Small, creeping, coastal, submerged annuals. Roots 4-7 from a node, simple, long, densely covered with long root-hairs. Rhizome branched, slender, with a deciduous, lanceolate, amplexicaul scale from each node. Stems simple, erect, up to 150 mm long, filiform. Leaves usually aggregated above, filiform-setaceous, up to 50 mm long; sheaths folded, membranous, about 5 mm long, forming a forked appendage above, 2-3 mm long, becoming frayed with age. Flowers in clusters. Male flowers solitary, on a pedicel which elongates up to 4-5 mm during anthesis so that anther is just exserted, often with 3, as yet undeveloped, female flowers beside them; perianth a small 3-toothed cup, with the 1-thecous anther sessile or subsessile; pollen globose. Female flowers usually 3 together; (1-)3-carpellate; carpels stipitate, each enfolded in a membranous bract, narrow-ovoid, unilocular, with long style and saucer-shaped apical stigma. Fruit a stipitate, cylindrical, slightly oblique, smooth drupelet crowned with persistent, pin-like style and stigma base.
Species 3, or 1 polymorphic species: Althenia filiformis F.Petit, Mediterranean coast, Atlantic coast of Morocco, Portugal, Spain, France, Asia Minor, central Siberia and the coastal waters of Namibia and South Africa as far as Port Elizabeth (Eastern Cape).