freshwater mussels reproduce by releasing glochidia, which attach to the gills or fins of a suitable host fish and get transported to new habitat
Source: US Fish and Wildlife Service, Extracting glochidia from freshwater mussel
Mussels are the freshwater equivalent of marine clams. They are the longest lived invertebrates in Virginia, with adults living up to 100 years in some species.
Like clams and oysters, they are filter feeders who pump water through their shells and extract food. That exposes them to pollutants in the water during their long lives, and the decline of mussel populations is a clear indicator of the decline of water quality.1
When rivers cease to flow in dry summer months, freshwater fish and other animals find refuge in the remaining pools. Freshwater mussels behave like the livers of rivers, keeping these refuges clean and ensuring animals can survive until the rains return. They maintain water quality by filtering and removing suspended sediments, nutrients, bacteria and algae. They also deposit nutrients on the river bottom, and their burrowing activity mixes and aerates the sediment.
Adult mussels remain in one spot on the riverbed. To occupy new habitat, they rely upon fish to transport their larvae, which are released into the water where they can parasitize a passing fish:2
Freshwater mussels have an unusual and complex mode of reproduction, which includes a brief, obligatory stage as a parasite on a fish... Inside the female mussel, fertilized eggs develop into microscopic larvae known as glochidia. Mussels need to "infect" a host fish with glochidia to complete the reproductive process. Once the glochidia are released from the female, they must attach to the gills or the fins of the right fish host and encyst to complete development...
Glochidia transform into microscopic juveniles and drop off. If by chance they settle into suitable habitat, a new mussel bed is created.
Virginia has about 80 species of mussels, but that number could drop. The US Fish and Wildlife Service declaredthe the green-blossom pearlymussel to be extinct in 2021, and:3
According to the Department of Wildlife Resources, only 30 percent of the state's freshwater mussels are considered stable. In the state's portion of the Tennessee River system, a staggering 31 mussel species are designated by Virginia or the federal government as endangered or threatened; in Virginia's portion of the Chesapeake Bay watershed, where freshwater mussels have received far less attention, six species bear those designations.
The Clinch River has more endangered species than any other river in the United States. One of the most significant kills of endangered species since passage of the Endangered Species Act in 1973 occurred when a tanker truck overturned in 1998 on U.S. Route 460. The accident in Tazewell County spilled a chemical which flowed into the Clinch River and killed tan riffleshell, purple bean, and rough rabbitsfoot mussels, which are three endangered species.4
Mussel restoration projects depend upon two state and one Federal hatcheries. The Virginia Department of Wildlife Resources has a propagation facility in Marion, the Aquatic Wildlife Conservation Center (AWCC), while Virginia Tech operates a Freshwater Mussel Conservation Center (FMCC) in Blacksburg. The US Fish and Wildlife Service raises mussels for transplanting into rivers at the Harrison Lake National Fish Hatchery in Charles City County.5