Species of Greatest Conservation Need: Western Ridged Mussel

by Emilie Blevins, Senior Conservation Biologist, The Xerces Society for Invertebrate Conservation

Oregon is home to a fascinating range of animals, from rare species found nowhere else on earth to iconic creatures like beavers. In this lineup of biodiversity, freshwater mussels are an important yet often overlooked group, supporting many other species in our rivers by burrowing into stream bottoms, filtering water, and improving habitat. They are a diverse group of animals that have a unique association with fish. While adult mussels keep the water cleaner and support the aquatic food web, they also go through a temporary parasitic larval stage associated with specific fish species. For just a couple of weeks, young mussels must attach to a fish’s gills or fins and undergo metamorphosis. Once complete, mussels will drop off the fish, landing, with luck, in just the right habitat and spend the next 10, 60, or even more than 100 years quietly filtering water along the river bottom. Where habitat is good, native fish are plentiful, and mussels are thriving, you might find thousands of them in a small area, called a mussel bed.

Western Ridged Mussel

As rivers and streams have been impacted by human activities, and as other native aquatic species have declined, so have freshwater mussels. At one time, North America was home to more than 300 mussel species. Now, many are extinct or highly imperiled. In Oregon, this means that our species—the western pearlshell, western ridged mussel, and floater mussels—are now found in smaller numbers and in fewer places. Drought, pollution, and other pressures are all negatively impacting populations, and as Oregon Conservation Strategy Species, mussels have been recognized by the state as species of greatest conservation need.

Oregon does not have many species relative to other regions in the country; our species are unique to the West. Our most imperiled species, the western ridged mussel (Gonidea angulata), is the only member of its genus on earth. As with other mussels, it uses specific host fish, most likely relying on sculpins. However, sculpin have a much broader range than the western ridged mussel, so what factors specifically govern where the mussel is found are still a bit of a mystery.

The western ridged mussel is a bellwether for Oregon’s rivers. While we may still find an abundance of animals in some places, such as on the Malheur National Wildlife Refuge, increasingly the species is dying out, whether slowly, as observed in some streams, or quickly, as in the case of the unexplained die-offs seen in the Crooked River. Even where the species may still be abundant, unexplained die-offs or human activities threaten its future. Little is known about its needs—what water temperatures it tolerates, what time of year it breeds, minimum viable population size—and time is running out. Even now, the U.S. Fish and Wildlife Service is reviewing the mussel’s status in consideration for listing under the Endangered Species Act.