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.

Trapa bispinosa var. iinumai
Trapa bispinosa var. iinumai
(water chestnut)

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Trapa bispinosa var. iinumai Nakano

Common name: water chestnut

Synonyms and Other Names: Trapa natans var. bispinosa (Roxb.) Makino

Trapa bispinosa Roxb.

Identification: According to Hsieh (1994), Chorak et al. (2019), Dodd et al. (2019), Rybicki et al. (2019), and Virginia Botanical Associates (2020):

Habit: floating, rooted, aquatic annual

Stems/Roots: submerged, flexuous stem and roots that anchor into the mud and extend upwards to the surface of the water

Leaves: rosette of floating, alternate, fan-shaped leaves, each leaf having a slightly inflated petiole (leaf stem) and biserrate (doubly serrated) leaf margins with leaf surfaces green above and red below; submerged leaves are opposite, linear, and die back to be replaced by roots

Flowers: four-merous, solitary, small, reddish sepals, pink petals, sprouting in the center of the rosette; flowering typically in June

Fruits/Seeds: large drupe or "nut" with two, opposing, sharp spines that develop from hardened sepals and two pseudo spines where sepals senesce

Look-a-likes: Ludwigia sedioides (Humb. & Bonpl.) H. Hara has similar leaf shape, arrangement, and floating habit, but is much smaller than Trapa and the flowers are yellow. Trapa bispinosa is distinguished from T. natans L. by the reddish undersides of the leaves, reddish sepals, pink petals, and two horizontally opposed pseudo-horns.

Size: fruit 3-5 cm wide

Native Range: All Trapa species are native to the Old World (Asia, Europe, and tropical Africa) (Muenscher 1944). Trapa bispinosa Roxb. is native to Taiwan, China, Korea, and Japan (Hsieh 1994).

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

Nonindigenous Occurrences: The initial U.S. discovery of Trapa bispinosa var. iinumai occurred in the summer of 2014, in the tidal Potomac River in Virginia. Earlier vouchers were considered Trapa natans until later verified as Trapa bispinosa var. iinumai by DNA tests (Chorak et al. 2019).

Table 1. States with nonindigenous occurrences, the earliest and latest observations in each state, and the tally and names of HUCs with observations†. Names and dates are hyperlinked to their relevant specimen records. The list of references for all nonindigenous occurrences of Trapa bispinosa var. iinumai are found here.

StateFirst ObservedLast ObservedTotal HUCs with observations†HUCs with observations†
MD202220231Middle Potomac-Anacostia-Occoquan
VA199520244Lower Potomac; Middle Potomac-Anacostia-Occoquan; Middle Potomac-Catoctin; Middle Roanoke

Table last updated 6/19/2024

† Populations may not be currently present.


Life history: Rosette numbers ranged between 34-70 per m2 with an average of five flowers per rosette (Nancy Rybicki pers. comm.). Trapa species senesce in the autumn due to frost, while seeds remain dormant in sediments for at least two years in the wild, and up to five years in stable conditions (Kunii 1998). Germination occurs above 12°C (Muenscher 1944).

Habitat: Full sun, sluggish, eutrophic, fresh water, and soft sediment (Winne 1950) with water depths from 0.3-3.6 m (Muenscher 1944). Plant growth is associated with nitrogen-rich waters (Vuorela and Aalto 1982).

Tolerances: Trapa species appear to be affected by density dependence; at low densities, plants produced 10 times as many ramets as those at high density (Groth et al. 1996). Seeds germinate best in alkaline substrates with a pH of 7.9-8.3 and where salinity is below 0.1% or 1 ppt (Vuorela and Aalto 1982).

Community and environment: The leaf beetle, Galerucella nipponensis (Coleoptera: Chrysomelidae), was found to prefer vertically grown leaves of Trapa japonica (Ikeda and Nakasuji 2002).

