“I can’t let you into the country with those.” The man at the U.S. Department of Agriculture checkpoint at the Los Angeles airport was talking about the freshwater snails-the all-important subjects of my master’s thesis research-that I was importing from New Zealand.
I smiled confidently and handed over a letter from the USDA stating that I didn’t need a permit. I also produced documents from the Centers for Disease Control certifying their harmlessness to human health.
He was not impressed, and my heart began to sink. He didn’t care that I had faxed my paperwork to them ahead of time or that I had spent countless hours over the previous weeks driving all over New Zealand and Australia collecting these snails. And he certainly wasn’t concerned that Prof. Mark Dybdahl, my advisor at Washington State University, would not be happy if I returned to the Palouse without our precious cargo.
I was not exactly an inconspicuous traveler. I lugged around a 55-pound portable freezer in a three-foot-tall, bright blue box. A little less obvious was my small soft cooler-the kind you might fill with sodas to take to the park-that held cold packs and a box full of snails wrapped neatly in paper towels. I didn’t expect anyone to understand why I cared so much about getting my 8,000 or so snails into the country, but I had hoped that my paperwork would hold some weight.
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.
There’s no question that an individual snail is better off without parasites. The most common of the 14 parasites is an unnamed trematode species that castrates its unlucky snail host. Like many parasites, this one has a complex life cycle, beginning when an unwitting snail eats a parasite egg. The egg hatches into a small baby worm that finds its way to the snail’s gonad (even the asexual snails have this sex organ). There, the parasite multiplies and forms hundreds of small round cysts that take over the inside of the shell, leaving hardly any room for the snail itself.
The parasite’s life doesn’t end there, however. In order for the parasite to become an adult and reproduce, an infected snail must be eaten by a duck or some other bird. In the duck’s gut, the parasite cysts hatch into adult worms. They mate, and the eggs are released in the duck’s feces for the next unsuspecting snail.
This might seem like a tough passage for the parasites, but they’re quite abundant despite such a circuitous life journey. In some New Zealand lakes, up to 50 percent of the snails are infected.
Is our invasive snail some sort of “super clone,” resistant to all parasites and therefore a better all-around invader? Is being parasite-free enough of an advantage to become invasive in a new region? Or is some other factor responsible for the transplanted snail’s success?
Whatever the answer to these questions, more and more people and agencies, including the U.S. Fish and Wildlife Service, the Idaho Department of Fish and Game, and the International Flyfishers Association, are interested in controlling snail populations. If we could find a parasite population capable of infecting the western U.S. clone, then we would know that the invader was not a completely resistant “super clone.” Also, we could investigate the parasite as a way to lower the snail populations here.
But using parasites to contain snail populations opens up a whole new can of worms, so to speak. By importing a new living thing to control an invader, the solution might become another problem if it has unintended consequences. On the other hand, if the “controller” lowers the invader’s numbers without harming anything else, then we could reduce the damage done by the snails. A lot of testing is needed to make sure biocontrol agents are both effective and safe before they are imported.
Meanwhile, none of these avenues of research-from studying why invasives are invasive to finding safe biocontrol agents-were going to be explored, if I couldn’t get my snails past the Los Angeles airport USDA checkpoint.
So I stalled, pointing out that we had followed the USDA and CDC rules. I was met with blank stares. “How do I know these snails are what you say they are?” the man asked. He and his sidekick pushed forms in front of me that meant they would be confiscating my critters.
Luckily, I was able to delay long enough to call my advisor, who said the magic words that let my portable freezer, my cooler full of live snails, and me get home to Pullman. Apparently, his words carried more weight than my forms. Still, I have to applaud the tenacity of the USDA employees. After all, their efforts are part of our defense against ecological invaders.