If you want to see a freshwater harbormaster freak out, walk up and tell him that you just found a zebra mussel on a piling. The joke also works on biologists, fish and game wardens, utility plant managers, budget analysts and informed politicians. For the tiny zebra mussel, a thumbnail-sized bivalve from the Caspian Sea, has wreaked legendary destruction—to the tune of billions of dollars—on ecosystems and infrastructures all around the Great Lakes. On the West Coast, people tremble at the sound of its name.

In the 20 years since its arrival, probably on a ship’s hull or in its ballast water, the zebra mussel’s population has exploded to some 50 trillion in freshwater habitats ranging from the Hudson Bay to the Gulf of Mexico. Voracious filter feeders, they’ve “cleaned” the water of the Great Lakes too well, forcing light-sensitive fish and plants into deeper water. They’ve become a primary but low-quality food source for whitefish; zebra mussels lack a nutrient that whitefish need, so now the fish are smaller and slower to reproduce.

The Invaders

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Undaria pinnatifida: wakame, edible brown seaweed

Watersipora subtorquata: bryozoan, or moss animal

Batillaria attramentaria: Japanese mud snail

Hymeniacidon sinapium: orange sponge

Ficopomatus enigmaticus: Australian tubeworm

Carcinus maenas: European green crab

Zebra mussels are known to pile up three and four generations deep on the slenderest of surfaces, clogging pipes and harbors so thoroughly that Great Lakes utilities spend $200 million a year just getting rid of zebra mussels.

There have been a handful of sightings, but the zebra mussel has not yet invaded California—its arrival in our aqueduct system would be a nightmare. Its absence is probably due in part to a volunteer effort to keep it that way. But California has its own similar problems.

Here and elsewhere, invasive species are fast gaining recognition as a serious threat. It’s evident on land, where pernicious invaders like Cape ivy and ice plant crowd out native species. But it’s harder to know what’s happening underwater, and much harder, if not impossible, to eradicate a marine invader once it’s become established.

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CALIFORNIA, home to three of the nation’s four busiest international ports, is a big shiny magnet for alien marine species. Exotic creatures hitch rides to Oakland or Long Beach on ships sailing from foreign ports, then are dispersed up and down the coast via regional boat traffic or on currents. This has created a crisis in San Francisco Bay. Add the dumped contents of home aquariums, the exotic wares for sale in fish markets, and the Bay Area’s history of oyster cultivation, and the cumulative effect is staggering. In parts of the San Francisco Bay, 95 percent of the biomass is nonnative.

“San Francisco Bay is viewed as the most invaded estuary in the world,” says Andrew Cohen, a marine biologist with the San Francisco Estuary Institute. “It’s invaded across most of its habitats. Even more impressive: across many of the largest habitats, exotics wholly dominate.”

By 1995, a series of surveys organized by Cohen had identified 234 nonnative species in the bay. An update will probably find 275. Among the residents: a sea squirt that covers every available surface with yellow slime; an abundant Asian clam that has disrupted the food web and is blamed for falling fish stocks; the Chinese mitten crab, which burrows into the earthen levees of the Sacramento Delta and pilfers food from fishermen’s nets; and, as of August, giant Japanese oysters.

None of them is the mighty zebra mussel. But the fact remains that 120 miles up the coast from Monterey, within spitting distance of three national marine sanctuaries, sits a wellspring of exotic species, with transportation routes, ocean currents and human folly standing at the ready to deliver them and their larvae far and wide. It sounds like a disaster that’s already happened but hasn’t been noticed.

“It’s the ghost of Christmas Future. It’s what could happen,” says James Carlton, an Oakland native and Williams College marine scientist who has done extensive research in California. “I don’t know what species will appear next in San Francisco Bay or Half Moon Bay or Elkhorn Slough. It’s really biological roulette.”

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IT’S AS IF invasive species don’t know how to swim, so they huddle near shallow water. There are no known exotics in the open water of the Monterey Bay. The open coast—places like Point Pinos—doesn’t have many, either.

John Pearse, a professor emeritus at UC Santa Cruz and a Pacific Grove resident, says no one really understands why. “The idea here is the coast is well populated with natives that have a pretty good hold on things,” he says simply.

But inside the Monterey harbor, where the water is shallow and protected, more invaders show up.

The graceful and edible seaweed Undaria pinnatifida (see story, pg. 26) has taken root here in the last five years, to some consternation. So has a creature called Watersipora subtorquata, which colonizes dock pilings, boat hulls and the like in clusters of mauve ruffles that look like brains.

Watersipora subtorquata only arrived in the 1980s, most likely by “fouling,” or sticking to boat hulls (it’s uniquely resistant to the copper used in anti-fouling paint), but it’s going gangbusters all around the bay, says Moss Landing Marine Laboratory graduate student Josh Mackie.

“It’s incredibly abundant in the shallow zone around here,” he says. “It’s the invertebrate that is increasing its range at the fastest rate.”

But nothing else in the Monterey Bay National Marine Sanctuary compares to Elkhorn Slough’s alien population.

In 1998, Kerstin Wasson, then a graduate student at UC Santa Cruz, undertook a survey of the slough’s nonnative invertebrates. There were thought to be just a handful.

After hours combing the literature—including Ed Ricketts’ 1939 classic Between Pacific Tides—and peering down at mudflats, Wasson determined the slough was home to 56 invasive species. Most are also found in the San Francisco Bay, and 38 are associated with oyster farming (done in Elkhorn Slough early last century).

