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 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).
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).
References: (click for full references)
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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.
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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.