Ceratophyllum demersum, commonly known as Hornwort, Rigid Hornwort, Coontail, or Coon’s Tail, is a species of Ceratophyllum. It is a submerged, free-floating aquatic plant, with a cosmopolitan distribution, native to all continents except Antarctica. It is a harmful introduced weed in New Zealand. It is also a popular aquarium plant.
Ceratophyllum demersum grows in lakes, ponds, and quiet streams with summer water temperatures of 15 – 30 °C and rich nutrient status. In North America, it occurs in the entire US and Canada, except Newfoundland. In Europe, it has been reported as far north in Norway. Other reported occurrences include China, Siberia, Burkina Faso and in the Volta River in Ghana (Africa), Vietnam, and New Zealand (introduced). Ceratophyllum demersum grows in still or very slow-moving water.
An aquatic plant, Ceratophyllum demersum has stems that reach lengths of 1 – 3 m (3 – 10 ft), with numerous side shoots making a single specimen appear as a large, bushy mass. The leaves are produced in whorls of six to twelve, each leaf 8 – 40 mm long, simple, or forked into two to eight thread-like segments edged with spiny teeth; they are stiff and brittle. It is monoecious, with separate male and female flowers produced on the same plant. The flowers are small, 2 mm long, with eight or more greenish-brown petals; they are produced in the leaf axils. The is a small nut 4 – 5 mm long, usually with three spines, two basal and one apical, 1 – 12 mm long.
This species is often used as a floating freshwater plant in both coldwater and tropical aquaria. Though without roots, it may attach itself to the substrate or objects in the aquarium. Its fluffy, filamentous, bright-green leaves provide excellent cover for newly hatched fish. It is propagated by cuttings. It is frequently used as a model organism for studies of plant physiology. One of the reasons for this is that it allows studies on shoot effects without the influence of a root, which often makes interpretation of nutrition and toxicity experiments difficult in terrestrial plants.
In aquaria, this plant appears to drop all its leaves when exposed to products designed to kill snails. The stems can recover relatively quickly, growing new leaves within a few weeks.