Musseling in: Small but successful invader

by Stephan Patten

Like experienced generals, two McGill biologists use a huge map of the world spread across the table in front of them to outline the zebra mussel's invasion of North America.

From its home in the Caspian and Black seas, the zebra mussel spread through much of Europe in the 1800s and was discovered in North America in Lake St. Clair in 1988.

Since then, it has rapidly expanded its sphere of influence to include the St. Lawrence, Hudson, Mississippi, Illinois, Ohio, Tennessee and Arkansas river systems.

When the zebra mussel, named for the stripes on its almond-shaped shell, arrives in a new location, it overruns it like an invader, achieving densities of 700,000 animals per square meter in some places. The small (under 4 cm) bivalve inhabits structures like drainage and intake pipes at power plants, fouling water systems and creating billions of dollars of clean-up costs.

It substantially alters the ecosystems of the lakes and rivers it inhabits through its filtration activity, which can deplete the water of phytoplankton. "Lake St. Clair used to be a walleye lake and now it's a muskellunge-bass lake," says McGill biology professor Joe Rasmussen. "The walleye don't even want to be there anymore. Instead of turbid water you have clear water, weeds started growing and then the whole fish community changed."

Rasmussen and PhD student Anthony Ricciardi have spent five years studying various aspects of zebra mussel biology.

Much of their early research (with master's student Eric Mellina) was devoted to identifying the preferred living environment of the zebra mussel. They identified both the chemical and physical properties a body of water had to have to become infested with zebra mussels. For example, the mussels will not inhabit water with a calcium concentration below 15mg/L and prefer hard surfaces for attachment.

This ability to attach to hard surfaces using adhesive filaments partly explains how zebra mussels managed to travel halfway around the globe to invade North American fresh waters.

"Attachment is a very important aspect of their biology that when combined with human ship traffic led to the dispersal of this species from its native area," says Rasmussen. "The zebra mussel went along for the ride."

The mussel's inclination for hard surfaces (as opposed to local varieties which burrow in soft sediments) also threatens native species such as freshwater clams. Hundreds, even thousands of zebra mussels have been known to attach to a single clam, causing death by interfering with the clam's normal feeding, breathing or movement.

"In the St Lawrence River, we have found that in only a few years after the zebra mussel has become established, native clam populations are being wiped out," says Ricciardi.

The zebra mussel's planktonic larval stage is also conducive to long distance travel. It is commonly believed that the free-swimming larvae travelled to North America in the freshwater ballast of European ocean-going ships.

More recent research has pointed out that on average, adult zebra mussels can withstand as much as five days out of water, at 20C and 50% relative humidity. If they attach to boats they can be transported to most lakes in North America, Ricciardi warns.

"Because of this mean five-day endurance the zebra mussel could be transported anywhere," said Ricciardi. "The fact is that California border authorities have already stopped a couple of boats that had zebra mussels attached. I've heard that the mussels were dead, but they could have just as easily been alive."

Rasmussen and Ricciardi are now applying what they have learned from the zebra mussel to be better prepared for another invasion.

"The zebra mussel is helping us understand what makes a good invader," said Ricciardi. "Now what Joe [Rasmussen] and I are primarily interested in is finding ways to predict what could come next and where they could come from."

They have their sights on a number of other overseas species that have the potential to invade North American waters and are working on identifying characteristics that successful invaders must possess.

Characteristics such as attachment, a planktonic larval stage, high fecundity, quick and early maturation and large numbers of offspring (one adult female zebra mussel can produce more than a million eggs a year) make for successful migration.

Global species dispersal has been occurring naturally over geological time scales, but is accelerating dramatically due to human activities such as ocean-going ships penetrating deeper into various freshwater bodies.

"This sort of thing happened once every million years or so, but now we're seeing species skip continents on a regular basis," says Rasmussen.

And this tendency towards species migration is putting the world's ecological stability at risk, they argue.

"What's happening is we're putting species all together in a small global bottle, shaking them up so that only a few are going to survive," says Ricciardi. "When you have low global biodiversity you have low ecological stability."

Stephan Patten, an undergraduate in the Department of Biochemistry, is a science writing intern for the Reporter.The internship program is funded by the Natural Sciences and Engineering Research Council.