The Recent Introduction of the Seagrass Zostera japonica Aschers. and Graebn. to the Pacific Coast of North America

The seagrass Zostera japonica Aschers. and Graebn., recently introduced to British Columbia, Washington, and Oregon, is smaller than the native eelgrass Z. marina L.; grows intertidally where Z. marina is rare; and is an annual, overwintering as buried seeds, or a short-lived perennial, whereas the longer-lived perennial Z. marina usually overwinters as leafy shoots. Establishment of new populations of Z. japonica occurs mainly by seed germination although the chance of a seedling surviving its first summer is small. Once established, patches expand in area exponentially in spring and may help to stabilize the sediment. The fact that several areas where Z. japonica is abundant are or were sites of intensive oyster cultivation suggests that the plant was introduced as a contaminant (probably seeds) in shipments of Japanese oysters in the first few decades of the 20th century. Now common on the central coast of Washington, in northern Puget Sound, and in the southern Strait of Georgia, Z. japonica occurs as far south as Coos Bay, Oregon, and has the potential to spread further north and south.Key words: biogeography, distribution, establishment, introduction, oyster culture, seagrass, Zostera americana, Zostera japonica