Species Description: Hydrilla, Hydrilla verticillata, is an aquatic weed not native to Florida but widespread within the state. It is a submerged, rooted (usually) perennial with long, sinewy branching stems that often reach the surface and form dense mats. The stems are covered in small, pointed, often serrate leaves arranged in 4-8 whorls. Leaf midribs are often reddish with one or more sharp spines. Small pale subterranean tubers occur attached to the roots. Small white flowers grow above the water line on stalks (Godfrey and Wooten 1979, Illinois-Indiana Sea Grant College Program undated).
H. verticillata exhibits a degree of phenotypic plasticity (variable physical appearance) in response to age, habitat conditions, and water quality (Kay 1992). Branching is generally sparse in submerged portions of the plant, tending to become profuse at the surface (Langeland 1996).
Potentially Misidentified Species: In Florida, Hydrilla may be easily mistaken for a related (confamilial) non-native aquatic weed Egeria densa with which it co-occurs in much of the state. The leaves of E. densa occur in whorls of 3-6 and have very fine serrations that can only be discerned under magnification.
Hydrilla can also be confused with a native aquatic plant, Elodea canadensis, although this species only occurs in northernmost Florida.
Regional Occurrence: Two distinct introduced Hydrilla verticillata biotypes exhibiting different reproductive traits (see below) occur in the United States. The dioecious southern form found in Florida appears to have originated from the Indian subcontinent, while the monoecious northern form (occurring north of North Carolina) appears to be derived from stock originating in Korea (Schmitz et al. 1991, Madeira et al. 1997).
Hydrilla occurs in freshwater throughout most of peninsular Florida.
IRL Distribution: Florida Exotic Pest Plant Council (FLEPPC) collection records confirm the presence of Hydrilla verticillata in Volusia, St. Lucie, Martin, and Palm Beach counties. Although not reflected in the FLEPPC records, collection records from the UF/IFAS Lakewatch water quality monitoring program and elsewhere suggest hydrilla is also widespread in freshwater systems within Brevard and Indian River counties.
Age, Size, Lifespan: The sinewy branching stems of Hydrilla verticillata regularly reach 2 m length and can attain lengths of more than 7.5 m (Cook and Luond 1982, Langland 1996).
Hydrilla is an herbaceous perennial that experiences seasonal winter dieback (Carter et al. 1994).
Abundance: Schardt (1994) refers to Hydrilla verticillata as the most abundant aquatic plant in Florida public waters, and hydrilla-infested water bodies occur in 70% of the state's freshwater drainage basins. Schardt (1997) reported hydrilla was present in 43% of Florida's public water bodies in 1994, representing an estimated coverage of 38,500 ha.
Reproduction: The southern Hydrilla verticillata biotype that occurs in Florida is comprised mostly of dioecious (one sex) female plants. Although extensive flowering may occur, the lack of male plants indicates that sexual reproduction is essentially non-existent. The northern (non-Florida) biotype consists of monoecious individuals (male and female flowers occur on the same plant), and at least some sexual reproduction resulting in the setting of fertile seed occurs in this population (Langland and Smith 1984, Madiera et al. 2000). Where reproduction through flowering does occur, wind pollination is the means of fertilization (Steward et al. 1984).
Reproduction of hydrilla in Florida is predominantly if not exclusively through vegetative means. Vegetative strategies include regrowth from stem fragments and clonal reproduction via runners, rhizomes, and tubers (Pieterse 1981, Hurley 1990). Sutton et al. (1992) indicate that one tuber can give rise to as many as 6,000 new tubers per square meter, and Van and Steward (1990) notes tubers can remain viable longer than 4 years.
Vegetative reproduction also occurs via specialized axillary buds called turions which reside on the stems in the water column and are somewhat smaller than tubers (Yeo et al. 1984, Spencer et al. 1994). Turions, which are actually underground tubers, become detached from the parent plants to disperse through water movement to new location where they can grow vegetatively into new plants (Hofstra et al. 1990). In colder climates, turions are important as overwintering organs.
Embryology: Where seed production occurs, seeds are poorly dispersed and usually sink to the sediment with perhaps some water current transport occurring as well. Seed set occurs around September and germination commences the following April-May. Maturation is rapid, and plants are capable of flowering just two months after germination (Steward et al. 1984). Hurley (1990) reports seed germination rates are generally less than 50%. Germination of turions occurs at around 18°C (Hurley 1990, Pieterse 1981).
Temperature: The U.S. distribution of Hydrilla verticillata extends into temperate regions where prolonged freezing winter temperatures occur. Freezing temperatures result in dieback of hydrilla stem, but overwintering turions and sub-sediment biomass including vegetative tubers survive to grow new shoots in the spring.
