There are not many freshwater mussels west of the continental divide in Montana; in fact, there is only one native species here, the western pearlshell.
Freshwater mussels are a very unique, diverse group of animals. There are about 300 species of freshwater mussels in North America, and 43% of those are in danger of extinction. Freshwater mussels filter feed and are therefore vulnerable to a variety of pollutants in their habitat such as toxins and sediment.
Where I used to live in Kentucky, the water quality is not the best, but with a little bit of looking, I could usually find some mussels in lakes and small rivers. I’ve been on a few rivers in western Montana like the Clark Fork, Blackfoot and Rock Creek and not once have I seen a freshwater mussel. I began to wonder why and decided to do a little bit of research. The reason I never saw them was because there are not many freshwater mussels west of the continental divide in Montana; in fact, there is only one native species here, the western pearlshell.
This mussel is part of a family commonly referred to as pearl mussels since they are capable of making pearls. The western pearlshell is the smallest freshwater mussel in Montana, usually just five to eight centimeters in length. Its shell is elongated and very dark on the outside and purple on the inside. It prefers cold water streams wider than 4 meters with low to medium gradient.
Filter feeding, which involves siphoning in water and filtering out the organic particulate materials, is the primary way western pearlshells obtain their food. Because of their feeding method, mussels provide an excellent indicator of water quality. A quick decrease in mussel populations in an area usually means a decrease in water quality.
The development of the western pearlshell, like other mussels in the same family, is a very complex process. Western pearlshells are hermaphroditic meaning each individual contains both male and female reproductive organs. When the embryonic mussels, or glochidia, are fully developed in their parent, they are released into the water. Once released, the glochidia have only a few days to attach to the gills of a fish which often has to be a specific species. The optimal species for the western pearlshell glochidia to attach to is the westslope cutthroat. If the glochidia attaches to a wrong host, it often dies.
A few days after attaching to the gills of the host, the host’s dermal tissue will grow around the glochidia forming a nodular cyst. The encapsulated glochidia develops in the dermal tissue for a few months. Afterward, it breaks off and remains in the river bottom for the rest of its life continuing to grow and develop. Surprisingly, there have been as many as 3,000 glochidia found on a fish at one time with no apparent harm to the fish. The western pearlshell adults can release millions of glochidia at one time, and for a good reason. There is a 99.99% mortality rate from the time the glochidia are released from their parent to the time they break off their host. That means only about one in one hundred million glochidia will survive to adulthood. Luckily, the ones that do become adults can be very long-lived, up to 70 years, and produce many offspring in their lifetime.
Freshwater mussels are one of the most endangered groups of animals in the world. The main threats to the western pearlshell and other freshwater mussels today is land practices that alter water quality. Construction, point source pollution, poor agriculture and forestry practices as well as mining can impact water quality by increasing the sediment or toxic chemicals in the water. The increased sediment and toxic chemicals in rivers affects mussels probably similar to how inhaling chloroform, ammonia or carbon monoxide affects humans. In Montana, the western pearlshell is listed as a species of concern by the Montana Fish, Wildlife and Parks.
It’s important to realize that in the creeks in rivers we live around, there are more than just fish and insects residing in them. I know the next time I go out fishing, I’ll keep a sharp eye out hoping to spot one of these mussels. Who knows, maybe the next time you find yourself on one of Montana’s magnificent rivers, you’ll stumble across this fascinating mussel.
"Field Notes" is produced by the Montana Natural History Center.