Sp. Pl. 2: 1015. 1753..
Stems branched distally, 6–45 cm × 0.5–4 mm; internodes 0.3–11 cm, usually with prickles. Leaves spreading to ascending with age, 0.5–3.9 cm, stiff in age; sheaths 2–4.4 mm wide, apex acute; blade 0.4–4.5 mm wide, margins coarsely serrate, teeth 8–13 per side, apex acute, with 1 tooth, teeth multicellular; midvein with prickles abaxially. Flowers 1 per axil, staminate and pistillate on different plants. Staminate flowers in distal to proximal axils, 1.7–3 mm; involucral beaks 2-lobed, 0.3–0.7 mm; anthers 4-loculed, 1.7–3 mm. Pistillate flowers in distal to proximal axils, 2.5–5.7 mm; styles 1.2–1.7 mm; stigmas 3-lobed. Seeds not recurved, reddish brown, ovoid, 2.2–4.5 × 1.2–2.2 mm, apex with style situated at center; testa dull, 10–15 cell layers thick, pitted; areoles irregularly arranged, not in distinctive rows, not ladderlike, 3–4-angled, longer than broad, end walls slightly raised. 2n = 12 (Europe).
Phenology: Flowering summer–fall.
Habitat: Brackish or highly alkaline waters of ponds and lakes
Elevation: 0–1000 m
Ariz., Calif., Fla., Ind., Mich., Minn., Nev., N.Y., N.Dak., Ohio, Pa., S.Dak., Tex., Utah, Wis., Mexico, West Indies, Central America (El Salvador, Panama), South America, Eurasia.
With its prickly internodes and prickles along the abaxial surface of the leaves, Najas marina is the easiest of our Najas to recognize. Over its entire range, the species displays considerable morphologic variability (L. Triest et al. 1986), giving cause for ten subspecies to be recognized (L. Triest 1988). In North America, however, variability is relatively minor, so I am recognizing the taxon at the specific level only (R. R. Haynes 1979). The species should be studied over its entire range, utilizing a variety of approaches, to determine adequately what, if any, infraspecific taxa should be recognized.