My dad was out kayaking on a slough near Newport, OR with some botanists and learned
about Japanese (or dwarf) eelgrass, Zostera japonica. It often occurs in the same locations as the native common eelgrass, Z. maritima, but higher in the intertidal if both species are present. It has narrower and shorter leaf blades than common eelgrass.
Japanese eelgrass probably arrived in the 1930s in shipments of Pacific oysters. It reproduces prolifically by seed and spreads by rhizomes. It has established from Humboldt Bay in CA north to the Strait of Georgia in Canada.
You can read an excellent summary about this eelgrass written by Levy Hay at University of Washington in 2011.