King County, Washington
When you hear of mussels, most people in the Pacific Northwest think of the edible, blue, marine mussels anchored by threads to rocks and exposed at low tides. Few folks are aware of the black-brown fresh water mussels that bury themselves in mud, sand, and rocks and lives on the bottom of our streams. These freshwater mussels are in an entirely different family of shellfish.
Freshwater mussels don't get a lot of attention—they aren't cute with fur, feathers, or fins. They sit hidden in the bottom of the streams, and their brown color helps them blend into the substrate. But they witness the stream on a daily basis as they filter and clean it for us. Their sensitivity to pollutants make them an indicator of stream health that can help us track sources of pollution.
How can you tell a mussel's age without asking?The surface of the shell has distinct black lines or ridges that represent winter rest periods. These ridges can be counted to estimate the age of a mussel, just as one counts the rings on a tree.
The United States has the richest diversity of mussels in the world with about 300 different species. Unfortunately, it is a very endangered group of animals. It is believed that about 10% of the species are already extinct and it is estimated that that 70% are at risk of disappearing.
Most of the US species live in the southeast, which is home to the richest collection of mussels in the world. In western Washington, we have only three species: western pearlshell (Margaritifera falcata), Oregon floater (Anodonta oregonensis), and western ridgemussel (Gonidea angulata). The western pearlshell is pictured above.
Mussels are long-lived—some species can live over 100 years. There have been individuals of our local species, western pearlshell (M. falcata), found to be as old as 85 years in Bear Creek (Toy 1998). The mussel pictured above is from Bear Creek and is about 50 years old.
From Shiny to Spiny
Mussels are a bi-valve—they have a two-part shell that is hinged together. They open and shut these "valves" using a strong muscle and ligament that works just like bending a knee. These valves or shells come in many sizes, shapes, thicknesses, and surface textures. The variety of shapes and surfaces that range from shiny to spiny has led to many quaint common names such as elk toe, shiny pig toe, fatmucket, heel splitter, and cats paw.
Toy, K.A. 1998. Growth, reproduction and habitat preference of the freshwater mussel Margaritifera falcata in Western Washington. Thesis. University of Washington. Seattle, Washington.
Rensel, J. Analysis of Stream conditions in Upper Bear Creek, Washington. Report prepared for Group Four, Inc., Lynnwood, WA.