Both species are thought to have arrived in North America in the ballast of trans-Atlantic cargo ships. By 1991 they appeared in the Hudson River, and within a year there were 500 billion between Troy and West Point, said David L. Strayer, an ecologist with the Cary Institute of Ecosystem Studies in Millbrook, N.Y.

The tiny mussels became a dominant species in the Hudson. Not even counting their shells, their total weight exceeded that of all the fish, plankton and bacteria combined, Dr. Strayer said, adding that they filtered “a volume of water equal to that of all the water in the estuary every one to four days.” There were no natural enemies to keep them in check.

None, that is, except scientists like Dr. Molloy. His fascination with water goes back to childhood summers on Lake Hopatcong, in New Jersey, where his father, an Irish-born lieutenant in the New York Fire Department, had built a cottage.

In 1956, when Dan was 8, his father and five other firefighters were killed when a wall collapsed in a storefront blaze — still the department’s worst disaster in the Bronx. Their children were given scholarships to Fordham University; the young Mr. Molloy got his bachelor’s and master’s degrees in biology at Fordham, and in 1972 entered the doctoral program at the State University College of Environmental Science and Forestry at Syracuse.

There he began working with State Museum scientists on one of their most vexing research challenges: finding an environmentally safe way to control a plague of black flies in the Adirondacks, where swarms and slashing bites were making life and tourism unbearable in the spring and early summer.

He began by converting a ramshackle fish hatchery in Cambridge, about 40 miles north of Albany, into what is now an internationally recognized field laboratory.