Submerged, monoecious, slender annual or short-lived perennial, to 1 m long, though usually much shorter. Stems c. 0.5 mm diam.; internodes not spinescent. Leaves linear, flat, 10–20 mm long, 0.3–0.4 mm wide (excluding spines), acute; margins serrulate with fine spines on multicellular excrescences; spines 0.05–0.16 mm long; septa present to leaf margins; fibres absent; midrib without spines; sheath shortly auriculate; auricle 0.3–0.5 mm long (including spines), serrulate or lacerate, obtuse, with 4–6 spines each side. Male and female flowers at different nodes, but males more common towards plant apex. Male flowers spatheate, c. 2 mm long; anther c. 1 mm long, unisporangiate. Female flowers espatheate, 1–4 mm long; ovary 0.5–0.7 mm long; style and stigma 0.4–0.7 mm long; stigma 2-or 3-lobed. Seed 0.7–1.7 mm long; style base persistent; testa distinctly pitted with areoles in regular longitudinal rows of 18–30.
Stems 20-30 cm tall, ca. 1 mm in diam. Leaves often in pseudowhorls of 3, linear, 1-2 cm × 0.5-1 mm; sheath shortly auriculate; auricles triangular, ca. 2 mm, minutely serrulate with several teeth on apex; leaf margin on each side minutely serrulate with 10-20 teeth. Plants monoecious; flowers solitary or 2 or 3 together. Male flowers ca. 1 mm; spathe with a short neck, with brownish spine cells at apex; anther 1-thecous. Female flowers linear-ellipsoid, ca. 1 mm; style short; stigmas 2-lobed. Fruit linear-ellipsoid, 1.5-1.7 mm. Seeds narrowly ellipsoid, with many rows of pits; areoles squarish to pentangular. Fl. and fr. Aug-Nov. 2n = 12*.