Ribbed mussels are similar to blue mussels in shape and size, but differ in appearance, color, and habitat.  The shell 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 slitlike siphons with frilled edges extend from the body to the muddy surface.  Gills aid the siphons in bringing in water and are lined with cilia, which remove oxygen from the water and trap plankton and organc matter.  Particles of organic nutrients are processed into inorganic matter helps to enrich the surrounding mud and contributes to salt-march growth.

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.  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, howerver, burrow in such a way that water is retained in the mantle when the tide recedes, an adaption 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.

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 among the root structures of cordgrass (Spartina alterniflora) and, in turn, provide essential nutrients that enhance the plants' growth.  Mussel beds also provide support and stability for the root structure of cordgrass, allowing the plants to withstand harsh storm or ice conditions.