Although most people just notice a sea of grasses when they look at a salt marsh, there are many other organisms that make their home in and among these plants. While many of these animals, such as fiddler crabs and snails are less commonly seen as the weather gets colder, there are others, like the ribbed mussel (Geukensia demissa) that are not as mobile; once settled, ribbed mussels are very sedentary. Ribbed mussels occur in salt marshes along the western Atlantic from the Gulf of St. Lawrence in Canada to the coast of Florida. Ribbed mussels are similar to blue mussels in shape and size, but differ in that they have corrugated ribs along their shells and are yellowy-brown, whereas the blue mussels have smooth shells and are bluish-black in color. They are found throughout the intertidal zone attached to hard surfaces or tucked into the sediment, where their byssal threads help anchor them in place. In fact, the ribbed mussel has a mutualistic relationship with the marsh grass, Spartina alterniflora.

Clusters of ribbed mussels along the edge of a salt marsh provide additional support and stability for the root structures of smooth cordgrass (Spartina alterniflora), allowing the plants to withstand ice conditions or high wave energy associated with storms. The dense cluster of ribbed mussels in the photos below is an extreme example of just how well mussels can armor the edge of a salt marsh; these mussels essentially formed a protective barrier, shielding the marsh from waves, boat wakes and ice. In addition to increasing stability, ribbed mussels also increase marsh productivity. As filter feeders, the mussels draw in water along with plankton, detritus and nutrients to their gills, and then deposit the nutrient-rich waste into the sediment where they’re attached. The resulting increased soil nitrogen in turn stimulates marsh growth.

Bivalves (mollusks, such as clams and mussels, that have two shells) in the intertidal zone typically retract their siphons and close their shell tightly when the tide recedes to avoid drying out. However, unlike many other intertidal bivalves, ribbed mussels will sometimes “air-gape”, opening their shell a bit while still exposed to the air, to promote gas exchange. This air-gaping behavior was occurring frequently enough during my walk along the edge of the salt marsh that the opening and closing of their shells was actually audible.