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common eel-grass, eel-grass, grass wrack, seawrack, zostère marine

dwarf eelgrass, Japanese eel-grass, narrow-bladed eelgrass

Habit Herbs, perennial. Herbs, annual, rarely perennial.
Rhizomes

2–6 mm thick;

roots 5–20 at each node.

0.5–1.5 mm thick;

roots 2 at each node.

Leaves

sheath tubular, rupturing with age, 5–20 cm, membranous flaps absent;

blade to 110 cm × 2–12 mm, apex round-obtuse or slightly mucronate;

veins 5–11.

sheath open, persistent, 3–5.5 cm, membranous flaps 2;

blade to 15 cm × 1–1.5 mm, apex obtuse or retuse;

veins 3.

Inflorescence(s)

peduncles with adnate portion 15–100 mm, free portion 2–3 cm;

spathes 10 or more, sheath 4–8 cm × 2–5 mm;

blade 5–20 cm;

spadix linear; staminate flowers 1–20; pistillate flowers 1–20, apex acute or mucronate.

peduncles with adnate portion 20–90 mm, free portion 1.2–3.1 cm;

spathes 2–5, sheath 2–4 cm × 2–2.5 mm;

blade 5–10 cm;

spadix lanceolate; staminate flowers 5–7; pistillate flowers 5–8, apex obtuse.

Staminate flowers

bracts absent or rarely 1 subtending most proximal flower;

pollen sacs 4–5 × 1 mm.

bracts subtending every flower;

pollen sacs 0.5 × 0.7 mm.

Pistillate flowers

ovary 2–3 mm, style 1–3 mm.

ovary 1.5–2 mm;

style 1–2 mm.

Fruits

ellipsoid to ovoid, 2–5 mm, often beaked.

ellipsoid, 2.5 mm, beaked.

Generative

shoots terminal, repeatedly branched, each branch with 1–5 spathes.

shoots lateral, mostly unbranched, with 1–5 spathes.

Zostera marina

Zostera japonica

Phenology Flowering late spring–summer. Flowering early fall.
Habitat Intertidal to sublittoral of marine waters Intertidal marine waters
Elevation -10–0 m (-0–0 ft) -2–0 m (-0–0 ft)
Distribution
from FNA
AK; CA; CT; DE; MA; MD; ME; NC; NH; NY; OR; RI; VA; WA; BC; NB; NF; NS; NU; ON; PE; QC; YT; Mexico (Baja California, Sinaloa, Sonora); Eurasia
[WildflowerSearch map]
[BONAP county map]
from FNA
WA; BC; Asia [Introduced in North America]
[WildflowerSearch map]
Discussion

Zostera marina is adapted to the cold waters of the North Atlantic and North Pacific. It extends southward to North Carolina in the Atlantic and Baja California in the Pacific. At the southern limits of its range, active growth mostly is in the cooler months of autumn and spring, with flowering and fruiting mostly in the spring and the plants dying in the hotter summer months, the vegetation becoming dislodged from the substrate and floating to the water surface. The fruits apparently remain in the floating vegetation for a period of time, eventually falling from the shoots to the substrate. Movement in dislodged vegetative material is the only adaptation the fruits have for dispersal (C. den Hartog 1970).

The species is found mostly in the sublittoral region, only rarely being exposed at low tide. It occurs in more or less sheltered areas on soft mud or firm sand. Plants of sandy substrates had narrower leaves than plants growing on muddy substrates (C. H. Ostenfeld 1905). Fruits fall from the floating vegetation to the substrate and settle on the substrate ripple marks, which run more or less perpendicular to the direction of current. Seedling establishment is parallel with the ripple marks, forming vegetated ridges separated by depressions, which gradually fill with sediments, and the plants then grow laterally into them, forming a meadow (C. den Hartog 1970). The vegetation lowers the velocity of current flow, causing some suspended particles to settle out and accumulate around the base of the plants, slowly building the substrate. As more particles accumulate, the substrate gets deeper over the rhizomes, since the rhizomes grow horizontally, not vertically. Eventually, the rhizomes are too deep, and the plants begin to die back, a phenomenon followed by erosion.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

The name Zostera americana was proposed for some of the collections by Neil Hotchkiss from Pacific County, Washington (C. den Hartog 1970). Because Z. americana resembled a previously published species, it was suggested the name should be placed in synonymy, at least until further study could be undertaken of at least the ecology and genetics of the complex (R. C. Phillips and R. F. Shaw 1976; P. G. Harrison 1976). A proposal that Z. americana was synonymous with Z. noltii was based upon the identical or overlapping ranges of most characteristics (R. C. Phillips and R. F. Shaw 1976). Zostera noltii is native to the Atlantic coasts of Europe and Africa and to the Mediterranean Sea area. Therefore, the suggestion implies that Z. noltii has been introduced into North America. No mode of introduction was discussed, however.

Similarly P. G. Harrison (1976) suggested an introduction of an exotic species, but he suggested Zostera japonica instead. A study of populations of Z. americana from Boundary Bay, south of Vancouver, British Columbia revealed no obvious differences between those plants and individuals of Z. japonica and Z. noltii. A comparison of the British Columbia specimens with illustrations by C. den Hartog (1970) of both Z. japonica and Z. noltii indicated the British Columbia plants resembled more the illustrations of Z. japonica than those of Z. noltii. A discussion of possible modes of introduction noted that a brown alga, Sargassum muticum, was introduced into the North American Pacific coast area with seed oysters. Zostera japonica occurs in areas where the oysters were obtained in Japan, and oysters were packed in Zostera species during shipment. Such shipments were possibly the means by which the species was introduced into North America. Harrison’s explanation is quite plausible, and I am accepting it until further research solves the problem.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Source FNA vol. 22. FNA vol. 22.
Parent taxa Zosteraceae > Zostera Zosteraceae > Zostera
Sibling taxa
Z. japonica
Z. marina
Synonyms Z. marina var. stenophylla Z. americana
Name authority Linnaeus: Sp. Pl. 2: 968. (1753) Ascherson & Graebner: in H. G. A. Engler, Das Pflanzenreich 31[IV,11]: 32. (1907)
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