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      Ceratophyllum demersum (Illustration: University of Florida, Center for Aquatic and Invasive Plants - used with permission) - Click for full size   Ceratophyllum demersum (Photo: Vic Ramey, University of Florida, Center for Aquatic and Invasive Plants - used with permission) - Click for full size   Ceratophyllum demersum (Photo: Ann Murray, University of Florida, Center for Aquatic and Invasive Plants -  used with permission) - Click for full size   Ceratophyllum demersum (Photo: Vic Ramey, University of Florida, Center for Aquatic and Invasive Plants- used with permission) - Click for full size   Ceratophyllum demersum (Photo: Kerry Dressler, University of Florida, Center for Aquatic and Invasive Plants) - Click for full size
    Taxonomic name: Ceratophyllum demersum L.
    Synonyms: Ceratophyllum apiculatum Cham., Ceratophyllum demersum var. apiculatum (Cham.) Aschers., Ceratophyllum demersum var. apiculatum (Cham.) Garcke
    Common names: common hornwort (English-United States of America), coon's-tail (English-United States of America), coontail (English-New Zealand), hornwort (English-New Zealand), rigid hornwort (English-United Kingdom)
    Organism type: aquatic plant
    Ceratophyllum demersum is a native of North America. It now has a worldwide distribution, at least in part due to the aquarium and pond trade. It is a submerged aquatic plant which is capable of forming dense monospecific beds, excluding other plant species, causing problems to recreational activities on waterways and in some cases causing blockages at hydroelectric power stations. C. demersum can spread rapidly, and grows in a large range of aquatic habitats.
    Ceratophyllum demersum is a submerged perennial macrophyte which will normally grow with the base of its stem buried in sandy or silty substrates. It does not form roots. It is prone to dislodgement, and its buoyant stems may become free-floating. It can form a dense subsurface canopy and reach of height of 5-6m and frequently grows as a mono-specific community (heights of 10m have been reported in Maraetai, New Zealand (Rohan Wells.,, 2005)) (NIWA, 2005b). C. demersum can form modified leaves when it is growing near the lake bottom, which it uses to anchor to the sediment (Keskinkan et al. 2004).
    Similar Species
    Myriophyllum spicatum

    Occurs in:
    lakes, water courses
    Habitat description
    Ceratophyllum demersum can be found in ponds, lakes, ditches, and quiet streams with moderate to high nutrient levels (Johnson et al. 1995; in Keskinkan et al. 2004). It will grow in waters that are clear or turbid, still or flowing, and warm or ice-covered (NIWA, 2001). In New Zealand, as well as overseas, C. demersum has performed well in eutrophic waters (Coffey and Clayton, 1988), however, evidence suggests that the success of C. demersumet al. 1997) and it may be able to invade a wider variety of habitats than previously thought. Janauer (2003) notes that the frequency of C. demersum in the Danube River is "surprisingly high for a species supposedly confined to still water habitats".
    Coffey and Clayton (1988) described C. demersum as a brittle, poorly attached plant, which is prone to dislodgement by water currents and wave action. However, Wells et al. (1997) state that the establishment of permanent monospecific stands of C. demersum at Lake Tarawera in New Zealand (a relatively exposed lake) is not consistent with this description.
    C. demersum occupies a wide depth range, between 0.5 and 15.5m (Wells et al. 1997). It is classified as a shade-adapted species, although in clear water, shade tolerance may enable it to form a dense canopy in deep water, thereby further reducing the light climate for lower stature or higher light demanding plants (Wells et al. 1997). Su et al. (2004) state that C. Demersum is adapted to high light conditions.
    C. Demersum tolerates a wide range of water levels (Barrat-Segretain et al. 1999; in Armstrong et al. 2003). Sculthorpe (1985; in Armstrong et al. 2003) state that it may have thread-like rhizoid shoots which penetrate the substrate to aid absorption and anchorage.
    General impacts
    Ceratophyllum demersum has been spread throughout the world via the aquarium and pond trade, and is considered to be a weed of waterways in many regions of the world, due to its ability to spread rapidly, invade a wide range of aquatic habitats, and grow to deeper depths than some other weed species (NIWA, 2005a). C. demersum is able to dominant waterbodies. Its presence can affect phytoplankton development in three ways: by competition for inorganic nitrogen, competition for light, and allelopathy (Mjelde and Faafeng, 1997). A dense bed of C. demersum can remove up to 0.1 g N per square metre per day during the early growth stage. Allelopathy, the inhibition of growth of a plant species by chemicals produced by another species, has been shown to occur by C. Demersum (Mjelde and Faafeng, 1997; Korner and Nicklisch, 2002; Gross et al. 2003). Chemical compounds isolated from C. Demersum have been shown to inhibit the growth of phytoplankton (Mjelde and Faafeng, 1997; Korner and Nicklisch, 2002) and nitrogen-fixing cyanobacteria (Gross et al. 2003). Allelopathy by C. Demersum can change waterbodies by stabilising the dominance of aquatic plants over phytoplankton (Scheffer et al., 1993; in Gross et al. 2003).

    C. Demersum can also cause problems in aquatic ecosystems because of its ability to form dense monospecific beds. In some lakes in New Zealand, C. Demersum can be found growing from the shore to 14.5m deep, with beds up to 7m tall, creating a dense underwater forest (NIWA, 2001) (heights of 10m have been reported in Maraetai, New Zealand (Rohan Wells.,, 2005)). This displaces all other plants from the area, including native submerged vegetation, and also impacts on boating, fishing and other recreational activities. This alteration of aquatic habitat may also affect the accessibility of invertebrate prey to New Zealand's native fish populations (Duggan et al. 2002). It has also caused problems to hydroelectric power stations in New Zealand (NIWA, 2005a).

