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Starry Stonewort

Nitellopsis obtusa
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Starry stonewort is an invasive macroalgae native to northern Eurasia. It was introduced in North America through the St.Lawrence River in 1974 and was first identified in Ontario in 2009 in Presqu’ile Bay as well as Lake Simcoe. Ballast water is the suspected method of entry and subsequent spread through the Great Lakes basin.

Starry stonewort forms dense mats in waters two to 10 meters in depth. Its white bulbils (seed-like structures) are deposited in the mud, allowing the species to be easily spread and to survive Ontario’s cold winters.

In North America, only male individuals of starry stonewort have been recorded. They have no roots, making them easy to spread to new areas. The species is spread primarily by boating, but it is suspected that species like waterfowl can transport fragments to other unconnected water bodies as well.


In North America, starry stonewort is present in the St. Lawrence River, Lake Ontario, Lake Erie, Lake Huron, LakeSt. Clair, Detroit River system, Michigan’s Lower Peninsula, New York, Minnesota, Wisconsin, Pennsylvania, Vermont, and northern Indiana.

In Ontario, starry stonewort has been recorded all along the Trent-Severn Waterway, from Lake Ontario to Georgian Bay/Lake Huron, and several non-connected lakes and stormwater basins.

For an up to date distribution map of starry stonewort in Ontario, visit

Impacts of Starry Stonewort

  • Starry stonewort reduces biodiversity by forming dense mats (sometimes referred to as “pillows”) and competes aggressively with native plants.
  • Dense mats of starry stonewort can impede movement of fish, spawning activity, water flow, and recreational activities such as swimming, boating, and fishing.
  • Once introduced to a new area, starry stonewort can establish and spread very quickly.

How to Identify Starry Stonewort

  • It is a macroalgae that resembles true plants with thin stems and branchlets similar to thick fishing line.
  • Whorls of 4-6 branchlets coming off the main shoots, with blunt tips.
  • White, star-shaped bulbils, which give the species its name, are produced at the nodes, generally 3-6 mm (0.1 – 0.2 in) wide.
  • Can form dense mats up to 3 m (9.8 ft) thick.

What You Can Do

  • Learn how to identify starry stonewort and how to prevent accidentally spreading this species with your watercraft or fishing equipment.
  • Once established, it is almost impossible to eradicate. Avoid infested areas or reduce your speed when travelling near starry stonewort infestations. Your propeller can break off fragments and spread the bulbils to new areas. New populations can grow from small pieces of the species.
  • Inspect your boat, trailer, and equipment after each use. Remove all plants, animals, and mud before moving to a new waterbody.
  • Avoid planting starry stonewort in your aquarium or water garden. Aquarium hobbyists and water gardeners should only use native or non-invasive plants and are encouraged to ask retailers for plants that are not invasive.
  • Never release unwanted aquarium plants or pets. Return or donate unwanted plants to a garden centre or pet store, or put them in the garbage. Aquatic plants can be composted or mulched on land more than 30 m from the nearest waterbody.
  • If you find starry stonewort or another invasive species in the wild, please contact the toll-free Invading Species Hotline at 1-800-563-7711, visit EDDMapS, or search for the ‘Invasive Species in Ontario’ project on to report a sighting.


OFAH/OMNRF Invading Species Awareness Program. (2021). Starry Stonewort. Retrieved from:
This factsheet may be reproduced for non-commercial purposes.

Header photo by Jim Grazio, PA DEP

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