Means of Introduction: Trapa bispinosa var. iinumai were likely introduced as intentional plantings, and further as escapees from ornamental ponds. The spiny seeds are dispersed by hitchhiking on large animals including waterfowl. Trapa bispinosa var. iinumai has been observed attached to resident Canada geese (Rybicki et al. 2019). Trapa species spread vegetatively by the rosettes detaching from their stems and floating to another area or, more often, reproductively by the nuts being swept by currents or waves to other parts of the lake or river (Bickley and Cory 1955; Mirick 1996; Hummel and Kiviat 2004).


Populations are established in the Middle Potomac drainage including many private and public ponds (N. Rybicki, pers. comm. 2018).

Status is unknown in the Lower Potomac drainage.

Impact of Introduction: The types and magnitude of possible ecological and economic impacts are currently unknown. The absence of data to evaluate effects does not equate to the lack of effects. A better understanding to adequately assess this species in its invaded range will require further research.

Trapa bispinosa var. iinumai is potentially similar to T. natans in negatively impacting native wildlife and obstructing navigation (Gwathmey 1945, Naylor 2003, Hummel and Kiviat 2004). Its dense growth may lead to detrimental effects, such as low dissolved oxygen, obstruction of water flow, and impediment to recreational use of water, as shown with T. natans (Strayer 2010). Plant biomass of T. natans beds was an order of magnitude greater than in the native Vallisneria americana beds it replaced in a study of the Hudson River, New York (Strayer 2010). Once established, T. bispinosa var. iinumai, like T. natans, may dominate other plant communities over decadal time scales (Gwathmey 1945; Kadono 2004). Through shading T. natans competes with native submerged aquatic vegetation that is considered preferable waterfowl habitat (Martin and Uhler 1939; Kiviat 1993; Groth et al. 1996; Hummel and Kiviat 2004).

The roots of Trapa natans absorb dissolved inorganic nitrogen from water and sediment (Tsuchiya and Iwakuma 1993).

Remarks: No hybridization was detected among various Trapa species that grew interspersed within the same water bodies within the native range in China (Li et al. 2017). Low intra-population genetic diversity was also found in the other annual aquatic plants in China, specifically, Ottelia alismoides L. (Chen et al. 2008) and Ceratopteris pteridoides (Hook.) Hieron (Chen et al. 2010). There is evidence that Trapa species are self pollinating (Sinjushin, 2018).

References: (click for full references)

Bickley, W.E., and E.N. Cory. 1955. Water caltrop in the Chesapeake Bay. Association of Southeastern Biologists Bulletin 2:27-28.

Chen, Y.Y., X.L. Li, L.Y. Yin, and W. Li. 2008. Genetic diversity of the threatened aquatic plant Ottelia alismoides in the Yangtze River. Aquatic Botany 88(1):10-16. https://doi.org/10.1016/j.aquabot.2007.08.002.

Chen, Y.Y., Q.X. Han, Y. Cheng, Z.Z. Li, and W. Li. 2010. Genetic variation and clonal diversity of the endangered aquatic fern Ceratopteris pteridoides as revealed by AFLP analysis. Biochemical Systematics and Ecology 38(6):1129-1136. https://doi.org/10.1016/j.bse.2010.12.016.

Chorak, G.M., L.L. Dodd, N. Rybicki, K. Ingram, M. Buyukyoruk, Y. Kadono, Y.Y. Chen, and R.A. Thum. 2019. Cryptic introduction of water chestnut (Trapa) in the northeastern United States. Aquatic Botany 155:32-37. https://doi.org/10.1016/j.aquabot.2019.02.006.

Dodd, L.L., N. Rybicki, R. Thum, Y. Kadono, and K. Ingram. 2019. Genetic and Morphological Differences of Water Chestnut (Myrtales: Lythraceae: Trapa) Populations in the Northeastern United States, Japan, and South Africa. US Army Corps of Engineers Engineer Research and Development Center, Vicksburg, MS. https://apps.dtic.mil/dtic/tr/fulltext/u2/1070329.pdf.

Groth, A.T., L. Lovett-Doust, and J. Lovett-Doust. 1996. Population density and module demography in Trapa natans (Trapaceae), an annual, clonal aquatic macrophyte. American Journal of Botany 83(11):1406-1415. https://doi.org/10.1002/j.1537-2197.1996.tb13934.x.

