Ribbed mussels, partially burrowed into the mud of a salt marsh. Courtesy: NOAA

Ribbed Mussel (Geukensia demissa)

Color: Yellowish-brown to brownish-black on the top of the shell, with glossy underside. The body is lemon yellow.
Size: Up to 4 inches long.
Habitat: Lodged within stems and roots of smooth cordgrass in estuaries and salt marshes.
Seasonal appearance: All year.

Description

Ribbed mussels are similar to blue mussels in shape and size but differ in appearance, color, and habitat. The shell of a ribbed mussel is shaped like a long rounded triangle with corrugated ribs along the length. Ribbed mussels are usually found partially buried in the sediment, unlike the blue mussel that attaches to a piling or dock. When buried in the sediment, two slit-like siphons with frilled edges extend from the body to the muddy surface.

Life History and Behavior

Ribbed mussels do not burrow completely into the muddy or sandy bottom but remain partially exposed. They anchor themselves with byssal threads, which are mucous strands attached from the mussel to the substrate. Gills aid the siphons in bringing in water. The gills are lined with cilia, which remove oxygen from the water, and trap plankton and organic matter. Particles of organic nutrients are processed into inorganic matter by the ribbed mussel. The inorganic material is recycled back into the mud. This concentrated inorganic material helps to enrich the surrounding mud and contributes to salt marsh growth.

Burrowers, such as soft-shelled clams, must retract their siphons and close the shell tightly when the tide recedes to avoid desiccation, or drying out. Ribbed mussels, however, burrow in such a way that water is retained in the mantle when the tide recedes, an adaptation of great importance to a life in the intertidal zone. They are able to withstand periods of drought and extreme fluctuations in temperature and salinity.

Ribbed mussels play an important role in Rhode Island salt marshes as filter feeders, removing bacteria, heavy metals, and toxins from the water column.

Special Notes

• Unlike their relative the blue mussel, ribbed mussels are rich in organic bacteria and are not commonly eaten by people.
• Ribbed mussels play a critical role in the health of a salt marsh by exhibiting a cooperative relationship with the plants and animals of a marsh. Mussels establish habitat within the root structures of smooth cordgrass and, in turn, provide essential nutrients that enhance the plants.
• Ribbed mussel beds also provide support and better stability for the root structures of smooth cordgrass, allowing the plants to withstand harsh storm or ice conditions.

Adapted from The Uncommon Guide to Common Life on Narragansett Bay. Save The Bay, 1998.