Threeridge

Distribution

The Threeridge occurs throughout the upper Tennessee River basin in southwest Virginia, where it is encountered in the Clinch, Powell, and North Fork Holston River watersheds. In 2024, shell material was observed in the Middle Fork Holston River, the first report of this species in the watershed.

Identifying Characteristics

• Shell is large, thick, and squarish in shape
• Both the anterior and posterior ends are round
• The ventral margin is slightly curved
• Three or more distinct ridges are present on the posterior half of the shell
• The outer covering of the shell (periostracum) varies from yellowish-green to green in color, but darkens to black with age
• The inner portion of the shell (nacre) is white and iridescent, sometimes containing shades of pink or purple

Habitat

The Threeridge lives in a variety of habitats, from small streams to large rivers, as well as reservoirs and lakes. It can be found in a variety of substrates ranging from clay, mud, sand, gravel, to a sand-gravel mixture.

Life History

Freshwater mussels have a very unique life cycle. Parasitic larvae, called glochidia, develop in the gills of the female mussel. Once mature, the glochidia are expelled from the gills of the mussel and must attach to the gills of a suitable host fish, where they complete their transformation into a juvenile mussel. They then detach from the fish and begin their free-living existence on the river bottom.

Some freshwater mussels are host-specialists, meaning that metamorphosis can only occur on a limited number of closely related fish species, while others are host-generalists, with the ability to complete their life cycle using many different species. Female mussels employ a variety of strategies that “trick” their host fish into becoming “infested” with their glochidia. Some have elaborate mantle displays that protrude out of the shell that a fish attacks, while others release their glochidia in packets called “conglutinates” that resemble prey items that fish will ingest.

The Threeridge uses a variety of fish species including Black Bass (Micropterus spp.), Crappie (Pomoxis spp.), Perch (Perca spp.), Sunfish (Lepomis spp.), and True Bass (Morone spp.). Female Threeridge likely use a more passive approach to infest their host fish by broadcasting their glochidia into the water column, which then attach to the gills of fish as they swim by.

Special Considerations

Freshwater mussels are extremely imperiled organisms. DWR regulates the take and possession of freshwater mussels occurring within the state. It is unlawful to take mussels from any inland waters in the Commonwealth except as provided for in §§ 29.1-418 and 29.1-568 of the Code of Virginia.