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Starry stonewort (Nitellopsis obtusa)
Photo credit:   Jim Grazio, PA DEP, 2015

Starry Stonewort

(Nitellopsis obtusa)

Report this Species!

If you believe you have found this species anywhere in Pennsylvania, please report your findings to iMapInvasives by submitting an observation record.

Species at a Glance

While starry stonewort resembles a true plant, it is actually a rooted alga descended from some of the earliest life forms on earth and thought by scientists to have been the ancestors to all plants. Its ability to degrade ecologically sensitive areas and proliferate rapidly makes it a highly invasive species.


This robust alga grows more than 2 m (7 ft) long. It is a light green color when actively growing. One way to distinguish starry stonewort is by the tiny, star-shaped, tan-colored reproductive structures called bulbils that are firm to the touch when compared to its soft branches. Its long, relatively straight branches are arranged in whorls of 4-6, which are attached at angles to the stem nodes. Stems can reach up to 80 cm (31 in) long and will “pop” when squeezed. Dark-red to orange reproductive structures called oogonia (female) and antheridia (male), can occur in the nodes of the branches although all North American colonies appear to be male plants. Colorless hair-like filaments called rhizoids, which act as roots, absorb nutrients and provide stability. Instead of forming uniform mats, this alga forms irregularly spaced “pillows” of dense vegetation in various heights. When growth declines, usually in summer, circular openings may appear in the mat that resembles a “Swiss cheese” pattern.

Similar Species

While it may be confused with native species of chara, like muskgrass (Chara spp.), starry stonewort’s light green color when growing distinguishes it from other charoid species. It can also be distinguished by the “squeeze test”, where the contents of the cell will “pop” out when squeezed. In addition, starry stonewort can grow to remarkable heights and depths. Its branching pattern is more irregular, giving the plant a characteristic ragged or “disheveled” appearance. Most charoid algae have a musky or garlic odor that’s not as pronounced in the starry stonewort. Star-shaped bulbils are a key characteristic and are best observed from late fall through winter.


Stoneworts live in fresh or brackish water and tolerate low nutrient and light levels. They are found at depths from 1 m (3 ft) to greater than 6 m (20 ft) in lakes or slow moving rivers, and prefer alkaline waters. They can grow on organic and inorganic substrates and have the ability to absorb nutrients through all surfaces, not just the rhizoids.


Starry stonewort was likely introduced to North America in the ballast water of ocean-going ships. It spreads rapidly by fragments and bulbils that are easily transported by boats, trailers, and water currents.


Native to Europe and western Asia, and classified as endangered in Great Britain, this alga was first reported in the St. Lawrence River in 1978 and the St. Clair River in 1983. In 2005, starry stonewort was reported in Oneida Lake in New York, and in 2006 it began to rapidly expand its range throughout inland lakes in Michigan. It has since spread to Wisconsin, Vermont, and Pennsylvania where it can be found in Lake Arthur in Butler County and Presque Isle Bay in Erie County.


Note: Distribution data for this species may have changed since the publication of Pennsylvania's Field Guide to Aquatic Invasive Species (Second Edition 2015), the source of information for this description.

Environmental Impacts

Dense mats of starry stonewort can completely cover the lake bottom and greatly reduce the diversity of aquatic plants. These mats can be up to 2 m (7ft) thick, and can impede the movement of fish and other animals, decrease available habitat for successful spawning activities, reduce water flow,and prevent recreational activities.

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