<u’ve ever walked along the Cannon River, or spent time on its sandy banks near the entrance of lower arb, you may have seen what look like large clam shells scattered about. These are the shells of freshwater mussels, a subclass of bivalves that have played an historically important role in filtering freshwater rivers and lakes. In North America, there are 297 species recorded, 213 of which are endangered. According to a survey by the Minnesota DNR in 1988, 13 different species of freshwater mussel have been found in the Cannon River.
So, why are they so universally endangered? There are a number of causes, all of which are man-made. For one, mussel shells are strong and the insides are shiny with mother-of-pearl. This made them the perfect material for making buttons. They were harvested, killed, and turned into buttons by the thousands, until plastic buttons became more profitable.
Dams are another factor, as they increase silt deposition, complicating filter feeding and sometimes covering mussels that can’t move fast. They also interrupt the mussels’ reproduction. Freshwater mussels have a very interesting reproductive cycle, which is dependent on two factors: currents and fish. The males release spermatozoa into the water, where they are whisked downstream and caught by the filter-feeding females.
They then fertilize their eggs and hold the larvae inside their shells. The female mussel puts out a lure (sometimes the mantle, sometimes strands of mucus) to draw in a fish, and then ejects the larvae at the fish when it is in range. Some of the larvae are drawn into its gills, where they stay and develop, using the fish as transport to a new site, possibly upstream. Without fish able to move freely through the system, mussels aren’t able to spread their populations evenly, and slowly get pushed downstream.
The mussels have had a rough time of it, but increasing awareness and efforts for river conservation are the best thing we can do for them right now. Hopefully in the future we will see some of their numbers return.