Stout aquatic or marsh-dwelling herb; stems to 2 m long, 4-5 mm diam. near base, glaucous, rooting freely from lower nodes, glabrous. Leaves all whorled, slightly dimorphic. Submerged leaves in whorls of (4-)5-6, oblanceolate in outline, rounded at apex, (1.7-)3.5-4.0 cm long, (0.4-)0.8-1.2 cm wide, pectinate, with 25-30 linear pinnae up to 0.7 cm long, the lower leaves usually decaying rapidly. Emergent leaves glaucous, in whorls of (4-)5-6, erect near apex, ± spreading in lower parts, narrowly oblanceolate in outline, rounded at apex, (1.5-)2.5-3.5 cm long, (0.4-)0.7-0.8 cm wide, pectinate, with (18-)24-36 pinnae in the upper 4/5 (lower 5-7 mm of leaf rachis naked), pinnae linear-subulate, 4.5-5.5 mm long, 0.3 mm wide, tips very shortly apiculate, slightly incurved. Numerous hydathodes at base of emergent leaves. Plant dioecious. Inflorescence an indeterminate spike with the flowers borne singly in the axils of the upper emergent leaves and subtended by 2 bracteoles. Leaves subtending flowers in whorls of 6, pectinate, ± identical to other emergent leaves in size and shape. Bracteoles subulate, 1.2-1.5.mm long, with (1-)2 short teeth in lower 1/3, sometimes almost trifid. Flowers (in New Zealand) only female, 4-merous, on pedicel c. 0.2-0.4 mm long. Sepals 4, white, deltoid, 0.4-0.5 mm long, 0.3 mm wide, denticulate with 1-several small teeth on each margin, smooth. Petals absent. Styles 4, clavate, 0.1-0.2 mm long, stigmas white, densely fimbriate. Ovary pyriform, 0.6-0.7 mm long, 0.6 mm wide, 4-ribbed longitudinally between sepals. Male flowers and fruits not developed in New Zealand.

Stout, glabrous, dioecious herb. Stems to 2 m long, 4-5 mm diam. near base, glaucous, rooting from lower nodes. Lvs all whorled, slightly dimorphic. Submerged lvs in whorls of (4)-5-6, (17)-35-45 × (4)-8-15 mm, oblanceolate or oblong with rounded apex, pectinate; pinnae 25-30, to 7 mm long, linear-subulate. Aerial lvs in whorls of (4)-5-6, (15)-25-35 × (4)-7-8 mm, narrowly oblanceolate or oblong with rounded apex, pectinate, glaucous, ± erect near apex; pinnae (18)-24-36, to 5.5 mm long, linear-subulate, slightly incurved. Base of aerial lvs with numerous hydathodes. Fls solitary in the axils of upper aerial lvs which are almost identical to other aerial lvs. Bracteoles 1.2-1.5 mm long, subulate, sometimes almost 3-fid. ♂ fls not seen in N.Z. ♀ fls with pedicel c. 0.2-0.4 mm long; sepals 0.4-0.5 × 0.3 mm, deltoid, white, denticulate with 1-several teeth; petals 0; ovary 4-ribbed between sepals. Fr. not seen.