{{Short description|Species of plant}} {{Speciesbox |image = Hydrilla USGS.jpg | status = LC | status_system = IUCN3.1 |status_ref=<ref>{{cite iucn|author1=Zhuang, X.|author2=Beentje, H.J.|year=2017|title=''Hydrilla verticillata''|article-number=e.T167871A65905991|doi=10.2305/IUCN.UK.2017-1.RLTS.T167871A65905991.en|access-date=12 August 2023|name-list-style=amp}}</ref> |genus = Hydrilla |parent_authority = Rich. |species = verticillata |authority = (L.f.) Royle |synonyms = }}
'''''Hydrilla''''' ('''waterthyme''') is a genus of aquatic plants, usually treated as containing just one species — '''''Hydrilla verticillata''''', but some botanists divide it into several species. It is native to the cool and warm waters of the Old World in Asia, Africa, and Australia, with a sparse, scattered distribution; in Australia it occurs from the Northern Territory to Queensland and New South Wales.<ref name=fe>Flora Europaea: [http://rbg-web2.rbge.org.uk/cgi-bin/nph-readbtree.pl/feout?FAMILY_XREF=&GENUS_XREF=Hydrilla&SPECIES_XREF=&TAXON_NAME_XREF=&RANK= ''Hydrilla'']</ref><ref name=ft>Flora of Taiwan: [http://tai2.ntu.edu.tw/udth/bin/fot1.exe/browse?bid=5&page=15 ''Hydrilla''] {{Webarchive|url=https://web.archive.org/web/20071126095944/http://tai2.ntu.edu.tw/udth/bin/fot1.exe/browse?bid=5&page=15 |date=2007-11-26 }}</ref><ref name="APNI">{{cite web |url=https://id.biodiversity.org.au/name/apni/88035 |title=''Hydrilla'' |website=Australian Plant Name Index (APNI) |publisher=Centre for Plant Biodiversity Research, Australian Government |access-date=15 March 2023}}</ref>
The stems grow up to 2 m long. The leaves are arranged in whorls of two to eight around the stem, each leaf 5–20 mm long and 0.7–2 mm broad, with serrations or small spines along the leaf margins; the leaf midrib is often reddish when fresh. It is monoecious (sometimes dioecious), with male and female flowers produced separately on a single plant; the flowers are small, with three sepals and three petals, the petals 3–5 mm long, transparent with red streaks. It reproduces primarily vegetatively by fragmentation and by rhizomes and turions (overwintering), and flowers are rarely seen.<ref name=ft/><ref name=fnwe>Flora of NW Europe: [http://ip30.eti.uva.nl/BIS/flora.php?selected=beschrijving&menuentry=soorten&id=4171 ''Hydrilla verticillata''] {{webarchive|url=https://web.archive.org/web/20071208024427/http://ip30.eti.uva.nl/BIS/flora.php?selected=beschrijving&menuentry=soorten&id=4171 |date=2007-12-08 }}</ref><ref name=blamey>Blamey, M. & Grey-Wilson, C. (1989). ''Flora of Britain and Northern Europe''. {{ISBN|0-340-40170-2}}</ref><ref name=rhs>Huxley, A., ed. (1992). ''New RHS Dictionary of Gardening''. Macmillan {{ISBN|0-333-47494-5}}.</ref> They have air spaces to keep them upright.
''Hydrilla'' has a high resistance to salinity compared to many other freshwater aquatic plants. It can grow up to an inch a day, producing dense mats of vegetation along the bottom of lakes and rivers. As it grows up to the water's surface, these mats can become several feet thick.
