Eastern Mud Snail
(Ilyanassa obsoleta)

Description: The shell of the eastern mud snail is stoutly built, with 6 whorls, and is more rounded than the tall, cone-shaped shell of the native mud snail, Cerithidea californica, which has more than 6 whorls. The shell, which reaches a length of 2-3 cm, is dark brown to black and has crisscrossed lines that have minute knobs at the intersections. The aperture is black-glazed. Ilyanassa can be distinguished from littorine snails (Littorina spp.) by the shape of the aperture opening: Ilyanassa has an ovate, teardrop shaped aperture while the opening is circular in the littorine species.

Habitat: This snail is found in mud flats and salt marshes in sheltered waters.

Origin: East Coast of North America.

Invaded Areas: San Francisco, and various bays in Northern California, Oregon, Washington, and British Columbia.

Concerns: The eastern mud snail was first discovered in San Francisco Bay in 1907, probably introduced with oysters. It has since become the dominant mudflat snail in the Bay. It also occurs northwards of San Francisco in other estuaries. This snail has been shown to have significant effects on mudflat community structure and sediment composition. Experiments have also revealed that it can competitively displace its native counterpart, Cerithidea californica. The snail acts as an intermediate host for the flatworm that is responsible for swimmer’s itch in humans.