Why are freshwater clams disappearing?

In one species of freshwater clam, the female "goes fishing" in order to send her progeny afield, dangling what looks like a minnow at the end of a clear, yard-long line. This, however, is no ordinary minnow, but really a packet of clams in the larval stage. When a passing fish bites the lure, the clam larvae hitch a ride on it for purposes of transportation to distant stream sites. This unusual method of dispersing the next clam generation is one of a series of new findings about North America's little-known freshwater shellfish, notes David Strayer, an aquatic ecologist at the Institute of Ecosystem Studies, Millbrook, N.Y., who has reviewed research done by many scientists in the past few years.

This research has been spurred by the fact that freshwater clams are at higher risk of extinction than any other native plant or animal group in North America. Of the approximately 300 freshwater clam species in North America, 10% already are extinct and another 65% are at some risk of extinction. "It is likely that many clam species will not be around for any of our grandchildren, except as shells preserved in museums," Strayer indicates.

He and other scientists have been working on clarifying stages of the life cycle of these freshwater animals. Earlier research established that clam larvae live for a few days attached to a fish's fins or gills. The larvae don't swim or crawl, so they must wait for the right...