Perennial, dioecious, submerged herbs, growing in the sea. Rhizomes present, herbaceous and monopodial or woody and sympodial. Leaves alternate, distichous, with distinct lamina and sheathing base; sheath auriculate; lamina linear, flat or needle-like; ligule present. Flowers enclosed by leaf-like bracts, usually solitary, sometimes in cymes (Syringodium); perianth 0. Male flowers consisting of 2 stamens borne on a filament. Female flowers consisting of two 1-ovulate carpels. Fruit indehiscent.