University of New Hampshire (UNH). Cottingham and Byers are both coauthors of the article.

The intertidal periwinkle snail, Littorina littorea, amid ephemeral algae and barnacles. (Photo courtesy Chelsea Wood)

Cryptocotyle lingua, a parasite that infects the snail Littorina littorea and causes it to eat less ephemeral algae, affecting the entire intertidal community composition. (Photo courtesy Chelsea Wood and James E. Byers)

The research examined Littorina littorea, a snail also known as the common periwinkle, which has become the dominant herbivore in the intertidal zone of the northwestern Atlantic coast since its introduction in the 19th century.

The snail eats ephemeral algae, which is softer and more palatable than hard, chemically defended perennial algae. It is also frequently parasitized by the flatworm Cryptocotyle lingua. Where shorebirds abound, so does C. lingua, which moves from snails, to nearshore fish, to the shorebirds that eat the fish. The shorebirds' feces contain the flatworms' eggs, which are eaten by the snails, completing the cycle.

Because C. lingua attacks the snail's digestive gland—as well as its gonad, neutering the creature—Wood and her colleagues reasoned that infected snails might eat different amounts of algae than uninfected snails. The researchers hypothesized that, in a given area, the abundance of ephemeral algae would be affected by the percentage of snails that were infected with the parasitic flatworm. Based on studies of other parasite-induced eating behavior, that could either mean the infected snails would eat more algae—just as intestinal worms can make a pet a voracious eater—or less algae, perhaps due to sluggish digestion or lower energy requirements because of the neutering.

The experiments showed that infected snails ate less algae, posing a puzzle. Often, a parasite causes its host to act in way that obviously contributes to the parasite's success as a species. For example, a parasite that makes its host eat more receives more nourishment and can grow faster. It isn't clear what, if any, benefit a parasite derives from causing its host to eat less, Wood said. "This may just be an incidental effect of parasitism, rather than one of the weird behavioral modifications we see in other host-parasite pairs," she said.

The research included a laboratory experiment in which the researchers placed one snail and an equal amount of ephemeral algae in each compartment of three plastic tackle boxes. After 13 days, they measured the amount of remaining algae in each compartment, and found that infected snails had eaten 40 percent less than uninfected snails. The team also conducted field research, using three bottomless cages set in the intertidal zone, variously containing infected snails, uninfected snails, and no snails, as a control. Over the three-and-a-half-week field experiment, ephemeral algae increased by 186 percent in the snail-free cages and by 59 percent in the cages of infected snails, and decreased by six percent in the cages of uninfected snails.

Wood explained, "Our experiments show that parasites in L. littorea potentially impact the entire intertidal community, including organisms, like algae, that never directly interact with the parasite. Since around 50 percent of the species on the planet are parasitic, this effect may exist in many habitats—not just the marine intertidal zone. So parasites are potentially very important and, at the same time, very overlooked."

Located seven miles off the coast of Portsmouth, NH, in an archipelago that is one of the region's biggest shorebird breeding areas, Shoals Marine Laboratory provided the ideal setting for such an experiment. While C. lingua infects approximately 10 percent of L. littorea living on the mainland, around half the snails on 95-acre Appledore Island, where the lab is located, are infected.

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