The agent's orders were to stop anything alive, other than people, from getting into the U.S., and he was not about to waver, paperwork or no paperwork. His job is important: he helps protect the U.S. from plants, animals, and other living things that harm native species, crops, and livestock. Conservationists are especially interested in preventing the spread of these living things, known as invasive species, because they can cause other species to go extinct.

Ironically, this is exactly why I study the tiny New Zealand mud snails. They have already invaded the Snake River, Yellowstone National Park, and lots of other sites in the western U.S., including areas where endangered U.S. snails live. They can reach population densities greater than 300,000 per square meter, carpeting stream beds and changing the way nutrients cycle through the ecosystem. They've also invaded parts of Australia, Europe, and Japan. It was a little difficult though, in my panic at LAX, to explain all of this to the gentleman who wanted to confiscate my snails.

Potamopyrgus antipodarum is no ordinary snail, but of course I'm a little biased. In New Zealand, the snails are common in lakes scattered throughout the North and South Islands. Some are spiny, some are smooth, but they are all tiny-about the size of a lentil-and admittedly unimpressive at first glance. In these lakes, two types exist: those that reproduce sexually and those that reproduce asexually. Among the asexuals, hundreds of different lineages exist, whose offspring are genetically identical clones of the single female parent.

Only one of these clones has become invasive in the western U.S. Why? It was this question that led me down winding roads to hunt snails in remote New Zealand lakes. On the surface, collecting snails is a straightforward affair: walk knee-deep along the lake's edge, bend down, pick up a rock, brush snails into a net, and repeat ad nauseam. It was only after hours and hours of practice that I learned the subtler nuances, such as maximizing my speed, avoiding vegetation tangles in the net, and snorkeling for snails.

Sometimes the lakes were clear, the mountainous scenery was amazing, and I marveled that my research had brought me to such wonderful places. Other times, the rain poured down, the lakes were full of leeches, and I suffered for days from parasite-induced "duck itch," a condition resembling mosquito bites.

So why are some species-or in our case, one clone within a species-invasive? One hypothesis is that they have left their parasites, pathogens, and predators behind in their move to the U.S. Without natural enemies to keep their populations in check, the invaders proliferate. In New Zealand, 14 different parasite species infect the snails. In the U.S., none do.

Continued

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