The freshwater mussel provides a good example of the interrelationships that go on “behind the scenes” in nature. Mussels live for a surprisingly long time—some as long as half a century. Their age can be estimated by counting shell ridges, much as we can tell the age of trees from growth rings.
To reproduce, freshwater mussels depend on fish. The female mussel broods larvae, called glochidia, within her shell. When she releases them, they fasten onto the gills and fins of certain fish. The glochidia extract nutrients from the host fish’s blood. In a few weeks they are ready to go it alone and drop to the bottom to begin their adult lives, filter-feeding on plankton in the water. As filter feeders, mussels are good indicators of the presence of pollutants in the body of water they inhabit.
If the glochidia larvae did not “hitch a ride” on fish, they would be swept downstream by the current. The parasitic phase of the mussel life cycle assures that mussels will be dispersed upstream as well as downstream.
Now, the funny thing is that the larvae of each species of mussel go for specific types of fish as hosts. When herring and alewives were able to reach Sharon’s now-inaccessible (due to dams on the Neponset River) Lake Massapoag, a type of mussel known as the alewife floater could live there. Since alewives no longer reach the lake, the alewife floater mussel has disappeared as well.
In 1989 the Town of Sharon constructed a well field on the shore of Gavin’s Pond. Since then, the outflow stream from Gavin’s Pond has dried up every summer. Freshwater mussels (elliptio complanata) that lived in the outflow pool died off. Some of these mussels were several decades old. This species relies on yellow perch, sunfish, and largemouth bass to complete their life cycle. Fish remain in the pond, but any fish that escape downstream are eradicated every summer when the stream dries up.
In the United States about 300 species of mussels can be found. Because of dams, pollution, and past over-harvesting (the use of shells to make buttons) many mussel species are endangered. In Massachusetts seven of 12 freshwater mussel species are protected by the Massachusetts Endangered Species Act.
If you are interested in mussels as well as other denizens of Sharon’s Lake Massapoag, there is now an informational kiosk with descriptions and pictures of aquatic life in the lake. It is located at the Pond and Beach Street rotary near the boat ramp area.