On June 1, 1988, Paul Hebert sent two of his undergraduate students into Lake St. Clair to collect clams, using a dredging tool called a "ponar grab." As head of the Great Lakes Institute at the University of Windsor, he wanted to measure pollutant levels in the bivalves. But what his students plucked from the bottom sent those plans famously awry.
"The device came up with a rock caught in its jaws, and on the rock, was the first zebra mussel," recalls Prof. Hebert, now director of the Biodiversity Institute of Ontario at the University of Guelph.
His initial reaction, he admits, was far from prescient. "I said: 'I've never seen one of those before; we'd better take a picture in case it's the last one I ever see.' "
Within two months, his team had found mussel densities as high as 200 per square metre. The creatures were all the same size, and the researchers deduced that they had been hatched from larvae dumped from a ship in the fall of 1986. Shortly after being discovered, the first generation spawned and the invasion became, in Prof. Hebert's words, "an explosion."
So did the media coverage.
First came a barrage of apocalyptic predictions. Female zebra mussels were capable of producing 40,000 eggs a year and would carpet the Great Lakes, eradicating countless native species. Early cost estimates to unclog power-intake pipes, dredge harbours of shells and absorb the loss to Ontario's fishing and tourism industries ran in the billions of dollars. The invasion was covered heavily for years. (Who can forget the encrusted "mussel car" pulled from the Detroit River in the early 1990s?)
And then, abruptly, the risk appeared to fade. There were sporadic reports of treatments -- both natural and chemical -- that might eradicate the mussels, and evidence that they would thrive only where the bottom was rocky. The colder water of the more northerly Great Lakes proved less hospitable than Erie and Ontario. Some researchers speculated the population would peak and taper off.
And then came an even more radical departure from conventional thinking. Just as some optimists hail global warming (heating bills will go down), reports appeared suggesting that Dreissena polyorpha was actually providing a service to mankind, by vacuuming up some of the pollutants clogging the Great Lakes.
So what, looking back 20 years after it all began, has really happened?
Early estimates of the financial impact were probably on track -- the most recent figures put the cost to Great Lakes utilities alone at $200-million to $500-million (U.S.) a year, on top of the damage to a $4.5-billion fishing industry and tourism in the region.
The environmental toll has been much more difficult to assess. For one thing, the zebra is not acting alone. Scientists now believe that it has been largely muscled out by its own cousin, the quagga mussel, Dreissena bugensis, which looks almost identical and arrived from the same part of the Caspian Sea in the early 1990s. There's no doubt that, together, they have virtually wiped out several species of insect and native mussels, while seriously jeopardizing certain fish populations. They are also, literally, changing the terrain of the Great Lakes.
It's true that Dreissena mussels clarify water. Like tiny vacuum cleaners, they suck in vast amounts of algae and other particulates, producing feces from the organic matter while binding the non-organic sand and pollutants with mucous to create an unappealing glop called pseudo-feces. Both wind up on the bottom of the lake, meaning that over time the mussels remove tonnes of material from the water.
The visual impact is stunning. "I'm old enough to remember when Lake Erie was dead -- remember those headlines? -- but it was, in fact, so alive, it was choking itself," says David Barton, a biology professor at the University of Waterloo. "The water was so green that sometimes in the summer, the wake from boats was green -- it was thick."
Prof. Barton says, "Things really improved when the mussels came along because they're so good at pulling stuff out of the water column, and incorporating some of it into their own tissues and the rest on the bottom, where it can be eaten by other organisms that are good fish food. So, ecologically, it's not such a bad thing."