Submerged, marine perennial
herbs, with creeping rhizomes. Leaves linear, alternate or apparently opposite,
distichous, sheathing at base, ligulate at the juncture of sheath and blade.
Flowers small, monoecious or dioecious, unisexual, hydrophilous, naked or
rarely minutely bracteate, borne in axillary cymes or racemes of cymes, or
solitary. Male flower of 2 long pedicelled, 2-celled anthers, anthers paired on
a common filament, or sessile. Female flower of 1 or 2 carpels, sessile or
stipitate, styles and stigma filiform, ovule solitary. Fruit a 1-seeded nutlet,
small, hard and indehiscent.