The Celibate Water Weed That’s Taking Over The State
Something strange is lurking below the surface of New York’s waterways— and no, it’s not another giraffe skeleton.
The culprit is Hydrilla verticillata, a perennial, invasive species of aquatic weed that can grow in water as shallow as a few inches and in as little as 1% of sunlight. Hydrilla may look familiar to the recreational boaters and pond-swimming enthusiasts among us; it can now be found in 28 states across the US, including New York.
Brought to Miami from Asia in the 1950s as an aquarium plant, Hydrilla quickly found its way into the waters of Florida. It has become one of the most serious invasive problems in the US, and some Floridian biotypes have even developed resistance to the herbicide used to eradicate it.
Hydrilla verticillata can grow up to 35 feet long in most freshwater systems, and is detrimental to native species of plant and animal life whose precious sunlight, space, and dissolved oxygen supply are consumed by the fast-growing weed. Dense infestations can block irrigation systems, provide a breeding ground for mosquitoes, and decomposed plant matter can negatively impact water quality.
Hydrilla doesn’t stop at habitat destruction. A cyanobacteria that grows on the plant, known as Aetokthonos hydrillicola, has been linked to vacuolar myelinopathy (VM), a fatal neurological disease linked to mass bald eagle deaths in the Southeastern US. VM can be transferred through the food chain, from plant consumption by herbivorous fish to birds of prey. More research to determine if VM is transmissible to humans is critical.
As temps rise, Hydrilla will flourish
In addition to melting glaciers and spreading wildfires, climate change will likely result in the increased spread of this invasive species.
A 2017 study combining the use of ecological niche models, or ENMs, and genetic studies, looked to predict the future of Hydrilla verticillata distribution in North America, and how climate change events may affect its growth.
The study determined that rising temperatures and shifting rainfall events resulting from climate change will increase Hydrilla’s presence in North America. Hydrilla is thermophilic, or flourishes best in high temperatures, and as global water temperatures rise, so will the plant’s growth. By 2070, Hydrilla will take up 1.5 times its current square kilometrage in North American water systems.
Results also showed that, surprisingly, the hermaphroditic (monoecious) Hydrilla biotype had little success reproducing on its own, and the female (dioecious) biotype has no male counterpart in North America with which to reproduce. However, Hydrilla propagules, or plant fragments, can detach and disperse, giving rise to new growth—and experts now believe the plant is Trojan-Horsing its way into new waters by clinging to the bottom of boats.
Not just another New York transplant
Since its discovery in an Orange County pond in 2008, Hydrilla has spread quickly throughout New York state—seemingly without sexual reproduction. But that hasn’t stopped New York conservationists from working to stymie the plant’s proliferation.
In 2013 the Department of Environmental Conservation launched a successful 9-year campaign to remove Hydrilla from the Croton River using the aquatic herbicide fluridone.
“We treated the water for a few years, and this year we came back and only found one plant in the entire river,” said Ashley Morris, Aquatic Invasive Species Coordinator at the New York State Department of Environmental Conservation, of the Croton River program. The program was also able to protect the Hudson River Estuary and its habitats from Hydrilla invasion.
Though the controlled efforts in the Croton were effective, Hydrilla continues to pose a threat to New York water systems, and there’s still more to learn about how this pernicious plant works.
A single, abstinent clone
On January 25, Cornell University Faculty Fellow Jose Andres presented new research on Hydrilla. Andres and his team set out to identify the different types of Hydrilla found in New York state through the genotyping of 104 hydrilla plants in 11 different locations—the single-largest population genetics analysis ever conducted on the invasive species. The research also explored which of these Hydrilla types, if any, were resistant to the herbicide fluridone, and how Hydrilla is being spread statewide.
The results were illuminating—not only did Andres’ research further confirm Hydrilla’s inability to sexually reproduce, but it found that one clone was moving across the state, resulting in a single genetically homogenous population. This genetic case is similar to that of the now-infamous self-cloning marbled crayfish—both Hydrilla and the crayfish are young triploid lineages, or hybrid species, and both invasive species reproduce asexually.
