Flower:

Separate male and female flowers on the same plant (monoecious). Flowers are few, inconspicuous, alternating with leaves at the leaf nodes, about 2 mm long, stalkless, the males with pink to red stamens, the females with a single, yellowish, spine-like style.

Leaves and stems:

Plants are entirely submersed. Leaves are whorled all along the stem with 6 to 12 leaves in a whorl, the whorls ½ to 2+ inches in diameter. Leaves are firm, mostly forked once or twice (rarely 3 times) with very narrow segments that have a few to several sharp teeth along the edge. The leaf base is stalkless and not inflated.

Whorls at stem and branch tips are often more crowded and compact than farther down the stem.

Stems are light green to reddish, brittle and easily break apart, smooth and much branched, the branches ascending to spreading. Plants have no roots and may be free floating but are more often anchored in the substrate by whitish modified leaves. Turions (winter buds) are formed later in the season, drop off the parent plant and form new shoots the next spring.

Fruit:

Fruit is an oval, slightly flattened seed with 3 spines, 2 at the base and one at the tip. Rarely the spines at the base are absent.

Fruit is dark green to reddish brown at maturity, 3.5 to 6 mm long, excluding the spines. Spines are straight to curved, the basal spines .1 to 12 mm (to ~½ inch) long, the terminal spine .5 to 14 mm long.

Notes:

Common Coontail is one of the most common aquatic plants in the world. Native to North America, it now has a global distribution primarily through the aquarium and pond trade. It produces very little seed and spreads mostly vegetatively, from stem fragments and turions. It can form large, dense mats in favorable conditions; it's been reported to reach lengths of 10 meters (~33 feet) in New Zealand, where it is invasive. In Minnesota, it's likely in a lake or pond near you, wherever you are.

Ceratophyllum species are recognized by forked leaves that are whorled all along the stem with 5 to 12 leaves in a whorl, flowers at the leaf nodes, fruit that have a spine at the tip and usually spines at the base and/or along the edges; plants lack roots and are completely submersed, commonly anchored in the substrate by modified leaves. C. demersum has 6 or more leaves in a whorl, leaves are fairly firm, visibly toothed, and forked only once or twice (rarely 3 times); fruit has a pair of basal spines and none along the edges. It most closely resembles C. echinatum (Spiny Coontail), which is much less common, has 5 or more leaves in a whorl, most leaves are forked 3 or 4 times and have few if any teeth, the leaf segments are more thread-like and limp, and fruit has spines along the edges as well as the tip and base.

The overall form of Ceratophyllum is like some other aquatic species, notably Myriophyllum (Water Milfoil), all but one of which have leaves whorled in 4s, the leaves compound with a central stalk and multiple spreading leaflets, and most with an emersed terminal spike of flowers and fruits.