Aquatic or mud-inhabiting herbs, with broad or narrow leaves borne on a very short stem. Flowers regular, monoecious or dioecious, arising from spathes of distinct or united bracts. Perianth superior, 6-parted, the segments usually all petaloid, the flower-tube adnate to the ovary in the pistillate flowers. Stamens 6-12, distinct or monodelphous. Ovary usually 6-9-celled. Styles 6-9. Ovules numerous. Fruit somewhat fleshy, usually indehiscent. Seeds numerous.

About 8 genera and 20 species, in temperate and tropical regions. Thalassia in Florida.