The Exotics Guide

6

Geukensia demissa in a peat bank under cordgrass (Spartina alterniflora) in a salt pond on Long Island, New York. The mussel occurs in similar situations in San Francisco Bay, in vertical banks on the edges of salt marshes.

Return to Top

Geukensia demissa Image 5

Dense growth of Geukensia demissa in San Francisco Bay.

Return to Top

4

Geukensia demissa in mud among stems of exotic cordgrass (Spartina alterniflora. and/or Spartina alterniflora-foliosa hybrids) in San Francisco Bay. Barnacles (Balanus glandula) are growing on several of the mussels.

Return to Top

3

A young Geukensia demissa attached to a high intertidal rock in San Francisco Bay.

Return to Top

Geukensia demissa Key Image

Geukensia demissa from San Francisco Bay. The head ends of the shells are toward the bottom of the image, and the hind margins toward the top. Note the black, glossy periostracum covering the shell, but worn away over the central portion revealing the white shell underneath.

Return to Top

Geukensia demissa Image 1

Geukensia demissa embedded in mud in San Francisco Bay. Note the strong radial ridges that are characteristic of this species.

Return to Top

Geukensia demissa (Dillwyn, 1817)

RIBBED MUSSEL, RIBBED HORSEMUSSEL

Kingdom: Animalia
Phylum: Mollusca
Class: Bivalvia
Subclass: Pteriomorpha
Order: Mytiloida
Family: Mytilidae

Geukensia demissa is a mussel that typically lies partly buried in the mud with its narrow, bluntly-pointed, head end directed downward and its broader, round hind end protruding from the sediment. It generally grows to about 10 cm in length, with the largest specimens reaching 13 cm. The umbo—the hump at the center of the concentric growth lines—is near but not right at the edge of the head end of the shell. Numerous strong ridges run from the umbo out to the hind edge of the shell.

Return to Top