They're nearly always pregnant, like the mythical tribbles of "Star Trek" fame. They pass through gullets of fish unfazed. And they could bring disaster to native bugs, frogs and steelhead restoration efforts in the Santa Monica Mountains.
New Zealand mudsnails have taken over four watersheds in the Santa Monica Mountains and are spreading fast, expanding from the first confirmed sample in Medea Creek in Agoura Hills to nearly 30 other stream sites in four years.
The invasive species, found in many waterways in the U.S. West, the Great Lakes and Canada, reproduces asexually, so "it just takes one to infest a water body," said Mark Abramson, a stream restoration expert for the Santa Monica Bay Restoration Commission.
Mudsnails now infest the Malibu Creek, Trancas Creek, Ramirez Creek and Solstice Creek watersheds. They've claimed Malibou Lake, Malibu Lagoon and Cold Creek.
In an ironic twist, the spread of the infestation locally may be the result of environmentalists' efforts to improve stream-water quality in the Santa Monica Mountains, an effort Abramson spearheaded.
Equipment used by contractors and volunteers to help test water for groups such as Heal the Bay may have spread the mudsnails from stream to stream. The tiny mollusks cling to gear like fuzz to Velcro. Mudsnail infestations have cropped up where monitoring was completed.
"I wish I could say" it's not from monitoring, "but I can't," Abramson said. "It is really depressing for me. I had a horrible time at first."
Mudsnails don't look like much: An adult is about the size of a grain of rice. But one can produce 40-million clones, said Danuta Bennett, a researcher at UC Santa Barbara. They quickly drive out insects and tiny animals that provide food for frogs, birds and fish.
With their hard shells, mudsnails make a poor food substitute. Trout fed a diet of mudsnails get thinner. Tadpoles in mudsnail havens starve, Bennett said.
Being eaten hardly matters to a mudsnail: They can be excreted alive, ready to reproduce some more.
It gets weirder. Except for their size, the snails seem in every way suited to play the role of a menacing alien in '50s horror movies: They give live birth and all descend from a single super-clone -- one snail, in a sense, replicated millions of times -- said Mark Dybdahl, a biologist at Washington State University.
They eat algae but produce ideal conditions for further algal blooms, and they are freakishly tough, thriving in water brackish and fresh, polluted and pristine. They can live out of water 90 days.
Researchers once thought they could kill them with Formula 409 cleanser. But it turned out the cleaning fluid was no match for the menacing mollusk. "One thing after another," Abramson said.
Bennett and fellow researchers are working to combat the mudsnails by introducing the parasites that keep their populations in balance in New Zealand. But such bio-control strategies require extensive research to ensure they don't cause a new, exotic infestation.
So for now, there appears no way to eradicate them. Officials hope only to stop their spread.
Hikers, bikers and horseback riders in the Santa Monica Mountains are being asked not to travel from one stream to another without taking steps to shed the tiny hitchhikers.
"Don't go from stream to stream," Abramson said. "Bring a separate set of gear. Clean your bicycle tires. Clean your horses' hooves. Clean your boots, and if your shoes get wet, throw them in a dryer for a couple hours."
Mudsnails have spread throughout the West in the last decade but remain less well known than invaders posing clear economic danger, such as zebra and quagga mussels. In Los Angeles County, a few signs have been posted warning mountain visitors. Abramson said he has a couple of hundred more in his garage and is trying to convince various agencies to post them.
The bay restoration commission, Heal the Bay and others have moved aggressively to make sure monitoring is done safely. Abramson has 40 pairs of wading boots to use in different streams. He has devoted himself to studying mudsnails and fighting their advance. "I never heard of a damn snail before this," he lamented.
On a recent weekday, he waded into Medea Creek in a bandanna and fishing vest to check for mudsnails. He turned over a rock. It was peppered with tiny black specks: mudsnails, hundreds of them. He sloshed forward and picked up a second: The same. He held up his hands. They were covered with black snails. The youngest ones are the size of a grain of sand.
"You can feel this grit on your hands," Abramson said. "It's creepy."
After each stop, he changed his boots and spent several minutes cleaning off the used ones with a barbecue brush, then stuffed them into a color-coded plastic bag.
Throughout the morning, he chatted with passersby, including two National Park Service rangers at Paramount Ranch. None had heard of the snails. Abramson enlightened them and moved on.
"I try to be optimistic," he said. "Even though there doesn't seem to be any good coming from them or any way to get a handle on them."