==Taxonomy and naming== ''Hydrilla'' closely resembles some other related aquatic plants, including ''Egeria densa'' and ''Elodea canadensis''.<ref name=WADoE/> Synonyms include ''H. asiatica'', ''H. japonica'', ''H. lithuanica'', and ''H. ovalifolica''.{{Citation needed|date=May 2021}}
==Ecology== ''Hydrilla verticillata'' is negative allelopathic to the common hornwort (''Ceratophyllum demersum'') and prickly hornwort (''C. muricatum''), that is, it produces compounds that inhibit growth of the latter two species.<ref>{{cite journal|author1=Manjula Kulshreshtha |author2=B. Gopal |date=1983|title=Allelopathic influence of ''Hydrilla verticillata'' (L.F.) Royle on the distribution of ''Ceratophyllum'' species|journal=Aquatic Botany|volume=16|issue=2|pages= 207–09|doi=10.1016/0304-3770(83)90095-5|bibcode=1983AqBot..16..207K }}</ref>
As aquatic macrophytes, ''Hydrilla'' play critical roles in the ecosystem. They influence nutrient cycles and the ecology of the body of water, as well as the sediments.<ref>{{Cite journal |last1=Barko |first1=John W. |last2=Smart |first2=R. Michael |last3=McFarland |first3=Dwilette G. |last4=Chen |first4=Rex L. |date=1988 |title=Interrelationships between the growth of Hydrilla Verticillata (L.f.) Royle and sediment nutrient availability |journal=Aquatic Botany |volume=32 |issue=3 |pages=205–216|doi=10.1016/0304-3770(88)90116-7 |bibcode=1988AqBot..32..205B }}</ref> ''Hydrilla'' interacts with other organisms, supplying food and nutrients as well as habitats and shelters.<ref>{{Cite journal |last1=Pelicice |first1=Fernando Mayer |last2=Agostinho |first2=Angelo Antonio |last3=Thomaz |first3=Sidiniei Magela |date=2005 |title=Fish assemblages associated with Egeria in a tropical reservoir: investigating the effects of plant biomass and diel period. |journal=Acta Oecologica |volume=27 |issue=1 |pages=9–16|doi=10.1016/j.actao.2004.08.004 |bibcode=2005AcO....27....9P }}</ref>
''Hydrilla'' can have negative impacts in aquatic communities. When abundant, they affect dissolved oxygen levels, which can lead to decline in populations of fish, invertebrates, and other plant species.<ref>{{Cite journal |last=Simberloff |first=Daniel |date=2021 |title=Maintenance management and eradication of established quatic invaders |journal=Hydrobiologia |volume=848 |issue=9 |pages=2399–2420|doi=10.1007/s10750-020-04352-5 |pmid=32836349 |pmc=7407435 |bibcode=2021HyBio.848.2399S }}</ref>
==Status as an invasive plant== ===Introduction=== left|thumb|Foliage detail ''Hydrilla'' is naturalized and invasive in the United States following release in the 1950s and 1960s from aquariums into waterways in Florida, due to the aquarium trade.<ref name=WADoE>{{cite web|url=http://www.ecy.wa.gov/programs/wq/plants/weeds/hydrilla.html|title=General Information About Hydrilla|publisher=State of Washington, Department of Ecology|access-date=December 20, 2014|archive-url=https://web.archive.org/web/20141220102925/http://www.ecy.wa.gov/programs/wq/plants/weeds/hydrilla.html|archive-date=2014-12-20}}</ref> It is now established in parts of southern Canada and in the United States from Connecticut to Texas, and also in California.<ref name=fna>Flora of North America: [http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=200024721 ''Hydrilla verticillata'']</ref> By the 1990s control and management were costing millions of dollars each year.<ref name=NEANS>{{cite web|url=http://www.northeastans.org/hydrilla/ecoleconhydrilla.htm|title=Ecological and Economic Costs Associated with Hydrilla (Hydrilla verticillata)|publisher=Northeast Aquatic Nuisance Species Panel|access-date=December 20, 2014|archive-url=https://web.archive.org/web/20141220102826/http://www.northeastans.org/hydrilla/ecoleconhydrilla.htm|archive-date=December 20, 2014|df=mdy-all}}</ref>