“To be honest, I was a little bit disappointed because I thought we were going to be able to build a map of connectivity,” said Andres of the results of the research.
“This general lack of genetic variation means that tracking the spread of Hydrilla through genome connectivity patterns is virtually impossible,” Andres continued, as DNA sequence comparisons revealed extremely low numbers of variants in the analyzed Hydrilla genomes.
“From a genetic point of view, the state was colonized by a [genetically identical] plant, with the exception of a few somatic mutations. Not all the cells in your body have the exact same genetic information. Take the genetic differences in your right and left hands, for example. That’s the level of difference that we’re looking at,” explained Andres.
What these somatic variations can tell us, however, is that none of the Hydrilla fragments surveyed in New York carried mutations that are linked to fluridone resistance. This may be good news for future Hydrilla elimination efforts across the state.
In Lake Sebago, another elimination project using the herbicide fluridone is underway. Fluridone, commonly used to control invasive plants, has low toxicity to animals, and, at safe concentrations, doesn’t affect drinking water sources.
“Generally what has worked best in our region is herbicide treatment. What [Andres’] research can really help us understand is what chemicals will be most effective on the target plant and least effective on the non-target plants. We don’t want to hurt the native species,” said Ashley Morris.
“We’re able to use the information that we don’t have genetic diversity [in Hydrilla in the state] to predict the outcomes of similar treatment in Lake Sebago. We’ve created a management plan and hopefully we’ll have very similar outcomes [to Croton Lake],” said Morris of current efforts to tackle the infestation in Harriman State Park.
Though Hydrilla is expected to spread across New York state and North America in coming years, Andres’ genetic research has given experts key insights into how the weed spreads, and what we can do to stop it in its tracks.
Sources cited
J. Andres, S. Bogdanowicz. Invasion genomics of Hydrilla verticillata in NY state, presented at New York State Water Institute Seminar Series, 25 January 2024.
Cornell University Cooperative Extension. (2019, July 2). New York Invasive Species (IS) information. New York Invasive Species Information. https://nyis.info/invasive_species/hydrilla/
Dec celebrates success of Croton River Invasive Species Control Project. Department of Environmental Conservation. (n.d.). https://dec.ny.gov/news/press-releases/2022/10/dec-celebrates-success-of-croton-river-invasive-species-control-project
Hetrick, S. A., & Langeland, K. A. (2012, March). Hydrilla Management in Florida Lakes - Aquaculture, fisheries, & pond ... Hydrilla Management in Florida Lakes. https://fisheries.tamu.edu/files/2013/09/Hydrilla-Management-in-Florida-Lakes.pdf
Jacono, C.C., M.M. Richerson, V.H. Morgan, E. Baker, and J. Li, 2024, Hydrilla verticillata (L. f.) Royle: U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL, and NOAA Great Lakes Aquatic Nonindigenous Species Information System, Ann Arbor, MI, https://nas.er.usgs.gov/queries/greatlakes/FactSheet.aspx?Species_ID=6&Potential=Y&Type=2&HUCNumber=N, Revision Date: 1/15/2020, Access Date: 1/30/2024
Pennisi, E. (2018). An aquarium accident may have given this crayfish the DNA to take over the world. Science. https://doi.org/10.1126/science.aat2188
Steffen Breinlinger et al. Hunting the eagle killer: A cyanobacterial neurotoxin causes vacuolar myelinopathy. Science 371, eaax9050(2021). DOI:10.1126/science.aax9050
University of Florida, IFAS. (n.d.). Hydrilla verticillata. Center for Aquatic and Invasive Plants. https://plants.ifas.ufl.edu/plant-directory/hydrilla-verticillata/
Zhu, J., Xu, X., Tao, Q., Yi, P., Yu, D., & Xu, X. (2017). High invasion potential of hydrilla verticillata in the Americas predicted using ecological niche modeling combined with genetic data. Ecology and Evolution, 7(13), 4982–4990. https://doi.org/10.1002/ece3.3072
