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   Egeria densa (aquatic plant)  français     
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      Egeria densa (Photo: Virginia Tech Weed Identification Guide Archives, www.forestryimages.org) - Click for full size   Egeria densa (Photo: Virginia Tech Weed Identification Guide Archives, www.forestryimages.org) - Click for full size   Egeria densa identification (Photo: Virginia Tech Weed Identification Guide Archives, www.forestryimages.org) - Click for full size
    Taxonomic name: Egeria densa Planch.
    Synonyms: Anacharis densa (Planch.) Victorin, Elodea densa (Planch.) Caspary, Philotria densa (Planch.) Small & St. John
    Common names: Brazilian elodea, Brazilian waterweed, Brazilian-waterweed, common waterweed, dense waterweed, egeria, leafy elodea, South American waterweed
    Organism type: aquatic plant
    Egeria densa is a submersed, freshwater perennial herb that forms dense monospecific stands that restrict water movement, trap sediment, and cause fluctuations in water quality. It has also affected the status of certain threatened species. It has been introduced worldwide through the aquarium trade, and even in its native range can become a nuisance species causing local economic impacts. Chemical control is the most effective option for management of E. densa. Mechanical control is not recommended because fragments of the plant left behind can readily re-colonize and move downstream. The introduction of grass carp offers biological control of this species, but care must be taken because carp can introduce their own negative effects on the environment.
    Description
    The leaves and stems of E. densa generally are a bright green, short internodes frequently give the plant a very leafy appearance. Leaves which are are minutely serrated and linear, are 1-3cm long, up to 5mm broad, and found in whorls of four to eight. The lowest leaves are opposite or in whorls of 3, while the middle and upper leaves are in whorls of 4 to 8. Stems are erect, cylindrical, simple or branched, and grow until they reach the surface of the water where they form dense mats. The 18-25mm white flowers have three petals, are dioecious and float on or rise above the water's surface on thread-like hypanthiums produced from apical double nodes. White or pale, slender roots are unbranched. Adventitious roots are freely produced from double nodes on the stem (The Washington State Department of Ecology, 2003).
    Similar Species
    Elodea canadensis, Hydrilla verticillata

    More
    Occurs in:
    lakes, riparian zones, water courses, wetlands
    Habitat description
    Pierini and Thomaz (2004) state that, "E. densa is primarily invasive in temperate environments." The Washington State Department of Ecology (2003) states that, "E. densa is a submersed, freshwater perennial herb, generally rooted on the bottom in depths of up to 20 feet or drifting. It is found in both still and flowing waters, in lakes, ponds, pools, ditches, and quiet streams."
    General impacts
    Barreto et al. (2000) state that, "In southeast Brazil E. densa, together with E. najas, causes great annual losses to the hydroelectric companies. Interruptions of electricity generation and damage to grids and equipment are common in reservoirs belonging to CESP in São Paulo." The Washington State Department of Ecology (2003) states that, "E. densa forms dense mono-specific stands that restrict water movement, trap sediment, and cause fluctuations in water quality. Dense beds interfere with recreational uses of a water body by interfering with navigation, fishing, swimming, and water skiing." Champion and Tanner (2000) state that in New Zealand, "E. densa exhibited the ability to rapidly recolonise de-vegetated areas following floods in the Whakapipi Stream and may be considered to be 'adversity selected' (Whittaker & Goodman, 1979, in Champion and Tanner 2000)."
    Uses
    E. densa has been introduced worldwide through the aquarium trade. E. densa until 1996 was commonly sold in Washington pet stores under the name 'anacharis' as an aquarium species. E. densa was firts offered for sale in the United States in 1915, where it was recommended as a good "oxygenator" plant (The Washington State Department of Ecology, 2003).

    Lara et al. (2002) states that, "Among the higher aquatic plants, E. densa has been the preferred material for a number of different studies in plant physiology. One of the main reasons is that its leaves contain a single longitudinal vascular bundle and the blade consists of two layers of cells only, allowing studies of the whole undamaged organ in a natural environment. In this plant, heterogeneity is reduced to a minimum; all leaf cells are in direct contact with the external medium and at the same developmental stage and thus in similar physiological condition. These properties, together with the leaf polarity displayed by E. densa, represent an advantage for different kinds of research and make this species one of the model organisms of the plant kingdom for experiments, such as electro-physiology."

