Zebra Mussel, Dreissena polymorpha
It's hard to believe that most Canadians had never heard of zebra mussels until the 1990s. Since they were first noticed in Lake St. Clair (a small lake connecting Lake Huron and Lake Erie) in 1988, we've heard a lot about the damage they're causing: clogging intake pipes, sinking navigational buoys, making beaches tricky to walk on with their shells' sharp edges, and displacing or even eliminating native freshwater mussel species.
The main reason these tiny (typically less than 4 cm long) molluscs are so damaging is that they are invasive -- in other words, they reproduce like crazy and disperse very rapidly. A single female can lay more than 40 000 eggs in a single reproductive cycle, and up to a million in one spawning season! A fertilized egg takes three to five days to become a typical bivalve (two-shelled) mollusc larva, called a veliger.
Various stages of development: veligers approximately 0.1 micron in size, to juveniles approximately 1 millimetre large (clockwise, from upper left corner).
Zebra mussel veligers really get around: they can be carried by currents over tens or even hundreds of kilometres. The veligers spend two to four weeks growing big enough to settle on pretty much any hard surface they find: pipes, rocks, the shells of live or dead freshwater native mussels and snails, sunken tree trunks and boat hulls.
Once they've found a place to attach themselves to (called a substrate), they secrete strong, glue-like fibres called byssal threads, and that's it -- they're stuck on, and getting them off is really hard to do.
And zebra mussels don't just stick around -- they stick together. Canadian Museum of Nature Research Scientist Dr. André Martel has found them in the Rideau River near Ottawa in densities much greater than 100 000 individuals per square metre. In one power plant in Michigan, their density was more than 700 000 per square metre!
Of course, by accumulating in such large numbers, they may kill pretty much any other aquatic animal they attach themselves to. Can you imagine trying to move, eat or even breathe if your weight had been tripled or quadrupled by other creatures that had attached themselves to you?
There's a tree trunk hidden under these zebra mussels!
Zebra mussels are not native to Canada. Scientists think zebra mussels were accidentally introduced to North America from Eurasia in 1985 -- probably in ballast water released from a cargo ship. Since then, people have (unknowingly) helped zebra mussels to spread into uninfested water by moving boats or equipment that is infested with juveniles or adults, or by releasing bilge water that is infested with veligers.
Even worse, juvenile and adult zebra mussels can live out of water for many days, especially if it's cool and damp. So boaters can't be too careful. Bilge water should be drained on site when a boat comes out of the water for overland transport. Hulls, ladders, centreboards, motors, fenders, dinghies and trailers all need to be inspected and cleaned every time they are moved from one body of water to another.
If the spread of zebra mussels is going to be contained, boaters will have to help. Scientists are doing their part: they're learning as much as they can, in the hope they can find a way to prevent further damage.