Atlantic Ribbed Mussel
(Ischadium demissum)

Description: The radial ribbing on the exterior of the shells is distinctive for this mussel. A native ribbed mussel (Septifer bifurcatus) is found in a different habitat, attached under rocks in the intertidal of the open coast. The beak of the Atlantic ribbed mussel is offset from the end of the shell, while S. bifurcatus and species of other mussels (Mytilus) have the beak set directly at the end of the shell. The shells are usually dark yellowish-brown or black, fairly thin, and can reach a maximum size of 10 cm in length.

Habitat: Anchored to marsh plants or other firm substrates in muddy habitats of bays and estuaries.

Origin: East Coast of North America.

Invaded Areas: San Francisco Bay and various bays in southern California.

Concerns: Ribbed mussels can form huge aggregations that cover large areas. In areas of high infestation (e.g., San Francisco Bay), this species poses a serious threat to native organisms and can alter the physical environment. In the San Francisco Bay, the Atlantic ribbed mussel directly competes for resources with native suspension-feeding groups of organisms belonging to many different taxonomic groups. This species has also been blamed for injuring and killing endangered California Clapper Rails that forage in mussel infested areas. The open shells of these mussels can close on rail feet, trapping the birds when the tide returns, resulting in death or amputation. The mussels can also clamp shut on the rails’ beaks, resulting in starvation. This hardy species of mussel has the ability to thrive in varying environmental conditions and can tolerate polluted waters.