Hydatellaceae are small, aquatic flowering plants. The family includes the genus Trithuria, which has been recently re-defined to include the genus Hydatella. The family consists of about a dozen species. These tiny (few cm tall), relatively simple, aquatic plants occur in Australasia and India. The simple leaves are concentrated around a short stem basally. The plants are submerged and emergent aquatic annuals, rooted in the substrate below the water.
The members of this plant family are monoecious or dioecious and are likely wind-pollinated (anemophilous), water-pollinated (hydrophilous) or self-pollinating (autogamous). Flower-like reproductive units (which may be pseudanthia) are composed of small collections of minute stamen- and/or pistil-like structures that may each represent very reduced individual flowers. The non-fleshy fruits are follicles or achenes.
The family was for many years assumed to be a close relative of the grasses and sedges and was even sometimes lumped under the poalean family Centrolepidaceae. Even as recently as 2003, the APG II system assigned Hydatellaceae to the grass order Poales in the commelinid monocots. However, research based on DNA sequences and morphology by Saarela et al. indicates that Hydatellaceae is the living sister group of the water lilies (Nymphaeaceae and Cabombaceae) and thus represents one of the most ancient lineages of flowering plants. This realignment is now recognized in the APG III system of classification; developers of earlier classifications were misled by the apparently reduced vegetative and reproductive morphology of these plants.