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: hydrilla

Synonyms and Other Names: Florida elodea, waterthyme

Taxonomy: available through www.itis.gov

Identification: Submersed perennial herb. Rooted, with long stems that branch at the surface where growth becomes horizontal and dense mats form. Small, pointed leaves are arranged in whorls of 4 to 8. Leaves have serrated margins and may have one or more sharp teeth under the midrib (see Godfrey and Wooten 1979). Development of these features may vary with location, age, and water quality (Kay 1992).

The distribution of biotypes is changing rapidly, however, southern populations were predominantly dioecious female (plants having only female flowers) that overwinter as perennials (the monoecious biotype has spread south through Georgia, South Carolina, Tennessee, and Alabama). Populations north of South Carolina were often monoecious (having both male and female flowers on the same plant) (Cook and Lüönd 1982; Madeira et al. 2000). Fertile seed production was reported in the monoecious type (Langeland and Smith 1984). Both biotypes depend on tubers for overwintering, although monoecious hydrilla exhibits a more annual habit than the dioecious type, with abundant tuber/turion production around September (Owens et al. 2012).

Morphologically similar species include exotic Brazilian waterweed (Egeria densa), native western waterweed (Elodea nuttallii), and native (except Alaska and Puerto Rico) Canadian waterweed (Elodea canadensis). E. densa, E. nuttallii, and E. canadensis have 3-6 leaves per whorl, with inconspicuous leaf serration and no dentation on midrib, but E. densa leaves are 2-3 cm long, and both E. nuttallii and E. canadensis usually has 3 leaves per whorl near stem base (Langeland et al. 2008, Wunderlin and Hansen 2011, Rybicki et al. 2013).

Recent research into molecular techniques for identifying hydrilla and its biotypes has proven successful (Verkleij 1983; Ryan et al. 1995; Madeira et al. 2004). An early method used isoenzyme patterns in hydrilla to distinguish origin and biotype (Verkleij 1983). A later method used a random amplified polymorphic DNA (RAPD) procedure to find DNA markers in hydrilla samples (Ryan et al. 1995; Les et al. 1997; Madeira et al. 1997, 2000). A relatively inexpensive alternative method used “universal primers” to sequence hydrilla DNA (Madeira et al 2004; Benoit and Les 2013; Rybicki et al. 2013).

Size: Stems grow up to 9 m in length; leaves are 6-20 mm long and 2-4 mm wide.

Native Range: The common dioecious type originates from the Indian subcontinent. Historical reports specify the island of Sri Lanka (Schmitz et al. 1991) while random amplified polymorphic DNA (RAPD) analysis points to India's southern mainland (Madeira et al. 1997). Korea appears the likely origin for the monoecious type (Madeira et al. 1997).

Alaska	Hawaii	Puerto Rico & Virgin Islands	Guam Saipan

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

Ecology: Hydrilla is found in freshwater lakes, ponds, rivers, impoundments, and canals. It mainly spreads vegetatively through dispersal of plant fragments, axillary turions, and tubers (Langeland and Sutton 1980). Tubers remain viable out of water for several days (Basiouny et al. 1978) and in undisturbed sediment for over 4 years (Van and Steward 1990). Viability remains after ingestion and regurgitation by waterfowl, although passage of vegetative propagules throught the digestive tract likely renders them non-viable (Joyce et al. 1980). Sexual reproduction among and between monoecious and dioecious strains is possible (Steward 1993), but its importance is unknown (Langeland and Smith 1984). Sites such as Lake Guntersville, Alabama have large co-occurring stands of monoecious and dioecious hydrilla. Pollination occurs when pollen from free-floating male flowers disperses on the water surface (epihydrophily) to female flowers (Tanaka 2000; Tanaka 2003). It has a low salinity tolerance (Carter et al 1987; Shields et al 2012).

Means of Introduction: The dioecious strain was imported to the United States in the early 1950s for use in aquariums. It entered Florida's inland water system after plants were discarded or planted into canals in Tampa and Miami (Schmitz et al 1991). The monoecious strain was a separate introduction, first found decades later in Delaware and the Potomac Basin (Environmental Laboratory 1985; Miller 1988; Madeira et al 2000).

