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 degreesC and a 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 as at a latitude of 66 degrees in Norway. 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 fruit is a small nut 4-5 mm long, usually with three spines, two basal and one apical, 1-12 mm long. Ceratophyllum demersum grows in lakes, ponds, and quiet streams with summer water temperatures of 15-30 degreesC and a rich nutrient status.