IT’S ONLY A PUNY freshwater shellfish, foul-smelling and unpalatable. But it may be the most feared of all the thousands of exotic plants and animals taking root on this continent. It is of course the nattily striped zebra mussel,an East European native that has spread like Spice Girls around the East and Midwest since it was first spotted in Michigan’s Lake St. Clair 10 years ago. This audacious bivalve clogs power plants, water-system intakes, engines, and boat hulls, costing what the National Oceanic and Atmospheric Administration estimates will amount to $5 billion by 2000 just to clear and safeguard waterworks.

Scarier yet, new NOAA studies find that zebra mussels are drastically changing the ecosystem in Michigan’s Saginaw Bay. They gobble up the phytoplankton at the base of the food chain, upon which fish and humans eventually depend, leaving the waters stunningly clear. This clearing promotes the blue-green algae Microcystis, which is toxic to fish and humans.

The good news: No confirmed sightings of zebra mussels have been made west of Oklahoma, yet. The bad news: Unconfirmed sightings have been reported in Lake Washington (last summer) and on boats docked along the Ship Canal. California authorities, who check boats entering their state as part of their agricultural-pest inspections, have intercepted zebra mussels 11 times—once on a boat that had been towed through Washington. Worse yet, the mussels’ pinhead-sized larvae can be invisibly transported in bilge and ballast water, which is how they first arrived from Europe. “It’s not a matter of if” zebra mussels will show up here, says Sarah Reichard, an invasives expert at the University of Washington’s Center for Urban Horticulture. “It’s a matter of when.”

To try to forestall Great Lakes­style infestations, state Sen. Ken Jacobsen last week introduced a bill banning both the zebra mussel and European green crab (a voracious little shellfish ravager that has appeared as near as Coos Bay, Oregon). Jacobsen’s bill authorizes the eradication of infestations, quarantine of infested water bodies, and public education and other prevention measures. Its most potent provision would let the state Fish and Wildlife Department set rules (presumably inspections and decontamination) for boats, trailers, and seaplanes from infested areas, and fine owners who ignore them.

This bill is a more concrete sequel to Jacobsen’s failed proposal last session for a task force to address the full range of eco-invaders statewide. Scott Smith, Fish and Wildlife’s point man on aquatic nuisance species, sounds one hopeful note: Washington could become “the first state to [undertake a prevention plan] before the zebra mussels arrive.” One more hopeful note: Linda Chalker-Scott, another UW researcher, reports getting good results zapping the larvae with ultraviolet radiation. Perhaps one day UV chambers—a sterilizing version of tanning parlors—will join the weighing stations on highways into the state.