Freshwater Mussels

The Most Ancient Animals in Park Streams – Western Pearlshell Freshwater Mussels

If you are from the West Coast of the United States, you probably assume that all mussels, clams, oysters, and other bivalves exist only in the ocean. They do not. Freshwater mussels are found throughout the world’s lakes, creeks, and rivers. Their diversity is highest in the United States, with over 300 species found nationwide. Some states within the Mississippi River basin contain more species than the rest of the world combined. In western states, however, the diversity is not as extraordinary, with fewer than ten to fifteen species known to occur west of the Rocky Mountains.

Western pearlshell mussels (Margaritifera falcata) are the only species found within Redwood National and State Parks. A large population exists in Mill Creek in the north end of the parks, while a small, remnant population is also found in Redwood Creek in the south end of the parks. Western pearlshells look very similar to marine mussels, with an elongated black shell extending up to 3.5 inches (9 cm) long. They are most often found on the back sides of boulders, outside stream bends, and in slow-moving riffles in fast-flowing creeks and rivers, where they wedge themselves in cracks or between cobble-sized rocks. Like marine bivalves, freshwater mussels feed by filtering small food particles suspended in the water.

Western pearlshells are the oldest freshwater invertebrates in the world, with some individuals living for more than 100 years! Their long lifespan is attributed to the cool, nutrient-poor, highly oxygenated streams they inhabit. The relatively small amount of food available in these clean, clear-flowing streams means that pearlshells must grow very slowly, much like a slow-growing desert tortoise.

Another interesting “problem” that pearlshells must deal with is reproduction. To mate, male mussels release sperm into the water, which drifts downstream until it randomly encounters a female mussel. After fertilization, female mussels expel millions of growing larval mussels, or glochidia, into the stream, where they too float downstream. If you ponder this reproductive strategy, you quickly realize that over generations, these sedentary creatures would eventually be eliminated from a stream, as each successive generation would be flushed farther and farther downstream.

To overcome this, freshwater mussels rely on fish hosts—specifically salmon and trout in the case of western pearlshells—for transportation. When glochidia are inhaled by a passing fish, they clamp onto the fish’s gills. There, they grow for a few months while getting a free ride back upstream. Once mature, they drop off, settle permanently on the stream bottom, and grow into adults.

Unfortunately, the freshwater mussel taxonomic order (Unionida) is one of the most endangered animal groups on the planet. Over 70 percent of known species are either extinct or close to extinction. Dams, pollution, sedimentation, overharvesting, and loss of host fish species have all contributed to their decline. Ironically, because mussels are filter feeders, they serve as "canaries in the coal mine" for measuring water quality and watershed health.

Park staff use western pearlshells as indicators to monitor environmental conditions in Mill Creek and Redwood Creek. Though seemingly inconspicuous, these mussels play a vital role in cleaning park streams through their constant filtering. In many ways, pearlshells are as ancient as the giant redwoods that line the banks of the streams they inhabit.