Dioec. Fls solitary, shortly pedicellate. ♂ with per. of 3 minute, membr. segs; stamen single or 3 connate by their connectives; pollen spherical. ♀ with per. of 3 membr. segs about as long as ovary plus style; carpels us. 3, shortly stipitate; ovary oval; style short; stigma cylindric to funnelform. Fr. ± cylindric. Rhizome threadlike, rooting at nodes and giving off slender lfy branches. Lvs alt.; internodes, except near infl., at least as long as sheathing base; lf-base pale, broader than lamina, with short, colourless auricles. 4 spp. of fresh to slightly brackish water, all confined to Australia except that 1 extends to N.Z.