Bivalve takeover: once-benign clams boom after crab influx.

In a rare analysis of one marine invader benefiting an earlier arrival, an ecologist says that European green crabs invading a California bay have triggered a population explosion of a previously marginal clam.

The European green crab gobbled up two species of native shellfish, says Edwin D. Grosholz of the University of California, Davis. After considering decades of monitoring data and the results of his recent experiments, Grosholz blames this feast for removing competition that had kept the eastern gem clam in check. This clam had lived in the harbor for 50 years or so, but it's only now replacing the native species.

"The key point is that the results of invasions are not easily predicted," comments James T. Carlton of the Williams College--Mystic Seaport Maritime Studies Program in Mystic, Conn. He says that the new results support toughening of protections against alien species in many places.

Typically, some 90 percent of immigrant species do no obvious harm to their new homes, says Grosholz. Ecologists had put gem clams in that group after the East Coast species hitchhiked to the West Coast in shipments of oysters a century ago. The clams, barely half a centimeter across, have occasionally been noticed for decades in California's Bodega Harbor near San Francisco Bay.

In contrast, the European green crab has long been recognized as a menace to marine systems (SN: 6/13/98, p. 373). It showed up in Bodega Harbor a decade ago. Grosholz' work in the harbor documented that the crabs devastated populations of two small native Nutricola clam species.

Grosholz now reports that in tests, the crabs strongly prefer the native clams to the gem clams. He also set up competition experiments between native clams and the gem clams. He found that the gem clams grew only half as fast in crowds of native clams as they did in dense colonies of their own. However, the low densities of the native clams currently observed at Grosholz' study site didn't suppress growth of gem clams in the tests, he reports in the Jan. 25 Proceedings of the National Academy of Sciences.

Grosholz checked various environmental factors, such as water and air temperatures, but he found no explanation in those data for the ascent of the gem clam.

This isn't the first example of a new invader trigging a population boom in a previous invader, says Dan Simberloff of the University of Tennessee in Knoxville. "But this was a particularly nice one," he says.

Carlton emphasizes a different aspect of the marine invasion. It's an indirect effect: A (the new invader) affects B (the local species), which affects C (the earlier invader). He notes that cases of such indirect benefits among invaders "are probably much more common than now documented."