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.
II. HABITAT AND DISTRIBUTION
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.
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.
III. LIFE HISTORY AND POPULATION BIOLOGY
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).
Schardt (1994) refers to Hydrilla verticillata as the most abundant aquatic
plant in Florida public waters, and hydrilla-infested waterbodies occur in
seventy percent 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.
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
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).
IV. PHYSICAL TOLERANCES
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.
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.
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).
V. COMMUNITY ECOLOGY
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
VI. INVASION INFORMATION
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 (Note that 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.
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J. Masterson, Smithsonian Marine Station
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