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Naiad comparison table
Naiad comparison table

Najas flexilis, Najas gracillima


NOTE: Maine is home to three native naiad species. One species, slender naiad (N. flexilis), also called northern water-nymph, is common in Maine. The others: southern naiad (N. guadalupensis) and thread-like naiad (N. gracillima) are quite rare. Though a rare species, thread-like naiad has been included in this guide because it has more features in common with the invader European naiad than the more common, slender naiad.

Slender naiad in-situ
slender naiad in-situ

Habitat: Both native species discussed below are found in the submersed plant community, often growing in the sandy or gravel substrates of lakes, ponds and slow moving streams. Thread-like naiad is particularly sensitive to pollution, and has disappeared in some parts of its natural range.

Description: The branching stems of both native naiads are slender and flexible (up to 1 meter long, but often much shorter), growing from slight roots. Thread-like naiad is a very delicate plant with wispy stems branching lightly near the tips. The habit of slender naiad is more variable: some plants are tall and sparse; others short and bushy. The leaf arrangement of both species is mixed. Leaves may occur in opposite pairs and/or whorls along the stem, and be clumped into delicate sprays at the stem tips. The leaves are slender (1 to 4 cm long), linear, serrated (actually spined) along their margins, and sharply pointed at the tip. The tiny, inconspicuous flowers, followed by slender fruits, develop in the leaf axils. The fruits are about 3mm long, cylindrical, and pointed at both ends. The surface may appear to be smooth but magnification reveals many tiny, shallow indentations, or pits, arranged in longitudinal rows. The fruits turn brown as they mature.

The comparison table compares four key features that help to distinguish the two native naiads from each other and from their invasive look alike, European naiad, (Najas minor). Magnification is generally needed to observe these features. To observe leaf bases, gently pull the leaf away from the stem.

Thread-like naiad leaf base
Thread-like naiad leaf base

U.S. Range: Both species are native to Maine and New England. Slender naiad's range includes much of the northern and western United States. The range of thread-like naiad includes most of the eastern and central-eastern United States and California. In Maine, the distribution of slender naiad is fairly widespread. The other two native species, including thread-like naiad, are rare.

thread-like naiad Range Map
U.S. range map of thread-like naiad

slender naiad Range Map
U.S. range map of slender naiad

Annual Cycle: Unlike most aquatic plants, naiads are true annuals, dying back completely in the fall and relying upon seeds to regenerate the following season. Seeds germinate in the spring and plants are generally visible by early summer. Vegetative reproduction may occur during the growing season. Tiny flowers, followed by seeds, are produced in the leaf axils. (Male and female flowers occur separately on the same plant.) Plants become brittle and begin to break down at the end of the growing season, fragmenting, drifting and eventually depositing their seeds on the sediments.

Value to the Aquatic Community: Naiads are an important food source for a wide variety of waterfowl and marsh birds. Muskrats also feed upon the stems and leaves. The slender branches provide food and shelter for fish and invertebrates.

Look Alikes: Maine's native naiads may be confused with European naiad, some fine-leaved pondweeds, and some Stoneworts.

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slender naiad specimen thread-like naiad leaf base thread-like naiad stem slender naiad range map thread-like naiad range map slender naiad illustration slender naiad leaf illustration thread-like naiad illustration thread-like naiad leaf illustration thread-like naiad seeds

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