Rhizomes very slender to slender. Stems up to 0.6 m, very slender, terete, richly branched; nodal glands absent. Submerged leaves 45-120(-175) x (0.2-)0.4-1.4 mm, 70-250(-500) times as long as wide, bright green or dark green, filiform to linear, elliptical in cross-section, sessile, obtuse to acute at the apex, the margin denticulate at the leaf apex, otherwise entire and plane; midrib bordered on each side by one large or several smaller air channels, the air channels sometimes absent from the younger leaves, lateral veins absent. Floating leaves absent. Leaf sheaths 10-25 mm, open and convolute, the side from which the leaf arises green, the opposite side hyaline; ligule absent; sheaths of involucral leaves dilated, hyaline when young, becoming brown with age. Turions absent. Inflorescences with 2 flowers c. 1.5 mm apart; peduncles 40-300(-770) mm, (1.6-)2-10(-30) times as long as the longest carpel stalk when the fruit is mature, very slender, terete, sinuous or spirally coiled. Flowers with 2-8 carpels, the carpels initially with a very short stalk so that they appear sessile at anthesis, the stalks then elongating after fertilisation, the stalk of mature fruits 4-32 mm (the longest on each peduncle being 14-32 mm), very slender, terete, straight. Fruits 2.7-3.4 x (1.2-) 1.4-1.9 mm, pyriform, symmetrical or slightly asymmetrical about the longitudinal axis, brown or grey-brown with raised, more or less elongated, wine-red tubercles on the surface; beak 0.5-0.95 mm, usually subapical or apical, occasionally ventral, straight.
Similar to R. maritima but leaf tips blunt or rounded and flowers on long, slender, spirally coiled peduncles, and usually found in more saline, deeper water. Mostly Dec.-Feb.
Ruppia cirrhosa grows on soft sediments in the brackish water of ditches, ponds, coastal lagoons, tidal inlets and in lakes near the sea. Unlike R. maritima, it is rarely found in very shallow water. It favours more brackish conditions than R. maritima and is, for example, dominant in Loch an-t-Saile, Outer Hebrides, where the conductivity measured by Spence, Alien & Fraser (1979) was 31,800 µs cm-1 (compared to 43,900 µs cm-1 for seawater). It is a rarer plant than R. maritima, being most frequent in south-east England but also occurring at scattered localities elsewhere in Britain and Ireland. It extends north to the Outer Hebrides, Orkney and Shetland.
Brackish, lagoons and coastal pools of moderate salinity.
Near Gariep Mouth, SW Cape to Port Elizabeth, cosmopolitan.