Stem/Rhizoids: Nitellopsis obtusa has long, variable-length, relatively straight branches arranged in whorls that attach at acute angles to stem nodes. Internodal cells of N. obtusa are quite large, often on the order of a few centimeters long (Steudle and Zimmermann 1977, Yoshioka and Takenaka 1979). Most stem and branch cells are around 1 mm in diameter, while stems can reach up to 80 cm long (Hargeby 1990, Sher-Kaul et al. 1995). Heights of 2 m have been observed at a depth of 9 m in one Michigan lake (Pullman and Crawford 2010), although rate of growth is uncertain. N. obtusa is light green when actively growing. Creamy white bulbils may occur at the base of the main axis just below the substrate-water interface and on branches of the main axis at nodes; rhizoids are star-shaped.
Reproductive structures: Plants are dioecious. Female oogonia, with bracts on either side, form at the upper nodes of branchlets. Plants can form gyrogonites, which are calcified, spiral-shaped fructifications (Bharathan 1983, 1987, Schloesser et al. 1986, Soulie-Marsche et al. 2002). Orange to red oocytes can occur at the nodes of branches (Pullman and Crawford 2010).
Look-a-likes: Chara spp. musk-grass; Nitella spp. brittlewort; other Nitellopsis spp. stonewort. Key differences are the star-shaped rhizoid, the orange-colored oocyte, irregular branching, and lack of a garlic odor (Pullman and Crawford 2010).
Size: Up to 2 m in height (Pullman and Crawford 2010); main stem up to 80 cm long (Hargeby 1990).
Native Range: Nitellopsis obtusa is native to Eurasia, from the west coast of Europe to Japan (Mills et al. 1993, Soulie-Marsche et al. 2002).
Ecology: Nitellopsis obtusa is sometimes found in deep, slow moving water where other plants are scarce, typically near docks and marinas (Midwood et al. 2016). Nitellopsis obtusa is known to maintain permanent populations in freshwater or brackish water with salinity up to 5%. It can tolerate salinity fluctuations up to 17% for around 1 week. Under high salt loading or unfavorable environmental conditions, it has the ability to shift cells from a high-energy state to a state of passive permeability. It experiences suppressed growth at water temperatures of 30°C. In such conditions, apical cells no longer form and some plant cells may die (Marchyulenene et al. 1982, Moteyunene and Vorob’ev 1981, Winter et al. 1999). In areas of dense vegetation, N. obtusa forms “pillows” of various heights; as growth slows, these pillowed mats may develop circular clearings (Pullman and Crawford 2010). Under eutrophic conditions, it often produces oospores (Bharathan 1987).
Nitellopsis obtusa occurs at depths of 1–3.5 m in relatively protected zones of the St. Clair-Detroit River system at water velocities of 3–11 cm/s; on soft substrates such as silt, sand, and fine detritus; and where light transmittance ranges from 1–50%. In this system, it first appears around July and reaches highest biomass levels in September, gradually declining until March of the following year, when it decomposes. It has been recorded in water temperatures of 0–24°C in this area. It occurs more in the Detroit River than the St. Clair River, while other stoneworts (Family Characeae) occur more frequently in the latter. The environmental parameters determining stonewort distribution in these two rivers are not known. In the St. Lawrence River, it is uncommon in early July but increases through September. It occurs at an average depth of 4.8 m depth and 6% light transmittance. In its native habitat, it is typically found at depths of 3–8 m, preferring deeper habitats with low light transmittance but relatively high calcium and phosphorus content, where other stoneworts generally occur less frequently (Berger and Schagerl 2004, Nicholls et al. 1988, Schloesser et al. 1986).