The plant was introduced when a Florida West Coast aquarium dealer shipped live ''Hydrilla'' from Sri Lanka under the common name "Indian star-vine."<ref name=SMS>{{cite tech report|last=Masterson|first=J.|url= http://www.sms.si.edu/irlspec/Hydrilla_verticillata.htm|title=''Hydrilla verticillata''|publisher=Smithsonian Marine Station at Fort Pierce|date=October 4, 2007|access-date=December 20, 2014}}</ref> These plants were considered unsatisfactory and were dumped into a canal near Tampa Bay, where they flourished.<ref name=SMS/> By 1955, the plants found their way from Tampa to Miami as they were transported for cultivation and pet trade sale.<ref name=SMS/> It is believed that several undocumented cases of accidental or careless releases followed, as there was extensive spread of the ''Hydrilla'' throughout Florida and the southeastern United States.<ref name=SMS/>
===Importance of accurate identification=== Accurate identification of ''Hydrilla verticillata'' is essential for regulatory reporting, ecological monitoring, and management planning. Misidentification may lead to unnecessary treatment of native vegetation or delayed response to invasive spread.<ref>{{Cite web |title=Hydrilla.org {{!}} Authoritative Resource |url=https://hydrilla.org/ |access-date=2026-02-25 |website=hydrilla.org}}</ref>
''Hydrilla'' is frequently confused with ''Elodea canadensis'' and ''Egeria densa'' due to similar submerged growth form.<ref>{{Cite web |title=Hydrilla.org {{!}} Authoritative Resource |url=https://hydrilla.org/#/identification |access-date=2026-02-25 |website=hydrilla.org}}</ref>
===Problems=== [[File:Hydrilla in the Mallows Bay–Potomac River National Marine Sanctuary.PNG|thumb|A thick bed of ''Hydrilla'' in the Mallows Bay–Potomac River National Marine Sanctuary in Charles County. Maryland.]] Due to the combination of herbicide resistance, high growth rates, high ecological adaptability, dispersion ability, and low resources required, ''Hydrilla'' is able to invade almost every region of the world and spread at an alarming rate, affecting the ecosystem in a negative manner.<ref>{{Cite journal |last=Langeland |first=Kenneth A. |date=1996 |title=Hydrilla verticillata (L.F.) Royle (Hydrocharitaceae), "The Perfect Aquatic Weed" |journal=Castanea |volume=61 |issue=3 |pages=293–304|jstor=4033682 }}</ref> The high photosynthesis rate of ''Hydrilla'' leads to the depletion in dissolved carbon dioxide during daytime, which raises the pH, and to higher concentrations of oxygen. At night, the oxygen is used for oxidative phosphorylation, resulting in anoxia, and carbon dioxide is replenished, which in turn lowers the pH.<ref>{{Cite journal |last1=Colon-Gaud |first1=Jose-Checo |last2=Kelso |first2=W. E. |last3=Rutherford |first3=D. A. |date=2004 |title=Spatial Distribution of Macroinvertebrates Inhabiting Hydrilla and Coontail Beds in the Atchafalaya Basin, Louisiana |journal=Journal of Aquatic Plant Management |volume=42 |issue=1 |pages=85–91}}</ref>
As an invasive species in Florida, ''Hydrilla'' has become the most serious aquatic weed problem for Florida and most of the U.S. Because it was such a threat as an invasive species, one of the first cost-effective broadscale herbicide controls developed was fluridone. Unfortunately, this single-use herbicide resulted in fluridone-resistant ''Hydrilla''. "As ''Hydrilla'' spread rapidly to lakes across the southern United States in the past, the expansion of resistant biotypes is likely to pose significant environmental challenges in the future."<ref>{{Cite web | url=http://tnga.granicus.com/MediaPlayer.php?view_id=401&clip_id=15895&meta_id=373566 | title=Aquatic Spraying Study Committee - SB2286 - Nov 15th, 2018}}</ref><ref>{{Cite FTP |doi=10.1614/P2002-081 |title=Predicting the potential invasive distributions of five alien plant species in North America |volume=51 |issue=6 |pages=863 |year=2003 |last1=Peterson |first1=A. T. |last2=Papes |first2=M. |last3=Kluza |first3=D. A. |s2cid=86208277 |server=Weed Science |url-status=dead |url=ftp://norbif.uio.no/pub/outgoing/runeho/KR/Pete&al03WeedSci51-863.pdf }}</ref>
