Beneath the clear brown water the mussel paused, resting softly on the sand. Behind it, a long arcing trail unfurled. Puff. The mussel exhaled, sending a small burst of water out of its body, and in doing so, it traveled one centimeter further across the river bottom. Rest, exhale, repeat. The water lapped gently against the sandbar and a heron glided across the water to perch on a snag. Rest, exhale, repeat. Overhead, the sun slid slowly down the sky and into the horizon. Hour by hour, the ribbon grew longer as the mussel in the river slowly, ever so slowly, traced its circles in the sand.
The St. Croix River watershed is considered to be one of the premier environments for freshwater mussels in the world. Forty species of mussels live in the St. Croix, including five that are listed under the Federal Endangered Species Act: the Higgin’s eye, sheepnose, snuffbox, spectaclecase and winged mapleleaf. Researchers believe that all of the mussel species that have lived in the river historically are still alive today. This diversity and abundance of mussels in the St. Croix is an indication of good water quality and a healthy ecosystem.
Elsewhere in the United States, almost two-thirds of all native mussel species are threatened, endangered, or already extinct. In the Mississippi River, twelve mussel species have disappeared, leaving 28 species remaining. These mussels help to tell the story of a river, nearly destroyed by pollution, that is slowly returning to good health.
When the U.S. Army Corps of Engineers and National Academy of Science surveyed the Mississippi River in the Twin Cities during the 1970’s, they found only nine species of mussels, and only in a small section of the river. Mussels are filter-feeders, which makes them particularly vulnerable to water pollution, and the Mississippi River was one of the most polluted during the early 20th century. Minneapolis and St. Paul dumped untreated sewage into the river until 1938, and factories spewed chemicals into the water until the Clean Water Act was passed in1972. Today, native mussels are making a comeback in the Mississippi, and researchers have even reintroduced Higgen’s eye and winged mapleleaf mussels in some locations.
A mussel breathes and eats by drawing water in through a body part known as an incurrent siphon. They absorb oxygen from the water as it passes over their gills and they also filter out tiny plankton and nutrients, which they absorb into their bodies and shells. Once this process is complete, they shoot the water out through their excurrent siphon, propelling themselves forward ever so slightly as they do. Rest, exhale, repeat. In addition to their slow forward progress, female mussels perform elaborate dances and use mimicry to lure fish to their sides when they are carrying mussel babies, known as glochidia. Intrigued by a flash of movement or what appears to be a minnow, a fish swims closer to investigate and the mussel releases her young with a burst. The glochidia attach to the fish’s gills and hitch a ride for a few weeks until they are large enough to drop off onto the river bottom and begin a life of their own.
The newest threat for North American freshwater mussels are invasive zebra mussels, which came originally from the Caspian Sea. The zebra mussels attach themselves to any surface they can find, including boat docks, submerged rocks and native mussels, and disrupt the natural food web. The National Park Service prohibits boats from traveling upstream on the St. Croix River past the High Bridge north of Stillwater to protect the upper reaches of the river where zebra mussels haven’t yet invaded.
The mussel’s journey is both slow and long. As the curving path in the sand grows steadily longer, years turn into decades, and sometimes even a century. Answering machines give way to cellphones, and typewriters to computers. Past the river bluff, a prairie becomes a farm becomes a subdivision. Still the mussel carries on, carving circles in the sand. Rest, exhale, repeat.