Submerged, monoecious or dioecious marine herbs with creeping shoots rooting at the nodes; nodal scales 2, one subtending a later shoot, the other embracing the stem. Leaves oppsite or clustered, petiolate or sessile, oblong-elliptic, ovate, lanceolate or linear, entire or serrulate, 3-nerved, midrib prominent and 2 lateral intramarginal nerves often connected by cross-nerves. Flowers unisexual, solitary. rarely 1 or more male and one female flower in one spathe. Spathes sessile, 2-bracted, bracts elliptic, obovate to orbicular, acute, obtuse to emarginate. Male flowers: pedicellate; sepals 3, imbricate; petals absent; stamens 3, alternate with the sepals, filaments minute, anthers linear-oblong, 2-4-celled, extrorse, pistillode absent. Female flowers: sessile; sepals 3, minute; ovary inferior, elliptic or ovate, unilocular, long beaked, placentation pariental; styles (2-) 3-5, filiform, papillate all over. Fruit ovoid, few to many-seeded, rostrate, membranous walled. Seeds globose or subglobose.