River Currents: Freshwater Mussels: An indicator species
A common question when someone sees their first littering of freshwater mussel shells at the edge of the Connecticut River or one of its tributaries is, "How did these get here?" People might not know that a raccoon, mink, or otter had a feast and that our watershed hosts 12 species of freshwater mussels – a small number compared to the 300 species living in the streams and lakes of North America.
Mussels may move no more than a few feet during their adult lives. They are an indicator of water quality since they are sensitive to excess siltation and low dissolved oxygen levels and cannot escape these unhealthy conditions. Any drop off in the total number or species diversity of the mussel population is an indication that water quality is declining. If the mussel population drops, the fish and terrestrial species that rely on clean water will drop too.
Mussels filter feed on algae, bacteria, and organic particles suspended in the water. They feed by taking water in through a siphon and passing it over mucous-covered gills where the food items stick and then are transferred to the mouth. The mussel will flush waste particles out through their exit siphon. They not only act as an indicator of high quality water but their filtering action improves water quality, cleansing lakes and streams. Unlike their saltwater relatives, however, freshwater mussels are no culinary delight for humans. According to those who have tried them, along with being rubbery to chew, they taste and smell much like the river bottom.
Mussels supplement higher trophic level species diets. A declining mussel population means these species lose food value since the mussel serves as a link in the food chain transferring the organic debris and plankton they filter out of the water up the food chain to species such as fish, muskrats, raccoons, and otters.
The freshwater mussel life cycle starts when male mussels release sperm into the water, and the female through its syphon then takes the sperm in to fertilize the eggs. This is a different strategy than saltwater mussels where they release both eggs and milt into the water for broadcast fertilization. There is no internal stage in saltwater mussels' reproduction.
Freshwater fertilized eggs develop into microscopic larvae called glochidia that look like tiny mussels. They are parasites that must attach themselves to the fins or gills of a fish. To increase the chances of their young making contact with a fish host, some females "go fishing." by displaying specially adapted tissues that look like fish prey to try to lure fish to swim near them.
Sensing a fish nearby, the female releases her young into the water, ready to fasten onto the fish. After attaching to a fish for one to several weeks, glochidia let go of the fish and sink to the bottom where they will spend the rest of their lives. Mussels are specific about the fish they parasitize. If the host fish disappears from a river or lake, mussels cannot reproduce.
Through microscopic examination of annual growth rings in the shell similar to the growth rings in trees, an observer can determine age and growth rate of individual mussels. Some of our freshwater mussels only live 3 to 5 years. The eastern pearshell mussel can live to be 100 years old but even though pearshell can live to a ripe old age, they grow and propagate rather slowly and will not breed until nearly 12 years of age thus the population growth rate is slow.
Mussels require certain water flows combined with a distinctive mix of cobble and sand in which to anchor themselves. They move little from where first deposited by the host fish. Reduced dissolved oxygen levels added to the threats posed by natural predators and the increasing amounts of silt and sediment from unnatural erosion make it understandable why scientists say that up to 50 species of mussels in the US could go extinct in the next two decades.
Three species listed as endangered or threatened in the upper reaches of the Connecticut River are the Dwarf Wedgemussel (Alasmidonta heterodon): federally endangered, endangered NH and VT; Brook Floater (Alasmidonta varicosa): endangered in NH, threatened in VT; Eastern Pearlshell (Margaritifera margaritifera): threatened in VT. Countering the loss of mussels is underway through the Connecticut River Watershed Council and their citizen science program that will work with citizens to repopulate certain reaches of the river with Brook Floater mussels now through October. Funding provided by the Massachusetts Environmental Trust. The partners in the program are U.S. Fish and Wildlife Service, MassWildlife, and the University of Massachusetts. Volunteers are more than welcome – contact CRWC at www.ctriver.org.
Dams represent barriers to fish hosts and therefore to the propagation of mussels. Dams trap sediment on the upstream side and once buried these bottom areas are generally without mussels. The tail waters below the dams, on the other hand, often have dense mussel beds. It is a mistake to believe this is a benefit of the dam since these beds in many cases represent the last remaining portions of the river that are still well oxygenated, have little sediment and free flowing water.
The next time you are riverside look for one of our species of freshwater mussels. If you find mussels, you have also found clean water.
David L. Deen is River Steward for the Connecticut River Watershed Council. CRWC is celebrating 60 years as a protector of the Connecticut River. The opinions expressed by columnists do not necessarily reflect the views of the Brattleboro Reformer.
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