The findings made a splash. Even without the aid of foreign vessels, Elkhorn Slough was mimicking San Francisco Bay’s dismal record on invasive species. The number of alien plants, fish and invertebrates in the slough now approaches 100.

“To our knowledge, this is by far the largest number of exotic invertebrates recorded for an estuary without international shipping,” Wasson and her coauthors wrote in the journal Biological Conservation.

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IN THE PEACHY LIGHT of a November afternoon, Wasson and UCSC graduate student Rikke Preisler don galoshes and follow a trail to a mudflat newly exposed by a rapidly falling tide. It’s warm and quiet on the slough’s back channel. Stilt-legged egrets preen in the glassy water.

At low tide, nonnatives outnumber natives in Elkhorn Slough. One reason is the number of Batillaria attramentaria, the Japanese mud snail. In a few square yards, thousands of inch-long shells emerge from the gray mud. Wasson puts the slough’s population at 1 billion. They crunch underfoot.

“Its native counterpart is extinct,” she says. “It’s circumstantial evidence, but side-by-side in cages Batillaria spanked it. It’s got to be affecting the food web because it’s grazing down the algae on the estuary.”

Near an outflow pipe, Wasson and Preisler point out orange sponges on the rocks: Hymeniacidon sinapium. It’s all over the upper slough, like splashes of paint. It looks nice with the fall colors, but it may be crowding out the slough’s small native oysters.

Manmade features in the slough host a huge number of exotics. One of these is the Australian tubeworm, Ficopomatus enigmaticus, which colonizes into hard reefs that encrust features like the slough’s old railroad pilings.

“A Stanford student did an estimate of how often the water in the slough is turned over through these reefs,” Wasson says. “It’s like once a week. In contrast, our native oysters filter the water like once every 300 years. So there’s a huge change between the native filter feeder and the nonnative filter feeders.”

One of the mysteries of Elkhorn Slough is the European green crab, Carcinus maenas. It’s the second marine creature listed on the Global Invasive Species Database. Preisler is studying the invader, which hails from the North Atlantic, near her home country of Denmark (where, she notes, the crab is such a nuisance that fishermen have started pushing recipes for green crab stock). The crab arrived in San Francisco Bay in 1989 and swiftly spread north to Puget Sound and south to Morro Bay via larvae-carrying currents.

Its arrival spread something close to panic. There were fears for the Dungeness crab fishery, owing to the aggressive green crab’s taste for juvenile Dungeness. In Bodega Bay, a study found that the green crab’s arrival coincided with decimated native clam and shore crab populations. The authors of that study predicted an eventual impact on shorebirds due to changes in the invertebrates available for food.

The puzzle is that Elkhorn Slough’s green crab population rose precipitously in 2001, then dropped. Meanwhile, the crabs are huge for their species, so it’s not like life in the slough is grinding. One possible answer hints at the complexity of the system at stake.

“One thing that we’ve observed is that sea otters can eat a lot of green crabs,” Preisler says. “And sea otters tend to specialize, in the sense that one individual will eat mainly sea stars, another will eat mainly urchins and so on. We’ve seen sea otters eating green crabs all day long.  

“So this threatened species keeping the nonnative in check—that would be a fancy story for a science journal.”

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JAMES CARLTON understands that the public has a hard time caring about mud snails, tubeworms and sponges. But natural systems are so complex, he explains, that it’s hard to predict what the impact of these introduced species will be. A seemingly insignificant invader could outcompete a native, setting off a reaction up the prey-predator chain that eventually takes out something noticeable—the egret population, for example, or seals.

“Something as modest as a worm could have a cascading effect that really could affect something the public is interested in,” he says. “A lot of people are interested in the esthetic effects of a lovely kayak ride. And it comes down to invertebrates in the mud.”

As in so many cases, California is well ahead of the rest of the country in its recognition of the threat. In September, Gov. Arnold Schwarzenegger signed a bill authored by Sen. Joe Simitian (D-Palo Alto) that addresses the most common method of exotics transport. The law requires ships to treat their ballast water before dumping it in California waters. Right now that generally means changing ballast water mid-ocean, a difficult and dangerous task. The new rule phases in strict standards starting in 2009, with the goal of zero stowaways entering via ballast water by 2020.

The Monterey Bay National Marine Sanctuary, along with its sister sanctuaries Gulf of the Farallons and Cordell Bank, is proposing a similar rule in its new management plan. The plan would make it illegal to introduce or release any nonnative species into the Sanctuary. That sweeping prohibition potentially covers everything from ballast discharges to the dumping of live bait. If the proposal becomes law, it will forbid aquaculture operations to farm nonnative species in the sanctuaries as well. No pale Atlantic salmon grown here.

The twin regulations are a good start, Carlton says—but there is an alarming number of alternate pathways that exotics can take into the state.

“It’s a world of many different vectors,” he says. “For example, one can buy in San Francisco live Chesapeake Bay blue crabs. And they’re flown in and cross state borders because they’re marked as food for human consumption. If I were to propose that I wanted to introduce the Chesapeake Bay blue crab, one of the most aggressive crabs known, into San Francisco Bay, there wouldn’t be enough paper in the world to write the Environmental Impact Statement. And yet it’s right there on Columbus Avenue.’”

Unfortunately it seems to take a zebra mussel-sized problem to get people to pay attention to the onslaught of invasive species and the cost to native ecosystems. Preisler, the graduate student working in Elkhorn Slough, is troubled by the bottom-line mentality.

“There are no money problems at all with the green crab,” she says. “It’s not really bothering anyone. The problems are ethical. If we keep introducing species everywhere, everything is going to be more homogenized. So we’ll get a more uniform world.”