The New Zealand mud snail has infested watersheds in the Santa Monica Mountains, posing serious threats to native species and complicating efforts to improve stream-water quality for the endangered steelhead. Within a period of four years, the snails expanded from their first known population in Medea Creek in Agoura Hills to nearly 30 other stream sites. Researchers at the Santa Monica Bay Restoration Commission believe that the snails' expansion may have been expedited after the mollusks traveled from stream to stream on the gear of contractors and volunteers.