Rhizomes very slender to slender. Stems up to 0.4 m, very slender, terete, sparingly to richly branched; nodal glands absent. Submerged leaves (20-)35-115 x 0.35-0.9 mm, 50-210 times as long as wide, bright green, filiform, elliptical in cross-selection, sessile, acute at the apex, the margin denticulate at the apex, otherwise entire and plane; midrib bordered on each side by an air channel, lateral veins absent. Floating leaves absent. Leaf sheaths 5-22 mm, open and convolute, the side from which the leaf arises green, the opposite side hyaline; ligule absent; sheaths of involucral leaves slightly dilated, hyaline when young, becoming brown with age. Turions absent. Inflorescences with 2 flowers c. 1.5 mm apart; peduncles (8-)12-26 mm, 0.5-1.8 times as long as the longest carpel stalk when the fruit is mature, rarely as much as 3.3 times as long in dwarf forms, very slender, terete, straight, slightly recurved, arcuate-recurved or coiled with a single turn. Flowers with 3-5 carpels, the carpels initially with a very short stalk so that they appear sessile at anthesis, the stalk elongating after fertilisation, the stalks of mature fruits 3-35 mm (the longest on each peduncle being (3-)10-35 mm), very slender, terete, straight. Fruits 2.0-2.8 x 1.3-1.8 mm, pyriform, asymmetrical about the longitudinal axis, dark brown, with slightly raised more or less elongated wine-red tubercles on the surface; beak 0.4-0.65 mm, apical, straight.
Slender aquatic with thin horizontal rhizomes rooting in the mud. Stems 10-50 cm. (or more) long and 0.2-0.8 mm. thick (but thicker across the leaf-sheaths, green, slender, filiform, much-branched. Leaves numerous, set all along the plant, usually alternate below, often opposite above; sheath light to medium reddish brown, 5-20 mm. long and 0.7-1.5 mm. wide, somewhat inflated, with a minute or up to 2 mm. long tooth on each side of its orifice; blade 2-15 cm. long and 0.2-0.9 mm. wide, linear-filiform with many small teeth along margin near the acute tip. Peduncles only 5-10 mm. long and 0.2-0.5 mm. thick when flowers open, but occasionally elongating to 6 cm. (rarely more than 2 cm.) after anthesis, straight or more commonly recurved in fruit, but not spirally coiled. Anthers elliptic, 0.5-1 mm. long. Ovaries usually 4, 1-1.5 mm. long, narrowly ovoid with a short stipitate base, which elongates at maturity. Fruits obliquely ovoid with a prominent beak, 2-3 mm. long (excluding stalk) and 1.5-2 mm. wide, medium to dark reddish brown or almost black; stipe 5-10(-30) mm. long and 0.2-0.4 mm. thick, triangular, light reddish brown to dark brown.
Submerged, green or brownish, grass-like aquatic perennial with slender, cylindrical, branched stems. Leaves linear to filiform, gradually narrowed to a fine point. Flowers in a small spike on short, slender, straight peduncles.
Ruppia maritima grows in a wide variety of shallow coastal waters, including lakes and ditches, rocky cliff-top pools, creeks and pools in saltmarshes, slowly-flowing streams and tidal estuaries. It is found all round the coasts of Britain and Ireland, and also occurs in brackish water by inland salt deposits near Nantwich and Sandbach in Cheshire (Lee 1977). It is tolerant of a wide range of salinity, growing both in coastal streams which at times are virtually fresh and in ponds which are "brackish, muddy, often slimy and stagnant" (Scott & Palmer 1987). Associated species are usually few, but may include Potamogeton pectinatus and Zannichellia palustris. R. maritima also grows on the muddy flats of saltmarshes, often with Zostera noltii. Stratigraphic evidence suggests that in Poole Harbour such mudflat communities disappeared after Spartina invasion (Hubbard & Stebbings 1968).
Sp. PL: 127 (1753); P. Graebner in E.P. IV. 11:142 (1907); Obermeyer in F.S.A. 1:71, fig. 20/1 (1966); Podlech in Prodr. Fl. SW.-Afr. 144:4 (1966); F.W.T.A, ed. 2, 3: 18, fig. 323 (1968); F.AC., Ruppiac: 2, t. 1 (1977); Fl. Mascareignes 199:1, fig. (1984).