NEW YORK, Jan. 21 (UPI) -- Native mussels are beating back invasive zebra mussels in New York's Hudson River, scientists say, although they're not sure what is causing the turnaround.
Zebra mussels -- striped, nickel-sized mollusks native to western Asia -- first appeared in the United States in 1988 as stowaways in ship ballast water. Their tendency to starve out native invertebrates and foul equipment has made them serious aquatic pests, AAAS ScienceMag.org reported Friday.
Zebra mussels hit the Hudson hard in 1991, quickly gobbling up most of the river's plankton while native mussels, clams, and other invertebrates plummeted to as little as 1 percent of their original populations.
"It looked really, really grim," David Strayer of the Cary Institute of Ecosystem Studies in Millbrook, New York says.
Then around 2001, Strayer says, the native mussels stopped declining.
Researchers feared it was a temporary recovery, but the trend persisted, and in 2007 scientists reported a solid, albeit incomplete, comeback, although they're not sure what might be driving it.
It could be native blue crabs or some other predator are eating more zebra mussels or their larvae, researchers say, or perhaps some undetected pathogen or parasite is keeping them in check.
The native invertebrates are approaching their pre-invasion numbers, scientists say.