The Willamette River and its tributaries sustain a wide array of native aquatic species, with some of these at risk. Much of the focus in recent years in the Willamette System has been on native fish such as Spring Chinook, given how such species have been greatly diminished by habitat destruction and dams. One family of species that are not very well known to most people, are freshwater mussels.
Native mussels in the Willamette River System include the Western Pearlshell (Margaritifera falcata), Oregon Floater (Anodonta oregonensis) and the Western Ridged Mussel (Gonidea angulata). The most frequently seen mussel in our experience on the Willamette are the Western Pearlshell Mussel.
Mussels are a very interesting species. They are filter feeders who remain in the same general area for most of their lives. They filter materials such as bacteria, algae, zooplankton from the water. Western Pearlshell are dependent on native fish to help spread their larvae. When the larvae are released (known as glochidia) they can make their way to the gills and fins of native fish where they can remain for weeks. Then, at just the right time, the larvae drop off the gills, and float down to the river bottom. Once there they burrow down and remain covered for several years as they grow. Once they are mature, they can be seen partially buried in the river sediment. Of course, only a small fraction of the larvae ultimately survive to maturity.
Along the Willamette small beds of Western Pearlshell mussels are typically found, interspersed here and there usually in shallow areas near the shoreline. Sometimes beds can be found in fast moving side channels where there is abundant food. As adults, the dark color of their shells can make them easy to mistake as rocks, with a long oval shell of averaging 5 inches. Empty shells from mussels that have been preyed upon can be seen in clusters here and there.
Animal species that typically prey on Western Pearlshell include River Otter, and Raccoon. The biggest threat to this species is the modification of habitat, toxic pollutants, and changing water quality conditions. Given their filtering of the water, they can easily uptake pollutants. In our view, they are deserving of additional study, and efforts to protect their habitat. Over the past few decades freshwater mussel populations have declined steadily, with little consideration for their well-being.
In the case of the Western Pearlshell, the species can live to be over 100 years old, and averages about 70. Questions that arise in relation this species today include: 1) Is the population reproducing at a sustainable rate? 2) Are bed sizes decreasing? 3) Are toxic pollutants bioaccumulating in their bodies?
In our view Western Pearlshell mussels are easy to appreciate, but they are also at risk. They have a unique role in the Willamette River’s ecology as do Oregon Floaters which are found in slow moving backwaters, and in fewer numbers. Western Ridged Mussels are also occasionally seen. These species can also benefit from increased research to better understand their distribution, overall health and more.
For more information check out: Freshwater Mussels of the Pacific Northwest, by Ethan Nedeau, Allan K. Smith and Jen Stone. Also take a look at the Xeres Society web page on Mussels: http://www.xerces.org/western-freshwater-mussels/