An invasive species native to New Zealand, Potamopyrgus antipodarum has become established at many sites in the western United States since the 1980s. In suitable aquatic habitat, especially in geothermal streams with high primary production, it can form dense colonies that crowd out the insect communities needed by immature trout and other native species. Its overall impact on algal resources is likely to affect entire stream food webs. With its protective shell, the mud snail provides little if any nutrition as prey and may pass through a fish alive. Scarcely a quarter-inch long, mud snails may cling to boats, waders, and other fishing gear by which they are inadvertently transferred to another watershed. Because the species can reproduce asexually, a single mud snail is all that is required to establish a new colony.