Taxonomic name: Zostera japonica Aschers. & Graebn.
Synonyms: Zostera americana den Hartog, Zostera nana Roth
Common names: dwarf eelgrass
Organism type: aquatic plant
Zostera japonica is one of approximately 60 seagrasses, or marine angiosperm species. The only documented invasive seagrass, it has invaded Pacific coast estuaries in Canada and the United States. Zostera japonica alters physical habitat structure as well as the richness and densities of resident fauna
Zostera japonica is a submerged hydrophyte of intertidal marine and estuarine habitats. It is a monoecious, predominantly annual, glabrous herb. The rhizomes creep or ascend, and produce roots and shoots at the nodes. Internodes can be variable in length but are more or less elongate. The stems are flattened and branched, and the branches are partly sterile and abbreviated. The roots are mostly un-branched and arranged in two groups with two or more roots in each group. The leaves are simple and distichously arranged. Leaves are typically flat, linear, entire, and exhibit 3 parallel veins. The inflorescence is a flattened spadix enclosed by a spathe. The flowers are small, hydrophilous, and unisexual and lack a perianth. Rhizomes are from 0.5-1.5mm in diameter. Internodes are from 1-3cm long. Leaves can reach lengths of up to 30cm, and widths of 0.8-1.3mm. The leaf sheath can be up to 6cm in length and membranous (Shin and Choi, 1997). NOAA (2004) reports that, "when this species grows on tidal flats, the leaves are short and narrow, but when growing completely submerged in lagoons, the leaves are longer and wider."
estuarine habitats, marine habitats
Shin and Choi (1997) state that, "Zostera japonica can be found in broadly sheltered bays on sandy or muddy coasts at depths of up to 1-3 metres. It is also found along cool to subtropical seacoast in its native range." NOAA (2004) reports that, "Z. japonica is commonly found growing on sheltered tidal flats, but can also be found in brackish (estuarine) coastal lagoons."
Zostera japonica was first documented on the North American Pacific Coast in the late 1950s, and has since colonised historically unvegetated tidal flats and significantly altered physical habitat structure (Baldwin and Lovvorn 1994). Durance (2002) concluded that Z. japonica has contributed to declines in shorebird foraging habitats by causing changes in benthic invertebrate community structure. Posey (1988) noted that the introduction of Z. japonica “… has changed the physical habitat as well as the richness and densities of resident fauna." Larned (2003) demonstrated that Z. japonica invasions alter water column-benthos nutrient fluxes.
Animals associated with Zostera japonica
beds can benefit from available food resources, i.e. primary production, detritus, and epiphytes. The above- and below-ground biomass of Z. japonica
also provide refuge from predators and buffer environmental stresses, such as tidal energy (Lee et al. 2001).
Native range: Asia (Shin and Choi, 1998).
Known introduced range: Pacific Northwest USA (NOAA, 2004).
Introduction pathways to new locations
Other: The fact that several areas where Zostera japonica is abundant are 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 (Harrison and Bigley, 1982). Z. japonica was introduced to Washington through the shipment of Japanese oysters” (Washington State Exotics Expedition, 2000).
Local dispersal methods
Consumption/excretion: Dispersal agents such as birds are required for long distance (>40 km) dispersal (Fong, 1998).
Water currents: New colonies may be formed if the seeds released from the detached flowering shoots encounter a suitable environment during drift (Fong, 1998).
Chemical: Entrix (2003) reported that, "Both imazapyr and glyphosate killed off the eelgrass canopy of both Zostera japonica
and Z. marina. These species were killed if herbicide was applied on dry specimens at low tide, although the imazapyr was more toxic. If applied with a film of water overlying the bed, then no effect was recorded. Within 12 months post-treatment, all impacted eelgrass beds had recovered."
Physical: During April 2003 in Humboldt Bay (California, USA) University of California (UC) Extension workers began removing Z. Japonica with the help of volunteers. They attempted to install sections of plastic over Z. Japonica to kill it by shading, but found that the bay's strong tides pulled the sheets away (Rushton, 2005). The author reported that digging up Z. Japonica resulted in rapid revegetation by native eelgrass Z. Marina.
Zostera japonica is a monoecious species that reproduces both vegetatively (clonally) and sexually (via seed production). New sites are primarily colonized by seeds.
Harrison and Bigley (1982) report that, "The establishment of new populations of Zostera japonica occurs mainly by seed germination although the chance of a seedling surviving its 1st summer is small. Once established, patches expand in area exponentially in spring and may help to stabilize the sediment."
Reviewed by: Jun Bando Genomic Variation Laboratory
University of California-Davis USA
Principal sources: Shin and Choi, 1997. Taxonomy and distribution of Zostera (Zosteraceae) in eastern Asia, with special reference to Korea
Compiled by: National Biological Information Infrastructure (NBII) & IUCN/SSC Invasive Species Specialist Group (ISSG)
Last Modified: Wednesday, 22 March 2006