''Hydrilla'' populations have caused economic, environmental, and ecological damage.<ref name=NEANS/> ''Hydrilla'' is known to be an aggressive and competitive plant, out-competing and displacing native species, such as pondweeds and eelgrass.<ref name=SMS/><ref name=UFIFAS/><ref name=CCE/> ''Hydrilla'' has thus created monocultures, areas dominated by a single species, rather than having a balance among many species, as in a normal ecosystem.<ref name=CCE/>
In Australia, ''Hydrilla'' can become invasive if the nutrient levels are raised in disturbed ecosystems, though is not generally known to be problematic.<ref name="Parsons">{{cite book|author1=William Thomas Parsons |author2=Eric George Cuthbertson |title=Noxious Weeds of Australia|publisher=Csiro Publishing|location=Collingwood, Victoria|date=2001|pages=67–70|isbn=978-0-643-06514-7|url=https://books.google.com/books?id=sRCrNAQQrpwC&q=The+biology+of+Australian+weeds.+7.+Hydrilla+verticillata+%28L.f.%29+Royle&pg=PA70}}</ref>
''Hydrilla'' can host a biofilm of the cyanobacteria ''Aetokthonos hydrillicola'', which can produce the brominated neurotoxin aetokthonotoxin — the causative agent of avian vacuolar myelinopathy, a fatal brain wasting disease of waterfowl and raptors.<ref name=Breinlinger>{{Cite journal|last1=Breinlinger|first1=Steffen|last2=Phillips|first2=Tabitha J.|last3=Haram|first3=Brigette N.|last4=Mareš|first4=Jan|last5=Yerena|first5=José A. Martínez|last6=Hrouzek|first6=Pavel|last7=Sobotka|first7=Roman|last8=Henderson|first8=W. Matthew|last9=Schmieder|first9=Peter|last10=Williams|first10=Susan M.|last11=Lauderdale|first11=James D.|date=2021-03-26|title=Hunting the eagle killer: A cyanobacterial neurotoxin causes vacuolar myelinopathy|journal=Science|language=en|volume=371|issue=6536|article-number=eaax9050|doi=10.1126/science.aax9050|pmc=8318203|issn=0036-8075|pmid=33766860|doi-access=free |bibcode=2021Sci...371x9050B }}</ref>
===Management=== [[Image:Hydrilla verticillata collection LakeSeminoleFL.jpg|right|thumb|''Hydrilla verticillata'' collection at Lake Seminole, Florida|150px]] ''Hydrilla'' can be controlled by herbicides, as well as grass carp,<ref name=UFIFAS>{{cite journal|last=Langeland |first=Kenneth A. |url=http://plants.ifas.ufl.edu/node/184 |title=The Perfect Aquatic Weed |journal=Castanea |volume=61 |publisher=University of Florida, Institute of Food and Agricultural Sciences |date=1996 |access-date=December 20, 2014 |archive-url=https://web.archive.org/web/20141102084523/http://plants.ifas.ufl.edu/node/184 |archive-date=November 2, 2014 }}</ref> itself an invasive species in North America. Insects used as biological pest control for this plant include weevils of the genus ''Bagous'' and the Asian hydrilla leaf-mining fly (''Hydrellia pakistanae'').<ref name=UFIFAS/> Tubers pose a problem as they can lie dormant for a number of years, making it even more difficult to remove from waterways and estuaries. ''Hydrilla'' can spread efficiently through both tubers and turions.<ref name=CCE>{{cite web|url=http://ccetompkins.org/environment/invasive-species/ecological-impacts-hydrilla|title=Ecological Impacts of Hydrilla|publisher=Cornell University Cooperative Extension|date=July 24, 2014|access-date=December 20, 2014}}</ref>
