Green Crab Research Illuminates Value of Parasites

By GAIL GALLESSICH

Globalization of commerce, especially by ships and air traffic, transports hitchhiking plants and animals around the world. In many cases they become pests in the new location, according to an article earlier this month in the journal Nature.

One advantage that could explain their success is that the invaders often arrive without the parasites that hold them in check at home.

First author Mark E. Torchin, assistant research biologist at UC Santa Barbara, said that the authors analyzed 26 invading animal species–mollusks, crustaceans, fishes, birds, mammals, amphibians, and reptiles–chosen randomly, and found that, in general, the introduced populations had only half as many parasites as native populations.

"On average, an animal has 16 parasites at home, but brings less than three of these to new areas that it invades," said Torchin. "In the new region, parasites are not well matched to novel hosts, and only about four parasites will successfully attack an invading species."

For this research, Torchin teamed up with UCSB coauthors Kevin Lafferty, an assistant adjunct professor of biology and a marine ecologist with the U.S. Geological Survey; Armand Kuris, professor of biology; his graduate student, Valerie McKenzie; and Andrew Dobson, a professor of ecology and evolutionary biology at Princeton University.

Kuris explained that parasites are so pervasive that parasitism is the most common lifestyle on Earth. Many parasites not only make animals sick but they also may castrate them, change their behavior, or even kill them. By leaving parasites behind, introduced species have a strong advantage over less fit native competitors, which remain fettered by a full complement of parasites.

Borrowing from popular culture, Lafferty said, "Parasites are to invasive species what kryptonite is to Superman. On his home planet Krypton, kryptonite was a regulator, keeping Superman ordinary. Freed from kryptonite on Earth he gained super powers. But, unlike Superman who used his power for good deeds, invasive species can be devastating."

The European green crab (Carcinus maenas) is one voracious predator that has been studied extensively by Torchin, Lafferty, and Kuris. It has had a devastating effect on East Coast fisheries and is now threatening fisheries on the West Coast. It has also invaded Australia, Japan, Tasmania, and South Africa. The green crab eats Dungeness crabs, rock crabs, mussels, oysters, and clams.

Torchin, Lafferty and Kuris found that in Europe barnacles castrated the crabs. Where the barnacles were common, the crabs were small and rare. Conversely, crabs were big and abundant in areas where barnacles were uncommon.

According to the authors, bringing in parasites from a pest's native range can hinder superpests. The benefits to this organic form of pest control are sustainability, low cost, and reduced dependence on pesticides. But the researchers cautioned that biological control of pests is risky if the parasites are not specific to the target pest.

"Suitable biocontrol agents should be harmless to native species, just as kryptonite is harmless to Earthlings," said Lafferty.