For a wide group of marine pests, invasion is coupled with a marked increase in body size, a new study has found.

Edwin (Ted) Grosholz, UC Cooperative Extension specialist at UC Davis, and Gregory M. Ruiz of the Smithsonian Environmental Research Center in Edgewater, Md., compared the body sizes of 19 species of nonnative marine and estuarine invaders. These included crabs, shellfish and starfish, in their native habitats and other parts of the world where they have become invasive pests.

Twelve of the 19 showed increases in maximum size of up to 40%. European green crabs and Chinese mitten crabs, both prominent nuisance species in U.S. waters, were about 20% bigger than in their native habitats. Only one, the gem clam, showed any sign of a decrease. The increases in body size were not clearly linked to differences in latitude between the native range and invaded areas or to the length of time since invasion.

The changes could be because the animals are no longer held back by predators or parasites, Grosholz says. “Animals and plants that are innocuous in their home environment can become rampaging pests when they invade a new area.”

The results could have implications for understanding both how modern-day nuisance species become successful, and for interpreting fossil evidence of changes in populations of marine animals over millions of years.

The findings, published in the August 2003 Ecology Letters, appear to be unique to marine animals, as research in other taxa shows invading species both increasing and decreasing in size. “For example, the data for European plants invading California suggests that nearly 30% of invading species got smaller in the introduced range,” Grosholz and Ruiz wrote.