In 2011 an inlet of Cayuga Lake, one of the Finger Lakes in New York State, faced a ''Hydrilla'' introduction. The city of Ithaca and other local officials used the chemical herbicide endothall to try to head off establishment in the Finger Lakes, which would have been disastrous for their ecosystems. The first year nearly $100,000 and many man-hours were spent trying to eradicate the ''Hydrilla'' infestation. Follow-up treatments were planned for at least five years.<ref>[https://web.archive.org/web/20150118022422/http://ccetompkins.org/environment/invasive-species/fighting-hydrilla2011 2011 Hydrilla Eradication Efforts – Cornell Cooperative Extension of Tompkins County]. Ccetompkins.org (2013-09-16). Retrieved on 2017-08-10.</ref>
In August 2021 ''Hydrilla'' was discovered growing in a small boat marina connected to the Niagara River in the city of North Tonawanda, New York. The New York State Department of Environmental Conservation is working to contain the infestation.<ref>https://www.wivb.com/news/local-news/niagara-county/north-tonawanda/invasive-hydrilla-plant-found-in-north-tonawanda-marina/ Invasive "hydrilla" plant found in North Tonawanda Marina (2021-08-20) Retrieved on 2021-08-23.</ref>
In September 2021, the Connecticut River Conservancy stated that ''Hydrilla'' has been overwhelming tributary rivers, coves, inlets and riverbanks along the river for years. Kelsey Wentling, a river steward at the Connecticut River Conservancy, stated, "''Hydrilla'' can spread through a process known as fragmentation, in which the plants, due to some sort of disruption, break apart and regrow elsewhere. Fragmentation often happens when boats go through a patch of ''Hydrilla''. For this reason, it is critical that those using paddle boats, power boats, jet skis, and fishing equipment in the Connecticut River be aware of invasive ''Hydrilla'' and then take steps to reduce its spread." It has also been reported by local marinas and municipalities that they can no longer access certain boat slips and docks due to the density of ''Hydrilla''. The Connecticut River Conservancy in September 2021 requested area boaters to complete a survey to help with the ongoing management of the invasive plant.<ref name="WTNH-Conn-2021">{{cite web | last=Gentile | first=Isabella | title=Connecticut River Conservancy seeks boater feedback on invasive plant preventing access on the river | website=WTNH | date=2021-09-21 | url=http://www.wtnh.com/news/connecticut/connecticut-river-conservancy-seeks-boater-feedback-on-invasive-plant-preventing-access-on-the-river/ | access-date=2021-09-24}}</ref>
==Phytoremediation== This abundant source of biomass is a known bioremediation hyperaccumulator of mercury, cadmium, chromium and lead, and as such can be used in phytoremediation.<ref>{{cite book|author1=McCutcheon, Steven C. |author2=Schnoor, Jerald L. |title=Phytoremediation: Transformation and Control of Contaminants|url=https://books.google.com/books?id=5zE-swQcoPQC|date= 2004|publisher=John Wiley & Sons|isbn=978-0-471-45932-3| page= 898}}</ref>
==References== {{Reflist|30em}}
==External links== {{Commons|Hydrilla verticillata}} * [https://www.invasivespeciesinfo.gov/profile/hydrilla Species Profile- Hydrilla (''Hydrilla verticillata'')], National Invasive Species Information Center, United States National Agricultural Library. Lists general information and resources for Hydrilla.
{{Taxonbar|from1=Q2720105|from2=Q164181}} {{Authority control}}
Category:Angiosperms of Western Australia Category:Aquatic plants Category:Flora of New South Wales Category:Flora of Queensland Category:Flora of South Australia Category:Flora of the Northern Territory Category:Flora of Victoria (state) Category:Hydrocharitaceae genera Category:Hydrocharitaceae Category:Monocots of Australia Category:Monotypic Alismatales genera Category:Phytoremediation plants