Placed in Najadaceae in Michigan Flora. Najas marina is dioecious, all others are apparently monoecious.
1. Leaves appearing conspicuously toothed to the naked eye, the broad-based teeth as much as 0.5–1 mm long; fruits very plump, 4–5 mm long at maturity.
1. Leaves appearing entire or minutely toothed or spinulose especially when seen under a lens; fruits more slender, 2–3.5 (–4) mm long.
2. Leaves with auriculate or broadly truncate basal lobes (these toothed at summit), the setaceous blade 0.2–0.5 mm wide.
3. Teeth of leaf margin visible with lens, sometimes to naked eye; fruits about 2–2.5 (–2.8) mm long, with longitudinal striations more prominent than the transverse ones, giving a ladder-like appearance to the reticulations; lobes at base of leaf ± truncate and herbaceous.
3. Teeth of leaf margin merely microscopic spinules; fruits 2.5–3.5 mm long, evenly reticulate (the longitudinal striations not more prominent); lobes at base of leaf often definitely auriculate and scarious.
2. Leaves expanded at the base, but expanded portion tapering to the slender blade (not auriculate or broadly truncate), minutely spinulose; blades 0.3–2.2 mm wide.
4. Styles (including stigmas) 1–2.5 mm long; fruit (2–) 2.5–3.5 (–4) mm long, smooth and glossy, with very fine and rather obscure reticulate pattern; leaves, at least most of the well developed ones, very slender (0.3–1 mm wide at the middle, 20 or more times as long, sometimes up to ca. 3 cm long), tapering to apex.
4. Styles (including stigmas) ca. 0.3–0.6 mm long; fruit ca. 2 mm long with a rather conspicuously reticulate and shallowly pitted surface; leaves 0.6–2.2 mm wide at middle, nearly always 7–15 (–20) times as long (up to 1.8 cm long), thus appearing more abruptly acute at tip.