New to the neighborhood? If you're a clam, being big helps carve out new turf.

The Invasion of the Giant Clams

BERKELEY, CALIFORNIA--Plants and animals hitch rides in planes and boats and invade new ecosystems around the world, conquering local critters. Predicting which species might run rampant has been difficult, but on 29 June, a trio of paleontologists here at the North American Paleontological Convention argued that size matters. Hefty clams were more likely than small but otherwise similar clams to have expanded their range.

Countless invasions have taken place in the geologic past. David Jablonski of the University of Chicago and colleagues looked for traits in marine clams that might distinguish invaders from other species that stayed put. First they looked at relatively recent data, for paleontologists--the distribution of 216 bivalve species along the California coast during the middle and late Pleistocene, between 750,000 and 10,000 years ago. Since then, 26% of the species have changed their range by at least 1 degree of latitude. These invaders didn't differ from other bivalves in most traits, such as whether they lived on the sea floor or embedded in sediment, or whether they spawned free-floating larvae. But they were bigger than species that didn't expand their turf.

The same pattern held when the group examined biological invasions that took place along the Gulf Coast of North America after the Cretaceous-Tertiary mass extinction, 65 million years ago. It was also true when they compared the median size of 25 invasive marine bivalve species that have become well established in recent history to 914 other species. Jablonski suggests that larger clams are so successful because they tend to produce more eggs and gametes--and consequently more larvae than can spread to and colonize new habitat.

"This is really a first for the marine realm," says Ted Grosholz, a marine and invasion biologist at the University of California, Davis. Knowing that size can help predict a species' tendency to take over a new ecosystem might help fisheries managers direct their attempts to prevent such disasters, Grosholz says.