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.
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).
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.
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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.
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Hsieh, C.F. 1994. Onagraceae. Page 899 in Flora of Taiwan. 2nd edition. Editorial Committee of the Flora of Taiwan. Taiwan, China.
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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.
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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.
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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.
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.