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Article: New Surprises from an Old Foe, the Zebra Mussel

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Dreissena polymorpha, the invasive zebra mussel, is now one of the most common freshwater animals in North America.

AMNH


In 1991, the larvae of a striped, thumbnail-sized European shellfish floated quietly into the Hudson River. Hordes of these mollusks, called zebra mussels, had already fouled water-supply pipes and the ecological health of the Great Lakes, the site of the mussel's introduction into North America in the water ballast of ocean liners in the mid-1980's. New Yorkers were justifiably worried, fearing that the "rogue species' out-of-control" spread would leave "no little baby fish left," or even "strangle the city."

While these horror-movie scenarios have not come to pass, the zebra mussel invasion has been both tenacious and transformative. Larvae have since floated to the end of the Mississippi River. Aided by humans, they've hopped waterways all the way to California inside bait buckets and on vegetation clinging to boat hulls, Today, the mussel is one of the most common freshwater animals in North America; an estimated 57 billion zebra mussels clutter the Hudson alone. Computer models predict that the species will eventually exploit its entire suitable habitat in North America. Few human impacts on the continent have been this extensive.

In an increasingly porous world of global transportation and trade, biological invasion is now one of the main drivers of environmental change globally, with estimated damage exceeding $1.4 trillion a year, or 5 percent of the world economy. The zebra mussel is the icon of invasion, helping launch extensive long-term studies, public education about alien species, and even a new scientific discipline, invasion ecology. Still, the mussel continues to surprise. 

"Our simplest expectation," says ecologist David Strayer, "was that what we were seeing in the mid- and late 1990's would continue forever. The zebra mussels had come in and eaten the Hudson River ecosystem, and it was going to stay that way." Apparently not. For several years now, Strayer and his colleagues have noticed that local mussels are only living half as long as they used to. Could the zebra mussel's reign in this river be ending?

Monitoring a Movement

Understanding biological invasion is a long-term affair. It's very difficult to assess the impact of an alien species without comparing years of field study with data on the ecosystem before the invader arrived. Luckily, the Hudson is one of the world's best-monitored rivers, with records of water quality going back well over a hundred years. Strayer, his colleague Stuart Findlay, and other ecologists at the Cary Institute for Ecosystem Studies in Millbrook, New York, have been scrutinizing the river's freshwater portion, the 150-kilometer tract north of the salty estuary portion at New York City, since the 1980's.

Throughout much of the invasion, the Cary team periodically sampled every 2 to 4 kilometers along this entire tract. At one monitoring station near Kingston, New York, the team still samples the water every other week during the ice-free part of the year. By logging levels of organisms at the base of the river's food chain—single-celled phytoplankton and larger zooplankton—as well as levels of dissolved oxygen, sediment, and other abiotic factors, the scientists are assembling a picture of the Hudson's ecological health before, during, and after the mussel invasion.

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Flowing from the Adirondack Mountains to New York Harbor, the Hudson River is wide, straight, and silty. Tidal currents that push from the river mouth keep the water well-mixed for much of its length.

AMNH


The Cary team noticed immediately how the species flourished once it entered the Hudson's silty, nutrient-rich currents. The smoking gun was a swift 80 percent drop in the river's phytoplankton, which the mussels feed upon. The zooplankton population also fell by half. "The phytoplankton crashed mid-September 1992," says Strayer. "We reckon the zebra mussel population took off just about September 1. It was almost instantaneous." The mass of all those mussels—about 550 billion of them—outweighed all the other animal and bacterial life in the river combined.

The River Reacts

While the mussel's initial impacts weren't a surprise (phytoplankton also crashed in the Great Lakes), the researchers couldn't anticipate exactly how the river's complex web of life would respond. The monitoring revealed a dynamic series of shifts.

After the invasion, the team noticed that growth rates of open-water fish such as shad, herring, and striped bass dwindled in zebra-mussel infested areas. Many of these fish also appeared far from their usual habitats, moving downriver to mussel-free areas to seek food. (Zebra mussels cannot tolerate much salt, so they have not colonized the brackish estuary at the lower portions of the river near Yonkers and New York City.)

Shallow-water fish, however, benefited from the invasion. With fewer phytoplankton to cloud the water, the Hudson's murky brew became about 40 percent clearer. Light could then penetrate deeper and stimulate production of aquatic plants. Shallow-water fish that prefer weedy environs, such as sunfish and large- and small-mouth bass, actually found more vegetated habitat after the zebra mussel invasion. These fish species have grown larger and have expanded their territory. "With almost all invasion cases," says Stuart Findlay, "there are winners and there are losers."

Another loser in this case was the river's native mussels, but not for reasons the scientists expected. The assumption was that local, endangered pearly mussels (which were gathered by the millions a century ago to make pearl buttons) would, like native mussels in the Great Lakes, become encrusted by the smaller zebra mussels and die. Yet in actuality, the zebra mussels have avoided the pearly mussels, preferring to adhere to bedrock, stones, and "clinker" (hardened residue from coal-fired steamships) on the river bottom. Still, native mussel populations in the Hudson dropped by 70 percent soon after the invasion, likely because the invaders were consuming much of their food.

For Strayer, the possibility was real that the mussels could cause pearly mussels to go the way of the passenger pigeon. The last individual pigeon was named Martha and lived in the Cincinnati Zoo. She died there in 1914. "At one point, I thought I was holding the mussel version of Martha in my hand, wondering if this was the last living animal of this species I'd ever see," says Strayer. "It turned out we were wrong."

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Scientists have noticed that zebra mussels in the Hudson River are shrinking, but they don’t know why.

AMNH


Curiously, around 2000, the Cary Institute's surveys showed that the native pearly mussels were actually improving, despite persistently low phytoplankton. Then, about 2005, the researchers noticed that the age of the zebra mussels they were collecting shifted dramatically, from about seven years old to about two years old. "We don't see big zebra mussels out there anymore," says Findlay. "It's mostly the smaller, younger animals. And smaller mussels eat smaller food." Sure enough, abundances of the larger of the plankton types, zooplankton, shot up around 2005.

Researchers don't know why the Hudson's zebra mussel populations are shrinking after years of dominance. A predator—maybe sturgeon, blue crabs, ducks, or a combination—could be reaching abundances that can finally take a bite out of the scourge of shells. More data are needed to clarify the reasons behind the shift, but so far, no other waterway affected by the zebra mussels has seen a similar decline.

The Cary Institute researchers attempt to keep up with this constantly changing, complex ecosystem by monitoring patiently, regularly, and if budget permits, indefinitely. "If we'd stopped ten years after the invasion, we would have described only the acute impacts," says Strayer. "All that would have been true, but it would only have been part of the story." The zebra mussel's next move in the Hudson may take years, even decades, to uncover.

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