If you’ve fished or floated either the Clinch or Powell rivers, you’ve no doubt seen shells lying around in shallow water. These are from freshwater mussels, and these two river systems have one of the highest varieties of mussel species in the world, making our area a noted biodiversity hot spot.
Freshwater mussels are kin to clams and snails. They may appear to be simple creatures, but are in fact fairly complex in their anatomy and lifecycle. Basically they have a soft inner body protected by two hard outer shells called valves. The shells are joined by an elastic ligament that forms a hinge. The shells of different species vary in size, shape, thickness, color, and the presences of or absence of “sculpturing” (ridges or bumps) on the outer surface.
The soft body inside includes an outer covering called the mantle that secretes a lime-based substance that hardens to form the shell. A new layer of shell is grown each year. There is a tongue-like feature called a foot used for moving short distances, and muscles for keeping the valves tightly closed. Mussels also have two pairs of multipurpose gills. They are used for respiration, for moving food particles to the animal’s mouth, and also for housing and nourishing mussel eggs and larvae.
Day to day living for a mussel involves lying wholly or partially buried in the river bottom and continually pumping water through its body and filtering out needed oxygen and microscopic plant and animal food particles. Water enters via an incurrent siphon and exits via the excurrent or anal siphon. The lifespan of mussels varies from 10 to 100 years.
Freshwater mussels have an elaborate life cycle. They usually have separate sexes, but some have both male and female reproductive organs. During spawning, males release sperm into the water. The sperm are drawn inside a female’s shell during normal filtering activities, where they fertilize eggs in her body. The fertilized eggs develop into a parasitic larval stage called glochidia that are stored for a time in the female’s gills. When the larvae mature, the female expels them into the water where they must attach themselves as parasites to the gills or fins of fish. The larva remains on the host fish for several weeks or months during which time they develop adult organs and structures. When fully developed the young mussels then detach from their host and drop to the bottom of the river. Thanks to the swimming fish, they may now be far away from their parents and have colonized new territory.
Several of our local mussel species are classified as endangered, including interesting names like the Appalachian monkeyface, the birdwing pearly mussel, the rough pigtoe, the rough rabbitsfoot, and the Cumberlandian combshell. It is important that we all help preserve clean water habitat for them and other river wildlife by using good conservation practices to keep soil erosion from polluting streams and rivers.