Gwathmey, J.H. 1945. Potomac River cleared of floating islands. Maryland Conservationist 22(1):21-23.

Hsieh, C.F. 1993. Trapaceae. Pages 968-969 in Huang, T.C., ed.-in-chief, Flora of Taiwan. Volume 3. 2nd edition. Editorial Committee of the Flora of Taiwan. Taipei, Taiwan 106, the Republic of China.

Hummel, M., and E. Kiviat. 2004. Review of World Literature on Water Chestnut with Implications for Management in North America. Journal of Aquatic Plant Management 42:17-28. http://apms.org/wp/wp-content/uploads/2012/10/v42p17.pdf.

Ikeda, K., and F. Nakasuji. 2002. Spatial structure-mediated indirect effects of an aquatic plant, Trapa japonica, on interaction between a leaf beetle, Galerucella nipponensis, and a water strider, Gerris nepalensis. Population Ecology 44(1):41-47. https://doi.org/10.1007/s101440200005.

Kiviat, E. 1993. Under the spreading water-chestnut. News From Hudsonia 9(1):1-6.

Kunii, H. 1998. Longevity and germinability of buried seeds in Trapa sp. Memoirs of the Faculty of Science, Shimane University 22:83-91. https://ir.lib.shimane-u.ac.jp/608.

Li, X.L., X.R. Fan, H.J. Chu, W. Li, and Y.Y. Chen. 2017. Genetic delimitation and population structure of three Trapa taxa from the Yangtze River, China. Aquatic Botany 136:61-70. https://doi.org/10.1016/j.aquabot.2016.09.009.

Martin, A.C., and F.M. Uhler. 1939. Food of game ducks in the United States and Canada. Volume 634. U.S. Department of Agriculture, Washington DC, US.

Mirick, P.G. 1996. Goose grief. Massachusetts Wildlife 46(2):15-16.

Muenscher, W.C. 1944. Aquatic Plants of the United States. Comstock Publishing Company, Inc/Cornell University, Ithaca, NY.

Naylor, M. 2003. Water Chestnut (Trapa natans) in the Chesapeake Bay Watershed: A Regional Management Plan. Maryland Department of Natural Resources. 35 pp.

Strayer, D. L., C. Lutz, H.M. Malcom, K. Munger, and W. H. Shaw. 2003. Invertebrate communities associated with a native (Vallisneria americana) and an alien (Trapa natans) macrophyte in a large river. Freshwater Biology 48:1938—1949.

Takano, A., and Y. Kadono. 2005. Allozyme variations and classification of Trapa (Trapaceae) in Japan. Aquatic Botany 83:108-118. https://doi.org/10.1016/j.aquabot.2005.05.008.

Tsuchiya, T., and T. Iwakuma. 1993. Growth and leaf life-span of a floating-leaved plant, Trapa natans L., as influenced by nitrogen flux. Aquatic Botany 46(3-4):317-324.

Virginia Botanical Associates. 2020. Digital Atlas of the Virginia Flora. Virginia Botanical Associates, Blacksburg, VA. http://vaplantatlas.org/.

Vuorela, I., and M. Aalto. 1982. Palaeobotanical investigations at a Neolithic dwelling site in southern Finland, with special reference to Trapa natans. Annales Botanici Fennici 19(2):81-92. http://www.jstor.org/stable/23725192.

Winne, W.T. 1950. Water chestnut: A foreign menace. Bulletin to the Schools 36(7):230-234.

Author: Pfingsten, I.A., and N. Rybicki.

Revision Date: 12/5/2023

Citation Information:
Pfingsten, I.A., and N. Rybicki., 2024, Trapa bispinosa var. iinumai Nakano: U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL, https://nas.er.usgs.gov/queries/FactSheet.aspx?SpeciesID=2974, Revision Date: 12/5/2023, Access Date: 6/19/2024

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Citation information: U.S. Geological Survey. [2024]. Nonindigenous Aquatic Species Database. Gainesville, Florida. Accessed [6/19/2024].

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