Mussels

  What is a Mussel?

Mussels belong to a phylum (a division within the animal kingdom) called Mollusca, that includes 50,000 species of clams, sea slugs, snails, oysters, octopus and squid. All mollusks have a body covering with a sensory organ and respiratory surface, a foot for locomotion and digging, and a visceral area containing organ systems. Some mollusks such as clams and oysters have a hard outer shell (Cummings 2003).

There are seven taxonomic classes of mollusks, based on the number and type of shells produced. Mollusks are in the Class Bivalva because they have two shells. This group includes clams and mussels (Cummings 2003).

Although clams and mussels can be considered as the same, there are differences. In freshwater, a mussel is a large, 2-shelled mollusk with one set of ridges or teeth on the inside edge of each shell. A clam is a freshwater 2-shelled mollusk with two sets of teeth. These teeth serve to keep the shells in line, preventing them from moving from side to side (Cummings 2003).

Figure 58: Mussel Life Cycle (Marietta College, 2004)

The reproduction of mussels occurs in a series of stages (Figure 58). First, the sperm enter the water column, and the female siphons the water, collecting the sperm to fertilize her eggs. It requires one to ten months for the larva, called glochidia, (Figure 59) to fully develop within the shell of the female. When they are ready, the glochidia are released into the water and drift until they find the appropriate fish species that will serve as a host. The larvae attach themselves to the gills of a fish. Notice the hook on the glochidia in Figure 11. The larvae feed from the host fish without harming the fish. The larvae transform to juvenile mussels in one to four weeks, at which time they fall off the host fish and attach or burrow into the substrate and begin feeding (Marietta College 2004).

In order for mussels to reproduce, they must have an appropriate host fish species to continue their life cycle. As illustrated in Table 43, host fish are more abundant in the lower Illinois River, thus enabling the mussels to reproduce more.

Figure 59: Glochidia of a Species of Pyganodon (Canadian Museum of Nature, 2003)

Table 43: Average numbers of common host fishes of mussels caught per 30 minutes of electrofishing in the navigation pools of the Illinois River (1962-1966) (Starrett 1971)

*Includes rock bass, warmouth, Lepomis gulosus (Cuvier); green sunfish; pumpkinseed; orangespotted sunfish; sunfish hybrids; smallmouth bass; largemouth bass; white crappie; and black crappie