Panarctic Flora

230101-02 The Zostera marina aggregate Z. angustifolia, Z. marina

2n= 12 (2x). - Europe, Canada (W), Greenland. - Numerous reports.
The majority of chromosome number reports from the northernmost regions may belong to Z. angustifolia.
Not included: A report of 2n = 24 (4x) from New Zealand may belong to another species.

Geography: Amphi-Atlantic & amphi-Pacific.

Notes: Tzvelev (PAF proposal, comment) accepted Zostera marina with two subspecies: subsp. marina and subsp. hornemanniana. European authors often accept two species as Z. angustifolia (= subsp. hornemanniana) and Z. marina (e.g., Markgraf 1972; Markgraf and Zoller 1981; Stace 1997; Elven et al. 2005). The northern European material can be divided into two taxa on several constant features, both quantitative and qualitative, vegetative and reproductive: leaves 3-10 mm broad with 5(-9) veins in Z. marina vs. 1-3 mm broad with 3 (rarely 5) veins in Z. angustifolia; shoots little branched, mainly basally vs. much branched and not only basally; stigmas twice as long as style vs. as long as style; and fruit 3-3.5 mm vs. 2.5-3 mm (see, e.g., Stace 1997; Elven et al. 2005). Populations with both taxa mixed are fairly frequent in northern Europe and without observed transitional forms. We have applied the same characters on Greenlandic, Canadian, and Alaskan plants and have found the same differences. Tzvelev suggested that the narrow-leaved plants (Z. angustifolia) may occur in more shallow and less saline waters than the broad-leaved plants. Both broad-leaved and narrow-leaved plants reach the Arctic but mainly narrow-leaved ones. In view of the co-occurrence, the lack of transition, and the distinction in several assumed independent characters, we accept two species.

The ranges of the two species are not known in detail. Inspected plants from northern Scandinavia (BG, O, TRH, TROM), Iceland (AMNH, ICEL), northern Canada (CAN), northwestern Alaska (ALA, O), and East Chukotka (ALA, O) belong to Z. angustifolia, whereas both Z. marina and Z. angustifolia are present in Greenland. Zostera marina reaches north to the boreal parts of northern Europe and eastern and western North America. Our investigations do not support the arctic ranges as reported by Tzvelev (he reported Z. marina s. str. from Iceland, Norway, European Russia, the Russian Far East, Alaska, Canada, and perhaps Greenland, and Z. angustifolia from the Russian Far East and perhaps Greenland).