starry stonewort

Starry stonewort is an invasive green alga that can grow tall and dense, forming mats on the surface that interfere with recreation and potentially displacing native plant species. 

starry stonewort

What it is 

The scientific name for starry stonewort is Nitellopsis obtusa. It is a type of freshwater green algae known as a charophyte, a group that also includes muskgrasses and stoneworts (Chara and Nitella species) that are native to Minnesota. Starry stonewort is native to parts of Europe and Asia. It is relatively uncommon in much of its native range and is considered endangered in Japan and of conservation concern in Britain.

Life cycle

Starry stonewort is dioecious, meaning that individuals are either male or female — unlike many plants and algae that have both male and female reproductive parts. Interestingly, the best evidence to date indicates that the populations in the U.S. are all male, though there may be undiscovered females. This means that spread of starry stonewort is probably through human movement of fragments from lake to lake. In particular, starry stonewort produces small, star-shaped structures called “bulbils” that allow it to reproduce vegetatively (clonally).

What it affects

Where starry stonewort grows densely and forms surface mats, it can interfere with boating and other recreational activities. Dense growth may also displace native plants and could potentially have impacts on fish and other animals. Starry stonewort’s ecological impacts are not well understood, and there has been little published research to date.

What it looks like

Starry stonewort can look quite similar to some native charophytes, but may appear larger and more robust. It is a green macroalga with whorls of long, narrow branchlets in groups of 4 to 6 coming off of main shoots. Orange reproductive structures (male antheridia) occur at branchlet nodes. Small, white, star-shaped bulbils are a distinguishing feature that gives it the name starry stonewort.

Where it's found

Starry stonewort was first found in North America in 1978 in the St. Lawrence River and has spread inland since. It is now found in much of Michigan’s Lower Peninsula and many locations in New York State. It has also been found in Indiana and was discovered in southeastern Wisconsin in 2014. It was first recorded in Minnesota in 2015 (Lake Koronis, Stearns Co.).

How it spreads

Starry stonewort appears to be spreading vegetatively in the U.S. (by bulbils and fragments). Accidental movement by people is the most likely means of dispersal. Many of the known infestations occur in high-use waterbodies and near boat accesses.