Freshwater Mussel Life History and Reproduction
Life in the Mud
Adult mussels don't have a very exciting life. They stay in one spot, bury their anterior end in the soft river bottom, and leave the posterior end and two siphons exposed (like snorkels). They continuously pump water through their bodies. The water enters through the incurrent siphon, makes a U-turn and exits via the anal siphon. The mussel uses its gills to filter oxygen and food from the water. Their food consists of plankton (microscopic plants and animals) and other organic matter suspended in the water. Essentially, they are doing us a favor—as they eat, they are cleaning our water.
Reproduction and the Youngins
Are mussels and clams different?Basically, bivalves, mussels, and clams are all the same—mollusks with two shells (or valves) that can be opened and closed. There are many species and some are more commonly called clams or mussels.
In the summer when mussels are ready to reproduce, the males merely release sperm into the water, and the females catch what they can. The sperm is siphoned by the female and used to fertilize her eggs internally. Obviously, if they aren't grouped fairly closely, reproduction is hard to achieve. After fertilization, the female then holds up to several thousand eggs at a time in her gills. There they can obtain oxygen and have a place to brood until they develop into glochidia—the larval stage of mussels.
It is the larval stage that is more exciting! In the late spring or early summer, the glochidia are expelled into the water where they have to fend for themselves. They need to attach themselves to the gills of a host fish within a couple days. Most freshwater mussels team up with only one type of fish. Our northwest species favor salmon and without salmon as hosts, mussels cannot successfully reproduce.
Once the larval mussels attach to the fish, the fish body reacts to cover them with cells—an unconscious action that forms a cyst, where the glochidia remain for two to five weeks (depending on the temperature). Hitchhiking on a fish is a baby mussel's only opportunity to travel and experience more of the world, and traveling this way results in a free ride to a new home. After the mussels change from the larval form and begin to resemble adults, they break out of the cyst and fall to the bottom of the stream where they bury themselves in the bottom and begin to live an independent life. Only one in a million survive to the adult stage, but to offset these low odds, mussels lead a very long reproductive life and produce millions of eggs per year!
The Local Yokels
Do mussels have blood?Yes. Most have clear or bluish blood and they have little hearts to pump their blood. A couple species have red blood because they use hemoglobin to carry oxygen just like in mammals rather than the hemocyanin that makes blood clear in the other species.
Although one would expect to see mussels in most of our streams and rivers, they often aren't there. They are only found in the cleanest streams and rivers with cool, clear water and bottoms that aren't muddy. Western pearlshell (Margaritifera falcata) live in rivers and streams with cool, flowing water. Adults prefer sand/gravel substrates, and the juveniles prefer well-oxygenated sand, which is usually found behind debris jams or at the base of woody debris. The most common mussel in our area is M. falcata with observations in an area of northern Big Bear Creek of up to 150 mussels per half square meter (Rensel 1992). An identification tip: most Margaritifera falcata have purple color on the inside of their shell and other local mussels do not.
Oregon floaters (Anodonta oregonensis) are found in slower moving water. They are commonly found in mud, sand, or fine gravel beds. The juveniles attach to gravel in well aerated, flowing waters but the attachment threads dissolve as they age and the mussels are washed downstream where they settle to the sandy bottom in slower moving water. The Anodonta has also been seen in our area.
Western ridgemussel (Gonidea angulata) are found in rivers and only in Pacific drainages (Toy 1998).