Alien snails’ pace bodes ill for wild river : Tiny New Zealand mollusks swiftly multiply in Yellowstone river famed for fishing. It’s the latest ‘insult’ to ecosystem.

Lifting a long-handled net out of the Madison River, David Richards studied the mesh, inspecting little black dots.

“The first time I looked, I saw gravel,” said Richards, an aquatic ecologist in Yellowstone National Park. “Then I said: ‘That’s not gravel. It’s moving. It’s snails.’ ”

More specifically, a species of New Zealand snail that invaded the Snake River in Idaho in the mid-1980s and, more recently, prime fishing waters in Yellowstone National Park. Although smaller than BBs, the conical snails multiply so rapidly that some scientists worry they may become a Western version of the zebra mussel, a small clam that has engulfed the Great Lakes, clogging pipes and coating boat hulls.

“It has the exact same flavor” as the zebra mussel epidemic, said Daniel Gustafson, a research scientist at Montana State University in Bozeman. He avoids taking live snails to his laboratory because of the slim chance they could evade quarantine procedures and escape into local waters.

It’s not the snails themselves, but their numbers, that are frightening. In parts of the Snake River, they grow in mats as thick as carpet and clog irrigation pipes. While the invasion has received little study since the mudsnails showed up, probably in an intercontinental shipment of trout, five native snail species have been listed as endangered since their New Zealand relative arrived.

“The foreigners are ubiquitous everywhere and there are fewer and fewer natives,” said Terrence Frest, an independent expert on mollusks who works with power companies to assess environmental impacts of hydroelectric dams.

From the Snake River, the mudsnails probably hitchhiked on a bird or angler to Yellowstone, where they were first detected in 1994 by a power company that operates dams downstream.

Almost unknown to Yellowstone park managers, the creatures have rapidly taken over as the dominant form of invertebrate life in the Madison River, one of the West’s best-known fly fishing streams. Hot springs that warm the river seem to cultivate even more snails. A glance in the crystal waters reveals tiny black spots that are snails dotting rocks, reeds and aquatic vegetation waving in the current.

Which leaves less room for the insect larvae that hatch into stoneflies, caddisflies and mayflies that nourish the river’s prized trout population, as well as waterfowl.

“When you pick up a rock and it’s covered with snails that didn’t used to be there, it obviously leaves less room available for the native insect species that are supposed to be there,” Gustafson said.

Straddling the Continental Divide, Yellowstone gives the snails footing in drainages on both sides of the continent.

Even Xerox could have hardly created a better reproductive machine. All but 1 in 100,000 of the tiny mudsnails are females that reproduce without mating. They almost constantly churn out live young, which are born pregnant and armored with sturdy shells.

“They’re reproducers extraordinaire,” said Daniel McGuire, a biologist who was the first to identify mudsnails in Yellowstone.

Trout that eat the snails come away hungry. Studies have found that the mudsnail--unlike native species--can shut a trap door in its tough shell and pass through fish undigested, alive and ready to reproduce. It’s a double insult to the unsuspecting fish: They not only waste their time and energy gobbling snails, but the snails also occupy space that could instead hold a satisfying meal.

“It’s pretty much like eating gravel,” Gustafson said. “They short-circuit the food chain.”

Because mudsnails do not overrun their native New Zealand, natural forces must check their population there, said biologist Pat Clancy of the Montana Department of Fish, Wildlife and Parks. Options to control the snails may include poisons that target snails specifically and biological controls, such as parasites that prey on snails.

But an invasion of exotic lake trout--also known as Mackinaws--in Yellowstone Lake and the possibility of devastating whirling disease in fish populations already have beset Yellowstone Park managers. They admit they know next to nothing about the snails.

“This is really not a good time to be dealing with another alien insult,” said park resource management chief John Varley.

If agencies do not deal with the mudsnails soon, however, they may lose their chance, Frest said. At minimum, they should impose a strict quarantine to keep unwitting anglers and others from giving the snails rides elsewhere. “The more widespread they become,” he said, “the less likely you can do anything to stop them.”