    C. demersum can be used as a measure of lake pollution, as it can contain trace metals such as cadmium and lead in plant tissue (Stankovic et al. 2000). It can also be successfully used for heavy metal removal under dilute metal concentration (Keskinkan, 2004).
    C. demersum is recommended for use in plantings for remediation of a dump site in Europe (Stilinovic and Hrenovic, 2000).
    Although C. demersum, and other weeds, will eventually dessicate and die if kept out of water for long enough, fragments can survive for months in wet spots underfloor, or in the anchor well of boats (NIWA, 2005a).
    Geographical range
    Native range: North America.
    Known introduced range: United Kingdom, Germany, Italy, Russian Federation, Norway, Sweden, Finland, Poland, Czech Republic, Slovakia, Romania, Hungary, Serbia and Montenegro, Australia, New Zealand, China, Japan, Vietnam, Iraq and Egypt.
    Introduction pathways to new locations
    Pet/aquarium trade: Ceratophyllum demersum can be spread by the intentional release of aquarium contents into waterways (NIWA, 2001).
    Translocation of machinery/equipment: Ceratophyllum demersum is spread by contaminated nets, boat trailers and anchors, and drainage machinery (NIWA, 2001).

    Local dispersal methods
    Boat: Ceratophyllum demersum is spread by contaminated nets, boat trailers and anchors, and drainage machinery (NIWA, 2001).
    Intentional release: Ceratophyllum demersum can be spread by the intentional release of aquarium contents into waterways (NIWA, 2001).
    Natural dispersal (local): The fragmentation of shoots, and turion formation on them, is an important means of distribution to new habitats (Fukuhara et al. 1997).
    Translocation of machinery/equipment (local): Ceratophyllum demersum is spread by contaminated nets, boat trailers and anchors, and drainage machinery (NIWA, 2001).
    Management information
    Chemical: Diquat is commonly used in the control of submerged invasive weeds, but it can become deactivated under turbid conditions and therefore be ineffective. Hofstra et al. (2001) showed that endothall was more effective in these conditions and state that other recent studies have shown promising results for the use of endothall in controlling C. demersum. However, Hofstra and Clayton (2001) found that endothall also killed native non-target species in New Zealand such as Myriophyllum and Potamogeton. Further research on endothall showed that it required an application rate of at least 4.0 mg/L active ingredient endothall (dipotassium salt of endothall) to be effective (Skogerboe and Getsinger, 2002).
    Smith and Pullman (1997) reported that C. demersum appeared to be highly susceptible to applications of Sonar ® A.S. aquatic herbicide (active ingredient fluridone), however Wells et al. (1986); Hofstra et al. (2001) found it was not.
    Cedergreen et al. (2004) showed that C. demersum is sensitive to the herbicide metsulfuron-methyl.

    Biological: An alternative method of C. demersum control in agricultural drains in New Zealand has been evaluated by Wells et al. (2003) - stocking with diploid grass carp (Ctenopharyngodon idella). It was found that grass carp generally provided continuous weed control in these drains, although they did not provide adequate control in smaller side drains, or in the shallow, upper parts of the main drain. However, subsequent to this research trial, a number of carp were found dead in Churchill East drain, emphasising that grass carp management in agricultural drains in New Zealand can be problematic due to periodic fish kills. In other countries, the use of grass carp may not be as successful. Greenfield et al. (2004) state that C. demersum is not a preferred food of grass carp in Californian waters, while Pipalova (2002) stated that stocking with grass carp resulted in a change in species composition, including an increase in C. demersum.

    Integrated management: Greenfield et al. (2004) provide information about controlling aquatic pests and weeds in California using non-conventional methods such as alternative chemical control, biological and mechanical control methods
    New Zealand's National Institute of Water and Atmospheric Research (NIWA) also provide detailed information about controlling aquatic plants such as C. demersum, please see Aquatic plant control

    Unrooted submerged vegetation such as Ceratophyllum demersum requires nutrient uptake from the water (Denny, 1987; in Mjelde and Faafeng, 1997). Goulder and Boatman (1971; in Mjelde and Faafeng, 1997) state that C. demersum requires high inorganic nitrogen levels in the surrounding water during rapid growth.
    Propagation of Ceratophyllum demersum occurs by fragmentation of its brittle stems. Flowers do occur on this monoecious species, although there is no evidence of seed production in New Zealand (NIWA, 2005b). A Japanese study found that almost 90% of young shoots in spring originated from turions, with active growth of main shoots being observed from June to July. Auto-fragmentation was observed at the end of August, and shoots had turions at the apices from October (Fukuhara et al. 1997).
    Reviewed by: Dr. Rohan Wells, National Institute of Water and Atmospheric Research Ltd, Hamilton New Zealand
    Principal sources: NIWA, 2005, 2005a, 2005b. National Institute of Water and Atmospheric Research, New Zealand. Available from:
    Compiled by: IUCN/SSC Invasive Species Specialist Group (ISSG) with support from the Terrestrial and Freshwater Biodiversity Information System (TFBIS) Programme (Copyright statement)
    Last Modified: Tuesday, 11 April 2006

ISSG Landcare Research NBII IUCN University of Auckland