Introduction To Mussels
What are Mussels?
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 mollusks live in freshwater and others live in the salty oceans. Species of mollusks include oysters, nautiluses, octopi, snails and a multitude of other well-known invertebrate animals. Mollusk fossils can be found as far back as the Cambrian period, nearly 500 million years 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 mollusks, 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?"
1. The male mussel releases sperm directly into the water, which enters the female via the incurrent siphon. After fertilization, the eggs develop into the larval stage called glochidia (gloh-kid-ee-uh). The glochidia grow in the gills of the female where they are constantly flushed with oxygen-rich water.2. After growing for two months to a year in the female, the glochidia are ready to leave their mother. At this point, the glochidia are very tiny, almost microscopic in size. To successfully make it to adulthood, the glochidia become parasitic, and must attach to a host fish. Freshwater mussels have a number of different ways of helping their glochidia find a host fish. Some species 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. A few varieties have tissue that looks like a minnow or crayfish (video, 1.5 min), complete with spots that look like eyes. When a fish approaches or bites the lure (video, 5 sec), the glochidia are expelled into the water near or on the fish. With luck, they attach themselves to the gills or fins of the host fish. Many species of mussel are "host specific", meaning their glochidia can only grow on one species of fish. Unfortunately, this makes it difficult for them to reproduce in large numbers.
3. When the glochidia come in contact with the right species of fish, they enclose themselves in cysts on its gills or fins and feed off of the fish's blood. With this extra nutrients, the glochidia transform themselves into tiny adults. During this transformation period, the fish will likely travel, providing the tiny mussel with transportation.
4. After a period a few weeks to months, the young mussels begin to drop off of their host fish. Once they fall off, they bury themselves in the bottom substrate of the river or lake they're in. If they land in a place that suits their needs, they will begin their independent life as an adult mussel.
Mussels are benthic animals, meaning that they spend their lives on the bottom of lakes, rivers, and streams. Healthy mussels are usually very difficult to see because most of the animal is buried in the riverbed. While some mussels live on sandy river bottoms 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 along using a fleshy foot that they extend outside their shells. Under good conditions, mussel tracks can be seen on the riverbed.
Small changes in habitat can drastically affect a mussel's health. 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 as preferable as the old bed. Channels and dams built for navigation and/or flood control change the nature of the river in ways that can help some species of mussels, but devastate others. Exotic species, such as the Zebra mussel, threaten native mussels by competing for food, oxygen and living space. Zebra mussels attach themselves to native mussels and can grow so thickly that the native mussel can no longer open its shell to feed, move, or breathe, eventually killing it.
Freshwater mussels are a favorite source of food for muskrats, otters, raccoons and other mammals, and shells from their "hunting trips" can be found in piles along the riverbanks. Called middens, these piles of shells often contain a surprising variety of mussels in a single location. Ducks, geese, and flatworms have also been known to eat these critters. Historically, Native Americans and settlers ate freshwater mussels, but a chewy consistency and unpleasant taste helped keep them out of American cookbooks.
In current times, freshwater mussel shells are ground into small beads which are, in turn, placed inside other mussels to act as seeds for the development of cultured pearls.
Other Interesting Facts
Mussels are bivalves. Bivalves are symmetric, meaning both of their sides look the same. Among species however, shells show a wide range of color, size, surface smoothness, and other characteristics.
The concentric lines that are seen on the outside of the shell are growth rings. The lines represent a slow in growth when the stream is cool (fall and winter). Typically, the age of the animal can be determined by how many concentric rings it possesses. The inside of the shell is usually covered with a smooth layer of nacre (mother of pearl) with few growth lines visible. Depending on the species, the color of the nacre can vary from white, to pink, purple, or even blue.
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 plants and animals floating in the water.
Under the right conditions, some mussels species live for 100+ years, making them some of the oldest creatures on earth. While mussels can be found alone 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 are parasitic and spend time with a host animal, usually a 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 quarter sized to the size of a small dinner plate!