Barko and Smart (1981) report the following responses of hydrilla to various temperature treatments. At 20-24°C, optimum rates of photosynthesis were attained. At 16°C, photosyntheis was diminished but some growth still occurred. At 0°C, the water column biomass dies back but sub-sediment biomass survives.
Although the majority of hydrilla in the temperate United States is derived from the monoecious strain, genetic studies by Les et al. (1997) concluded that an established population in Mystic, CT, is actually derived from the dioecious Indian strain. With additional testing, the dioecious strain may be revealed to be more capable of temperate range extension than previously believed.
Salinity: Hydrilla verticillata exhibits moderate salinity tolerance, persisting in a laboratory environment at 7 ppt when transitioned in one step from fresh water, and at up to 12 ppt when the transfer was gradual (Haller et al. 1974, Twilley and Barko 1990).
Moderate halotolerance allows H. verticillata to occupy the upper reaches of estuaries such as Chesapeake Bay (Carter et al. 1994) and the Lower St. Johns River.
Light: Ramey (2001) indicates that hydrilla can grow in turbid waters receiving light at only 1% of surface sunlight conditions. In temperate climates, this allows it to start growing in early spring low light conditions before co-occurring species can (Van et al. 1976, Bowes 1977).
Trophic Mode: Autotrophic (photosynthetic).
Associated Species: Freshwater anglers recognize the role of hydrilla beds as fish attractors and regularly fish over beds and at their margins. In particular, dense hydrilla beds are common aggregation sites for chain pickerel (Esox niger), whose ambush style of predation benefits from the presence of vegetative cover. Experimental research has indicated that the presence of dense vegetation in general may result in a shift in foraging strategy in fish species, away from a pursuit predator strategy in favor of ambush predation that takes advantage of vegetative cover (Crowder and Cooper 1982, Savino and Stein 1982).
In Florida, Hydrilla verticillata often occurs in mixed beds with another non-native aquatic weed Egeria densa. The two plants are very similar in appearance.
Invasion History: The broad native range of Hydrilla verticillata is believed to include parts of Asia and India, Australia, and possibly Africa, and the plant has been subsequently introduced occurs on every continent except Antarctica (Pieterse 1981, Cook and Luond 1982, ISSG).
The first introduction of the species in North America was through a Florida west coast aquarium dealer in the early 1950s who shipped live H. verticillata from Sri Lanka (dioecious, exclusively female strain) for the aquarium trade under the common name Indian star-vine. More recent genetic studies, e.g., Madeira et al. 2004, suggest the original material may have come from Bangalore, India. The plants were deemed unsatisfactory and were disposed of into a canal near Tampa Bay, where they survived and thrived (McCann et al. 1996). By 1955, samples from this introduced Tampa population had been transported to Miami for cultivation and pet trade sale. Subsequent undocumented accidental/careless releases no doubt followed, as evidenced by the extensive spread of the Sri Lanka biotype throughout Florida and elsewhere in the southeastern U.S.
Introduction of the monoecious (Korean) strain of H. verticillata to the eastern seaboard occurred perhaps two decades after the initial Florida introduction. This hydrilla biotype was first reported from Delaware in 1976, and from the Potomac River around 1980 (Madeira et al. 2000).
Considering both biotypes together, Hydrilla is now present throughout the southeast, on the east coast from Florida north to Massachusetts, west into Texas, and in Arizona and California as well (Pieterse 1981, Cook and Luond 1982, Langeland 1996).
Potential to Compete With Natives: Hydrilla verticillata is an aggressive invader that has been shown capable of displacing native submersed plant communities (Haller and Sutton 1975, Bowes et al. 1977). Dense beds of hydrilla alter the community structure at multiple levels. Water chemistry is altered, zooplankton populations decline, and fish population and community structures are altered as well (Colle and Shireman 1980, Canfield et al. 1983, Schmitz and Osborne 1984).
Possible Economic Consequences of Invasion: Hydrilla has been recognized as one of the most invasive weeds in the world and infestations are capable of choking waterways and public water supplies (Illinois-Indiana Sea Grant undated). It is listed as a Category I invasive exotic plant in Florida, indicating that the species is currently altering native plant communities by displacing native species and changing community structures or ecological functions.
Worldwide economic impacts of Hydrilla verticillata include impacts relating to infestation of rice fields, irrigation canals, fishponds and public waterways (Cook and Luond 1982). Oxygen depletion is a potentially serious consequence of decomposition of large amounts of hydrilla plant biomass in infested lakes (Engel 1995).
Hydrilla control and management is expensive. The state of Florida spent approximately 14.5 million dollars on H. verticillata control in 1994-1995. The economic cost of lost recreational dollars is also considerable. Recreational activies worth $11 million were lost just in Orange Lake (Marion County) in those years when hydrilla infestations entirely choked the lake. (Langeland 1996).
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