Seeing Mussels

Mussels are benthic animals from the word benthos which refers the bottom of a sea, river or lake. Healthy mussels are usually very difficult to see because most of the animal is buried in the riverbed.Some mussels live on river bottoms that are sandy and where the current moves quickly, others prefer a muddier bottom where there is less current. Mussels don't move around very much, but when they do, they pull themselves using a fleshy foot that is extended outside its shell. Under good conditions, mussel tracks can be seen on the riverbed. Living mussels are a favored source of food for muskrats, otter, raccoon and other mammals; shells from these “hunting trips” can be found along the riverbanks. Called middens, these piles of shells often contain a surprising variety of mussels in a single location.

Scientifically Speaking

Freshwater mussels are part of the phylum mollusca; one of the oldest, largest, and most diverse phyla in nature with more than 110,000 species. Some molluscs live in freshwater and others live in the salty oceans. Species of molluscs include oysters, nautilus, octopus, snails and a multitude of other well-known invertebrate animals. The fossil record of molluscs goes back to the Cambrian period, 500,000,000 year ago.

Mussel biologists often disagree about the relationships among distinct mussel species. Consequently, scientific names and categories continue to change. Their taxonomy, or how scientists classify molluscs, follows:

• The Phylum Mollusca, meaning "soft-bodied" belongs to the Kingdom Animalia.
• The Class Bivalvia, meaning "two shells" belongs to the phylum Mollusca.
• The Family Unionidae contains only the animals from the Class Bivalvia whose larvae require a host animal.

For more information about naming mussels, see "What's in a Name?"

Life Cycle

1. The sexes of most species of freshwater mussel are separate, male and female. Sperm released by the male directly into the water enters the female via the incurrent siphon. After fertilization, the eggs develop into the larval stage called glochidia (glow-kid-e-a). Theglochidia grow in the gills of the female where they are constantly flushed with oxygen-rich water.

2. After two to ten months of growth in the female, the glochidia are ready to find their host fish. At this point the mussel is very tiny,almost microscopic in size. Various species of mussel have differentways of helping the glochidia find a host fish. Some simply release the glochidia into the water where they float until they come in contact with a fish of the right variety. Others attract fish by waving parts of their tissue in the water. Sometimes this tissue looks like a minnowand for other species, it looks like a fishing lure. When a fish approaches the lure, the glochidia are expelled into the water. With luck, they attach themselves to the gills or fins of the host fish to form cysts.

3. Some species of mussel are host specific, meaning they can only grow on one kind of fish. Others can use different types of fish as hosts. The mussel larvae attach themselves to either the gills or the fins of their host. Not only does the fish provide food and shelter for the glochidia, but also helps move the mussel up or down a stream or across a lake. During the cyst stage, the larvae changes form and comes to resemble a small adult.

4. The small young mussels break free from the host and drop to the bottom of the lake or riverbed. If they land in a place that suits their needs, they will begin their independent life.

Just the Facts

Mussels are bivalves. Bivalves are symmetric. That is, one side looks like the other side. Among species however, shells show a wide range of color, size, surface smoothness, and other characteristics.

Throughout its life, a freshwater mussel continuously pumps water through its body. Water enters via the incurrent or branchial siphon and leaves via the excurrent or anal siphon. Food is filtered from the water during this process. Mussels feed on plankton, microscopic plant sand animals floating in the water.

The concentric lines that are seen on the outside of the shell are growth rings. The lines represent a slow down in growth when the stream is cool (fall and winter). The inside of the shell is usually covered with a smooth layer of nacre (mother of pearl) with few growth lines visible.

Under the right conditions, some mussels species live for as many as 10 to100 years, making them some of the oldest creatures on earth. While mussels can be found singly on a stream or river bottom, they often live closely together in communities called beds. A single bed may contain many species of mussel.

Mussels and clams are not the same. Clams cannot make pearls, but mussels can. Mussel larvae, called glochidia (glow-kid-ee-ah), are parasitic and spend time with a host animal, usually fish. Clams do not have a parasitic stage. Minnesota has only a few tiny species of clams but has nearly 50 species of mussels. Mussels in Minnesota vary from the size of a fingernail to the size of a small dinner plate!

Mussels Live in a Dangerous Environment

Many factors affect the health of mussels. Excessive silt can cover a mussel or a mussel bed smothering the animals. High water and fast current can wash a bed downstream to a place that may not be distrias attractive as the old bed. Channels and dams built for navigation and/or flood control change the nature of the river in ways that help some species of mussels but devastate other species. Exotic, introduced species such as the Zebra Mussel threaten native mussels by competing for food, oxygen and living space. Zebra Mussels attach them selves to native mussels and can grow so thickly that the native mussel can no longer open its shell for feeding and movement causing the native animal to die.

Economic Uses

People know mussels best from their shells and the pearls they produce. Throughout history, pearls have been treasured and used in jewelry and other decroation. Pearls are found in a rainbow of colors and shapes. But mussel shells are also important. Native Americans used mussel shells for many purposes. Mussels were a source of food and shells were fashioned into bowls, spoons and a variety of other utensils and tools. Beautiful shells were traded and sometimes these shells are found in archeological digs far from their native environment. Before the invention and widespread use of plastics, shells were harvested and cut for use as buttons. During the late 19th and early 20th centuries, factories lined the Mississippi dedicated to this industry.

In current times, freshwater mussel shells are ground into small beads with are, in turn, placed inside other mussels to act as seeds for the development of cultured pearl.