    Notes
    The Washington State Department of Ecology (2003) observes that, Elodea canadensis an attractive aquarium plant, could be used as a good substitute for E. densa since it is native to Washington's lakes, ponds, and rivers. They however point out due to Elodea canadensis availability in the aquarium trade, it has been introduced to several countries where it is not native, and is now considered a noxious weed in those regions (parts of Europe, Australia, Africa, Asia, and New Zealand).
    Geographical range
    Native range: South America (USDA-GRIN, 2003)
    Known introduced range: Asia, Australia, Europe, New Zealand, North America (Washington State Department of Ecology, 2003)
    Introduction pathways to new locations
    Other: The Washington State Department of Ecology (2003) states that, "E. densa was offered for sale in the United States in 1915, where it was recommended as a good "oxygenator" plant."
    Pet/aquarium trade: The Washington State Department of Ecology (2003) states that, "E. densa is a popular aquarium plant and can be found for sale in most pet shops, usually under the name anacharis." E. densa is one of a list invasive aquatic palnts sold in aquarium or water garden dealerships, advertised on commercial websites, or occuring as contaminants among plants that are offered for sale (Kay and Hoyle, 2001).


    Local dispersal methods
    Natural dispersal (local): The Washington State Department of Ecology (2003) states that, "The plant fragments readily and each fragment containing a double node has the potential to develop into a new plant."
    Management information
    A Risk assessment of Egeria densa for Australia was prepared by Pacific Island Ecosystems at Risk (PIER) using the Australian risk assessment system (Pheloung, 1995). The result is a score of 22 and a recommendation of: reject the plant for import (Australia) or species likely to be a pest (Pacific).

    For details on chemical, physical, biological control options, please see management information.

    Nutrition
    In Japan the following nutritional and environmental requirements were noted in a scientific study by Haramoto and Ikusima (1998): "The seasonal activity of photosynthesis and respiration was measured in March, August and December. The optimum temperature of net photosynthesis of the summer-type plants reached a high 35°C similar to that of the C4 plant. The compensation for light intensity at 35°C was 340 lux. Each photosynthesis-temperature curve suggested that E. densa had the ability to adapt to the seasonal changes in temperature in the natural habitat. The maximum starch concentrations reached 25.4% in the leaf and 22.6% in the stem in December. The shortage in the balance of organic matter for over-wintering was found to be maintained by stored starch in the leaf and the stem."
    Reproduction
    The Washington State Department of Ecology (2003) states that, "The absence of sexual reproduction in introduced populations of E. densa emphasizes the importance of the vegetative growth phase of the plant. Specialized nodal regions described as double nodes occur at intervals of 6 to 12 nodes along a shoot. A double node consists of 2 single nodes separated by a greatly shortened internode. Double nodes produce lateral buds, branches, and adventitious roots. Only shoot fragments of E. densa, which contain double node regions, can develop into new plants. The plant fragments readily and each fragment containing a double node has the potential to develop into a new plant. Plant root crowns also develop from double nodes along an old shoot. When a shoot sinks to the bottom during fall and winter senescence, a new root crown may develop at one or several double nodes along the new shoot. Egeria densa lacks specialized storage organs such as rhizomes or tubers and stores carbohydrates in stem tissues."
    Lifecycle stages
    In North America the Washington State Department of Ecology (2003) states that, "E. densa initiate growth when water temperatures reach 10 degrees centigrade. Getsinger describes the life cycle of Egeria densa in Lake Marion, South Carolina as follows: Two major growth flushes occur in spring and fall. Each of these flushes are followed by periods of senescence, with a loss of biomass through sloughing and decay of tips and branches. Flowers are produced in late spring and again in the fall. The intensity of flowering varies from year to year. During the summer, profuse branching forms a canopy. The branches form dense, tangled mats on the water's surface." In Japan the following Life cycle stages were noted in a scientific study by Haramoto and Ikusima (1998): "The seasonal activity of photosynthesis and respiration was measured in March, August and December. The optimum temperature of net photosynthesis of the summer-type plants reached a high 35°C similar to that of the C4 plant. The compensation for light intensity at 35°C was 340 lux. Each photosynthesis-temperature curve suggested that Egeria had the ability to adapt to the seasonal changes in temperature in the natural habitat. The maximum starch concentrations reached 25.4% in the leaf and 22.6% in the stem in December. The shortage in the balance of organic matter for over-wintering was found to be maintained by stored starch in the leaf and the stem."
    Reviewed by: Robin W. Scribailo Ph.D. Aquatic Botanist Professor of Biological Sciences Director of the Biological Sciences Field Station Director of the Aquatic Plant Herbarium Biological Sciences, Purdue University North Central. USA
    Compiled by: National Biological Information Infrastructure (NBII) & 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