Disclaimer:

The Nonindigenous Occurrences section of the NAS species profiles has a new structure. The section is now dynamically updated from the NAS database to ensure that it contains the most current and accurate information. Occurrences are summarized in Table 1, alphabetically by state, with years of earliest and most recent observations, and the tally and names of drainages where the species was observed. The table contains hyperlinks to collections tables of specimens based on the states, years, and drainages selected. References to specimens that were not obtained through sighting reports and personal communications are found through the hyperlink in the Table 1 caption or through the individual specimens linked in the collections tables.

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Common name: Brazilian waterweed

Synonyms and Other Names: Anacharis densa (Planch.) Victorin, Elodea densa (Planch.) Caspary, Philotria densa (Planch.) Small & St. John, leafy elodea, dense waterweed, Brazilian elodea

Taxonomy: available through www.itis.gov

Identification: Egeria densa is a dioecious, submersed perennial found in lentic and lotic freshwater systems. Only male plants are found in the U.S. and in its native range male plants outnumber female plants by as much as 6:1 (Cook and Urmi-König 1984). Leaves and stems are generally bright green (often dark green when below the surface), and the short internodes give it a very leafy appearance. Leaves which are minutely serrated (needing magnification) and linear, are 1-3 cm long, up to 5mm broad, and found in whorls of four to eight. The lowest leaves may be opposite or in whorls of 3; 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. Flowers (18-25 mm diameter) have three petals which are white and float on or rise just above the water's surface on a slender peduncle. Slender roots are unbranched and typically a white to pale color. Adventitious roots are freely produced from double nodes on the stem (Washington State Department of Ecology 2003). It can be distinguished from Hydrilla verticillata (L.f.) Royle by the absence of turions (dormant buds from above ground stems) and tubers (dormant buds from below ground stems), and by the presence of showy white flowers that are produced above the water surface (Hoshovsky and Anderson 2001). It is usually rooted in the bottom mud, but can be found as a free-floating mat or as fragments with stems near the surface of the water.

Size: 3-5 m long, up to 8 cm diameter (Cook and Urmi-König 1984).

Native Range: South America (central Minas Gerais region of Brazil, coast of Argentina, and coast of Uruguay) (Cook and Urmi-König 1984)

Alaska	Hawaii	Puerto Rico & Virgin Islands	Guam Saipan

Hydrologic Unit Codes (HUCs) Explained
Interactive maps: Point Distribution Maps

Ecology: Egeria densa is an aquatic plant in the waterweed family that inhabits mild to warm freshwaters, such as slow flowing streams of warm, temperate, and tropical regions (Parsons and Cuthbertson 2001). It occurs at depths as deep as 7 m. It grows in thick mats of intertwining stems (Parsons and Cuthbertson 2001), which alter the light and nutrients available to the biota where it occurs (Yarrow et al. 2009), acting as an ecosystem engineer (Jones et al. 1994). 

Egeria densa can inhabit waters with a wide range of temperatures, low CO₂ levels, and low light levels. This species can survive in waters with temperatures of 3-35°C (Yarrow et al. 2009). The plant can overwinter as seeds (although female plants are not found in the U.S.), dormant shoots, or semi-dormant shoots until temperatures rise above 15°C (Parsons and Cuthbertson 2001). Egeria densa exhibits the C4 pathway and utilizes HCO₃^-; thus it is able to photosynthesize in waters with low CO₂ levels (Casati et al. 2000). Egeria densa can tolerate high phosphorous levels, but is susceptible to iron deficiency (Parsons and Cuthbertson 2001).

This species has a low light requirement and can thrive in turbid environments (Parsons and Cuthbertson 2001). Optimal light intensity is about 100 lux. Egeria densa cannot tolerate high light intensities or high levels of ultra-violet and blue light, as it experiences chlorophyll damage to light levels of 1250 lux. Egeria densa cannot tolerate high UV-B radiation, as it can damage the enzymes involved in photosynthesis and can reduce photosynthetic capacity (Casati et al. 2002).

Flowers float above the water surface and are pollinated by insects (Parsons and Cuthbertson 2001). It reproduces asexually in Australia (Parsons and Cuthbertson 2001) and in the U.S. (Hoshovsky and Anderson 2001), where only the male plant has established. Egeria densa is capable of vegetative fragmentation; stems of at least two nodes can break off from the parent colony and disperse by stream flow (Parsons and Cuthberson 2001). Stem fragments that break off can take root in bottom mud or grow as free-floating mats (Hoshovsky and Anderson 2001). Fragmentation can occur as a result of the mechanical shearing of water flows, wave action, waterfowl activity, and boating.

Means of Introduction: Introduced world-wide through the aquarium trade - sold widely as good "oxygenator" plant and dispersed secondarily by boat trailers and vegetative dispersal downstream.  

This species may be transported by hitchhiking on recreational gear; E. densa grows in thick mats that can become entangled on boat propellers and trailer wheels, or can be captured in bilge water (Washington State Department of Ecology 2013). Attached fragments can be transported between water bodies. Egeria densa is not known to be taken up in ballast water. As a popular ornamental plant, E. densa is planted in water gardens (Indiana Department of Natural Resources 2013), however, there is not enough information available to determine the frequency of E. densa plantings.

Status: Established in all states listed above.

Impact of Introduction:

Summary of species impacts derived from literature review. Click on an icon to find out more...

Ecological	Economic	Human Health	Other

Dense stands of E. densa may restrict water movement, trap sediment, and cause fluctuations in water quality (Hoshovsky and Anderson 2001; Parsons and Cuthbertson 2001). Severe infestations may impair recreational uses of a water body including navigation, fishing, swimming, and water skiing.  In Brazil, E. densa (as well as E. najas, Ceratophyllum demersum, and Eichhornia crassipes) have severely infested hydropower reservoirs. It was estimated that 48,000 cubic meters of aquatic weeds were removed from water intake structures in Jupia Reservoir (Marcondes et al. 2000). 

Remarks: Egeria densa can be detected using digital imagery, though this ability is highly specialized and pertains to populations near the water surface (Mandvikar and Liu 2004; Jenifer Parsons, WA Dept. of Ecology, pers. comm.). It is often sold in the name “Anachris” and is advertised to aquarium customers as an oxygenator. It is on Oregon’s and Washington’s quarantine lists as Class B noxious weeds, thus illegal to sell or ship to those states (Hamel and Parsons 2001; ODA 2015; WDA 2015).

Nonindigenous E. densa populations in Río Cruces, Chile have similar genotypes as populations in Western Oregon, suggesting that the two populations experienced similar bottlenecking events at introduction, or there is a lack of genetic diversity in the native population (Carter and Sytsma 2001).

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Other Resources:
US Fish and Wildlife Service Ecological Risk Screening Summary for Egeria densa

Author: Pfingsten, I.A., D.D. Thayer, V.H. Morgan, and J. Li

Peer Review Date: 4/4/2016

This information is preliminary or provisional and is subject to revision. It is being provided to meet the need for timely best science. The information has not received final approval by the U.S. Geological Survey (USGS) and is provided on the condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from the authorized or unauthorized use of the information.