T.E.R:R.A.I.N - Taranaki Educational Resource: Research, Analysis and Information Network

Ceratophyllum demersum (Hornwort)

Kingdom: Plantae
Clade: Angiosperms
Order: Ceratophyllales
Family: Ceratophyllaceae
Genus: Ceratophyllum
Species: C. demersum
Binomial name: Ceratophyllum demersum
Common names: Hornwort, Rigid hornwort

Ceratophyllum demersum is a species of submerged, free-floating aquatic plant, with a cosmopolitan distribution, native to all continents except Antarctica. Since it is a submerged oxygenator it is a popular aquarium plant. 

In New Zealand, it is a harmful introduced weed and is classed as an unwanted organism. Its dense growth habit crowds and shades out native underwater vegetation, leading to loss of biodiversity. Here in New Zealand, it has caused problems with hydroelectric power plants. It is rapidly spreading in the North Island. It was eradicated from two sites in the South Island. It is spread by plant fragments in moving water, by contaminated boats, trailers, machinery, nets and during the liberation of aquarium fish.

Ceratophyllum demersum is a floating, freshwater, perennial, aquatic plant. It does not have roots though it may attach itself to the substrate or objects. Its stems can reach lengths of 1–3 m, with numerous side, shoots making a single specimen appear as a large, bushy mass. In some clear water lakes, it can be found growing >16m deep.
The dark green 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.

All sightings should be reported to the Department of Conservation or the local regional council.

A job inspecting a catfish cordon in Te Weta Bay, Lake Rotoiti, Rotorua was stopped when an ROV backed into the resident forest of Hornwort.

Thanks to Wikipedia for text and information: https://creativecommons.org/licenses/by-sa/3.0/