Hydrilla is mainly introduced to new waters as fragments on recreational boats, their motors and trailers and in live wells. Stem pieces root in the substrate and develop into new colonies, commonly beginning near boat ramps. Once established, boat traffic continues to break and spread hydrilla throughout the waterbody. Both biotypes propagate primarily by stem fragmentation, although axillary buds (turions) and subterranean tubers are also important. Tubers are resistant to most control techniques (Schardt 1994) and may be viable as a source of reintroduction for years (Van and Steward 1990).

Hydrilla may be unknowingly transplanted into private ponds as a contaminant in water garden plants. It was found spreading after extensive 2,4-D use in Tennessee Valley Authority reservoirs and Lake Seminole, Georgia, once heavily populated with Eurasian water-milfoil (Myriophyllum spicatum) (Bates and Smith 1994). This has not yet been observed in northern lakes (M. Netherland, USACE pers. comm. 2015).

Status: Currently established in 28 states of the US (AL, AZ, AR, CA, CT, DE, FL, GA, ID, IN, LA, ME, MD, MA, MS, MO, NJ, NY, NC, OH, OK, PA, SC, TN, TX, VA, WV), as well as Guam and Puerto Rico; occurrences in Iowa, Kansas, Washington, and Wisconsin were removed or controlled in isolated ponds (Sample 1972; Kansas Department of Wildlife, Parks and Tourism 2009; Herrera 2011; Asplund 2007).

Impact of Introduction:

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

Ecological	Economic	Human Health	Other

It is commonly reported that once established, hydrilla results in an array of ecosystem disruptions. Hydrilla grows aggressively and competitively, spreading through shallower areas and forming thick mats in surface waters that block sunlight penetration to native plants below (van Dijk 1985). In the southeast, hydrilla effectively displaces beneficial native vegetation (Bates and Smith 1994) such as wild-celery (Vallisneria americana) and coontail (Ceratophyllum demersum) (van Dijk 1985; Rizzo et al. 1996). However, others reported that in a community of submerged aquatic vegetation including monoecious hydrilla, other exotic and native species responded to fluctuations in weather and water quality in the fresh tidal Potomac River (Carter et al. 1994) and the Upper James River in the Chesapeake Bay watershed (Shields et al. 2012). Hydrilla does not necessarily displace native species and may be beneficial to wildlife (Rybicki and Carter 2002; Rybicki and Landwehr 2007).

Hydrilla has been shown to alter the physical and chemical characteristics of lakes. Colle and Shireman (1980) found reduced weight and size in sportfish when hydrilla occupied the majority of the water column, suggesting that foraging efficiency was reduced as open water and natural vegetation gradients were lost. Stratification of the water column (Schmitz et al. 1993; Rizzo et al. 1996), decreased oxygen levels (Pesacreta 1988), and fish kills (Rizzo et al. 1996) have been documented in waters with hydrilla. Changes in water chemistry due to hydrilla may also be implicated in zooplankton and phytoplankton declines (Schmitz and Osborne 1984; Schmitz et al. 1993). However, other studies find a lack of negative effects of hydrilla on other biota, such as plants, fish, and aquatic bird communities (Killgore et al 1989; Hoyer et al 2008).

Dense beds of hydrilla affects water flow (Rybicki et al 1997) and water use. Beds in the Mobile Delta are reducing flow in small tidal streams and creating a backwater habitat (J. Zolcynski pers. comm. 1998). Its heavy growth may obstruct boating, swimming and fishing in lakes and rivers and may block the withdrawal of water used for power generation and agricultural irrigation. However, because of the resilience of hydrilla to control efforts and its competitive success and comparative vigor in stressed systems and capacity to provide at least some beneficial services combine to suggest it may have a useful role in some systems (Herschner and Havens 2008).

Remarks: Hydrilla verticillata HUC8 distribution by biotype and population status

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Other Resources:
The Maine Invasion

Monoecious hydrilla literature review

US Fish and Wildlife Service Ecological Risk Screening Summary for Hydrilla verticillata

Great Lakes Hydrilla Risk Assessment

US Fish and Wildlife Service Ecological Risk Screening Summary for Hydrilla verticillata

Author: C.C. Jacono, M.M. Richerson, V. Howard Morgan, I.A. Pfingsten, and J. Redinger

Peer Review Date: 10/27/2015

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.