Wednesday, July 07, 2010

do parasites rule the world?

Discover | On a clear summer day on the California coast, the carpinteriasalt marsh vibrates with life. Along the banks of the 120-acre preserve, 80 miles northwest of Los Angeles, thousands of horn snails, their conical shells looking like miniature party hats, graze the algae. Arrow gobies slip through the water, while killifish dart around, every now and then turning to expose the brilliant glint of their bellies. Fiddler crabs slowly crawl out of fist-size holes and salute the new day with their giant claws, while their bigger cousins—lined-shore crabs— crack open snails as if they were walnuts. Meanwhile, a carnival of birds— Caspian terns, willet, plover, yellowleg sandpipers, curlews, and dowitchers— feast on littleneck clams and other prey burrowed in the marsh bottom.

Standing on a promontory, Kevin Lafferty, a marine biologist at the University of California at Santa Barbara, watches the teeming scene and sees another, more compelling drama. For him, the real drama of the marsh lies beneath the surface in the life of its invisible inhabitants: the parasites. A curlew grabs a clam from its hole. "Just got infected," Lafferty says. He looks at the bank of snails. "More than 40 percent of these snails are infected," he pronounces. "They're really just parasites in disguise." He points to the snowy constellation of bird droppings along the bank. "There are boxcars of parasite biomass here; those are just packages of fluke eggs."

Every living thing has at least one parasite that lives inside or on it, and many, including humans, have far more. Leopard frogs may harbor a dozen species of parasites, including nematodes in their ears, filarial worms in their veins, and flukes in their kidneys, bladders, and intestines. One species of Mexican parrot carries 30 different species of mites on its feathers alone. Often the parasites themselves have parasites, and some of those parasites have parasites of their own. Scientists have no idea of the exact number of species of parasites, but they do know one fact: Parasites make up the majority of species on Earth. Parasites can take the form of animals, including insects, flatworms, and crustaceans, as well as protozoa, fungi, plants, and viruses and bacteria. By one estimate, parasites may outnumber free-living species four to one. Indeed, the study of life is, for the most part, parasitology.

Most of the past century's research on parasites has gone into trying to fight the ones that cause devastating illness in humans, such as malaria, AIDS, and tuberculosis. But otherwise, parasites have largely been neglected. Scientists have treated them with indifference, even contempt, viewing them as essentially hitchhikers on life's road. But recent research reveals that parasites are remarkably sophisticated and tenacious and may be as important to ecosystems as the predators at the top of the food chain. Some castrate their hosts and take over their minds. Others completely shut down the immune systems of their hosts. Some scientists now think parasites have been a dominant force, perhaps the dominant force, in the evolution of life.