Montana Field Guide

PLANTS: A dioecious aquatic annual. Branched stems are lax and floating, 6 to 45 cm long by 0.5 to 4 mm wide with 0.3 to 11 cm between the internodes. Stems and leaves are prickly. Source: Haynes in Flora of North America (FNA) 2000

LEAVES: Blades are 0.5 to 3.9 cm long, 0.4 to 4.5 mm wide, and have coarsely serrated margins. There are 8 to 13 teeth on each side of the leaf blade. Leaves stiffen with age and spread or ascend. Sheaths are 2 to 4.4 mm wide with a pointed tip. Source: Haynes in FNA 2000

INFLORESCENCE: The inconspicuous male and female flowers occur on separate plants (dioecious). Single flowers arise from the leaf axils. Male (or staminate) flowers are 1.7 to 3 mm long. Female (or pistillate) flowers are 2.5 to 5.7 mm long. Seeds are reddish brown, ovoid, 2.2 to 4.5 mm long, and 1.2 to 2.2 mm wide. Source: Haynes in FNA 2000

Najas comes from the Greek word naias meaning water-nymph (Giblin et al. [eds.] 2018).

Flowers arise in summer and fall (Haynes in FNA 2000).

Prickly stems and leaves make Spiny Naiad easily distinguishable from other Najas species. To evaluate the seed surface characters for any Najas species, remove the pericarp layer (Giblin et al. [eds.] 2018).

Spiny Naiad – Najas marina, Exotic
*Leaves: Coarsely serrated margins with 8 to 13 teeth on each side of the blade. Prickles can be found on the abaxial (lower) surface of the blade.
*Seeds: Ovoid, reddish brown, and pitted with areoles (outer two coatings of the testa) irregularly arranged, not in distinctive rows or not ladder-like. Seeds are longer than broad with the end walls slightly raised.

Guadalupe Water-nymph - Najas guadalupensis, SOC
*Leaves: 5-15 mm long with minutely toothed margins. Ribbon-like with short tapering tips and expanded bases. Additional leaves are often found in the axils. Prickles lacking.
*Seeds: Dull and coarsely pitted, with a honeycomb surface.

Slender Naiad - Najas flexilis, Native
*Leaves: 10–35 mm long with a dilated base that is 2–3 mm wide. From about their mid-section, leaves have a long taper that terminates in a long slender point. Prickles lacking.
*Seeds: Smooth, shiny, and sometimes minutely spotted.

Brittleleaf Naiad - Najas minor, NOT DOCUMENTED IN MONTANA
*Leaves: At maturity, blades recurve and become stiff with age. Prickles lacking.
*Seeds: Broader than long and with areoles arranged in longitudinal rows – like the rungs of a ladder.

TAXONOMY
The Najas genus is sometimes placed in the Family Hydrocharitaceae, also known as the Waterweed or Frog’s-bit Family.

Over its entire worldwide range, Spiny Naiad displays considerable morphologic variability (Triest et al. 1986), giving cause for ten subspecies to be recognized (Triest 1988). In North America, however, variability is relatively minor and Spiny Naiad is only recognized at the specific level (as Najas marina) (Haynes 1979; Haynes in FNA 2000).

Spiny Naiad is often associated with saline, brackish, or alkaline waters of ponds and lakes (Moyle 1945; Stuckey 1985; Haynes in FNA 2000; Les 2020). It is found in waters that have a pH average of 8.6, but range from 7.8 to 10.2 (Les 2020).

In Montana, plants were found in two adjacent ponds in the vicinity of Frenchtown Pond State Park. Frenchtown Pond was created from gravel excavation which helped to build Interstate 90 in the 1960s (Freeman and Pfingsten 2021). This pond fills from groundwater and has no outlet (Freeman and Pfingsten 2021). The pond north of Frenchtown Pond apparently also has no outlet and fills with groundwater. A single test of the northern pond in 2018 found the water to have a pH of 9.17 (Freeman and Pfingsten 2021).

PLANT GROWTH and DISPERSAL
Germination and maturation of Spiny Naiad is regulated by water temperature. Water temperatures of more than 15 degrees Celsius are required for germination and more than 20 degrees Celsius is needed for maturation (Van Vierssen 1982; Agami and Waisel 1984; Hoffman et al. 2013).

Studies have shown that waterfowl consume Najas species in North America and could be a possible mechanism for its dispersal (Meriläinen 1968; Stuckey 1985; Les 2020). Spiny Naiad seeds have been consumed by blue-winged teal (Spatula discors), mallards (Anas platyrhynchos), and redheads (Aythya americana) (Les 2020). In a study of mallards it was found that the seeds of Spiny Naiad can remain in the gut for up to 10 hours, then pass through unharmed, and that this process improves the germination rate (Agami & Waisel 1986). Spiny Naiad has hard-coated seeds which can remain viable for up to four years (Agami and Waisel 1986) or in lake sediments has been shown to stay viable for up to three years (Handley and Davey 2005).

Plants reproduce by seed.

FRUITS
The fruit is an achene. Seeds are reddish brown, ovoid, and longer than broad (2.2 to 4.5 mm long; 1.2 to 2.2 mm wide) with the end walls slightly raised. The seed surface is pitted with areoles that are irregularly arranged.

Spiny Naiad has the potential to expand its distribution and population in Montana (Freeman and Pfingsten 2021). Alkaline lakes and ponds are common; however, some large lakes, high elevation lakes, and many rivers and streams in Montana may not reach warm enough temperatures for this plant to establish [refer to Habitat and Ecology sections]. In Frenchtown Pond, surveys and observations found that within two years Spiny Naiad had rapidly increased its population to become the dominant aquatic plant.

The mechanism for how Spiny Naiad colonized the ponds in Montana is unknown, but two plausible explanations exist (Freeman and Pfingsten 2021). Recreational use of Frenchtown Pond is heavy in the summer. Although there is no boat ramp, hand-launch of small watercraft is common. At the same time data from eBird (2021) shows that Frenchtown Pond is known to be used by at least 15 species of waterfowl and waterbirds, including mallards and redheads (Freeman and Pfingsten 2021).

Management actions that assess the potential risk of Spiny Naiad to native species and water quality, as well as to contain and eradicate this non-native plant, are needed (Freeman and Pfingsten 2021):
* Development and dissemination of educational materials can bring forth distribution
information on Montana’s three rare, common, and exotic Najas species [refer to the
Diagnostic Characteristic].
* Continued efforts to survey and sample aquatic plants in vicinity of Frenchtown Pond State
Park can help determine when plants are detectable and estimate seasonal and yearly
population sizes.
* Containing and eradicating Spiny Naiad would greatly reduce and maybe prevent its spread.

Contact information for Aquatic Invasive Species personnel:
Montana Fish, Wildlife, and Parks Aquatic Invasive Species staff
Montana Department of Natural Resources and Conservation's Aquatic Invasive Species Grant Program
Montana Invasive Species Council (MISC)
Upper Columbia Conservation Commission (UC3)

Spiny Naiad is exotic to Montana, and has the potential to expand its distribution and population within the state (Freeman and Pfingsten 2021). Refer to the Management section.