Western Ridged Mussel

The western ridged mussel is narrower at one end more than the other (obovate) to trapezoidal in shape. Western ridged mussels rarely exceed 5 inches in length. The species has an angular ridge that runs from the beak (near the hinge) to the other edge of the shell. There are no lateral teeth but the right valve has one pseudocardinal tooth and the left valve has either one or no teeth.

Western ridged mussel are found more commonly in streams than lakes and prefer constant water flow and well-oxygenated stable substrates in areas of low gradient. They can be found in substrates ranging in size from silt, clay, and sand to boulders. They are rarely found in waters that are continuously turbid such as glacial streams.

Like all freshwater mussels, western ridged mussels are filter feeders that siphon suspended particles from the water column. They may consume plankton, bacteria, dissolved organic matter, or algae. This filtering provides an important water quality service by reducing turbidity and controlling nutrient levels, especially where there are dense mussel beds. Particles not used by the mussel are often re-formed and expelled as larger particles that are in turn used as food by other aquatic life.

Deliberate lateral and vertical movements by mussels does occur, but the movement is usually limited. Among the mussel species in western North America, western ridged mussels are considered among the least mobile. Because mussels don’t have vision, lateral movements can seem random and be multidirectional.  In some locations where ice can occur attached to the substrate (anchor ice), mussels may move vertically, burying into the substrate during the winter, and moving to the surface in the spring. Vertical movements during the spring to early fall may also be related to reproduction or a response to disturbance by other animals, predators or people. Lateral mussel displacement due to high flows or dislodgement due to predators or human activity also likely occurs. Mussel larvae have a parasitic phase where they attach to a host fish and can be moved by that host within the aquatic system.

The western ridged mussel matures at about seven years of age. Male mussels release sperm into the water and the females filter the sperm onto a special portion of their gill called the marsupium. The reproductive process probably begins in the spring. Females with larvae have been found in some locations from March through July. The embryos grow into larvae and are released by the female. The female releases the larvae into the stream as a mucous to rubbery white mass called a conglutinate which occurs in some locations in June. From the conglutinate the larvae release at which point they’re called glochidia and look like tiny mussels. At this stage the glochidia attach to a host fish’s gills or fins where they remain for weeks to months until the detach and settle to the substrate. Glochidia have been found on fish from March to early August. Mussel species are specific regarding what species of fish act as hosts. Western ridged mussel glochidia have been found in different species of fish from the minnow, sculpin, trout, and sunfish families.