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. iinumaii
Trapa bispinosa var. iinumaii
(water chestnut)
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Trapa bispinosa var. iinumaii Nakano

Common name: water chestnut

Identification: According to Rybicki et al. (2015):

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: solitary, small, white to pink flowers with four petals sprout 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.

Size: fruit 3-5 cm wide (Rybicki et al. 2015)

Native Range: All Trapa species are native to the Old World (Asia, Europe, and tropical Africa) (Muenscher 1944).

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

Nonindigenous Occurrences: The initial U.S. discovery of Trapa sp. occurred in the summer of 2014, in the tidal Potomac River in Virginia (Rybicki et al. 2015). Earlier vouchers were considered Trapa natans (L.) until later verified as Trapa sp. by DNA tests (N. Rybicki, USGS, pers. comm.).

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. iinumaii are found here.

StateYear of earliest observationYear of last observationTotal HUCs with observations†HUCs with observations†
Virginia199520203Lower Potomac; Middle Potomac-Anacostia-Occoquan; Middle Potomac-Catoctin

Table last updated 2/29/2020

† Populations may not be currently present.


Life history: Rosette numbers ranged between 34-70 per m2 with an average of five flowers per rosette (Rybicki et al. 2015). Plants senesce in the autumn due to frost, while seeds remain dormant in sediments for up to 10 years (Muenscher 1944). 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 1937). 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 are introduced as intentional plantings, as escapees from ornamental ponds, or by hitchhiking on waterfowl. They further spread either by the rosettes detaching from their stems and floating to another area or more often 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 roots 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).

The Trapa sp. found in Virginia was not morphologically different from the description of T. japonica found in Japan (Takano and Kadono 2005). However, there is insufficient DNA sequence data of Trapa accessions available in the Genbank database to determine whether Virginia Trapa sp. is genetically identifiable as T. japonica (Rybicki et al. 2015).

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.

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.

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.

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.

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.

Rybicki, N., E. Striano, and R. Thum. 2015. A Novel Invasive Species of Water Chestnut Found in the Potomac River near Washington, D.C. Pages 42-43 in 55th Annual Meeting of the Aquatic Plant Management Society, July 12-15, 2015. The Aquatic Plant Management Society, Inc. Gainesville, FL.

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.

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: 4/5/2019

Citation Information:
Pfingsten, I.A., and N. Rybicki., 2020, Trapa bispinosa var. iinumaii Nakano: U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL, https://nas.er.usgs.gov/queries/FactSheet.aspx?speciesID=2974, Revision Date: 4/5/2019, Access Date: 2/29/2020

This information is preliminary or provisional and is subject to revision. It is being provided to meet the need for timely best science. The information has not received final approval by the U.S. Geological Survey (USGS) and is provided on the condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from the authorized or unauthorized use of the information.


The data represented on this site vary in accuracy, scale, completeness, extent of coverage and origin. It is the user's responsibility to use these data consistent with their intended purpose and within stated limitations. We highly recommend reviewing metadata files prior to interpreting these data.

Citation information: U.S. Geological Survey. [2020]. Nonindigenous Aquatic Species Database. Gainesville, Florida. Accessed [2/29/2020].

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