Submerged marine or fresh-water herbs with fibrous roots and slender, branching stems. Internodes spiny or unarmed. Leaves alternate, opposite, or whorled, the blades linear, spiny on the margins or the back, or both, acute or obtuse at the apex, sometimes tipped with one or more subulate spines, sheathing at the base. Leaf-sheaths rounded or auriculate, entire or with spinulose teeth. The 2 intra-vaginal scales minute and hyaline. Flowers monoecious or dioecious, solitary in the axils. Staminate flowers with a double perianth-like envelope, the inner hyaline, the outer 4-horned or entire. Stamen 1, sessile. Anthers 1-4-celled. Pistillate flowers of a single pistil. Style short or wanting. Stigmas 2-4 and sometimes intermingled with 1-3 stigmoids. Fruit a sessile drupelet, the solitary seed filling the pericarp. The seed apparently smooth and shining, or reticulated with angled or roundish areolae. Embryo straight. Only the following genus: