Starry stonewort (Nitellopsis obtusa)

Synonyms: Chara obtusa, Nitellopsis stelligera

Description:

Starry stonewort (Nitellopsis obtusa) is an underwater green alga from Europe and Asia that looks like a grassy plant but is actually a large alga (a charophyte) [1][2]. It reached the Great Lakes ? likely in ballast water ? and has since spread to inland lakes [1][2]. MISIN tracks it because it forms dense underwater mats that crowd out native plants and degrade fish spawning and feeding habitat, and it is very hard to remove once established [1][2].

Habit:

Starry stonewort is a bright to translucent green, grass-like alga that grows up from the lake bottom in whorls ? rings of several long, smooth branchlets (usually about 5 to 8) spaced widely along a central stalk, giving it an open, airy look [2][3]. It is anchored not by true roots but by colorless thread-like rhizoids [1][2]. The surest clue is the small, white, star-shaped bulbils (about 2?5 mm, five-pointed) found on the rhizoids and lower plant ? no native look-alike has these [2]. Unlike the native muskgrasses it resembles, starry stonewort usually lacks a gritty lime crust and the skunky, musky smell [2].

It is easiest to find in summer and early fall, when it grows tall and forms dense mats that can reach the surface, and when the white star-shaped bulbils are present [1][2]. Telling it from native stoneworts can be tricky, so a sample and an expert help (see Similar Species) [2].

At-a-glance cues:

1. Bright, translucent green, grass-like alga with whorls of long, smooth branchlets and wide gaps between whorls [2][3]
2. Small white, star-shaped bulbils (about 2?5 mm) on the lower plant and rhizoids ? the key feature [2]
3. Anchored by colorless thread-like rhizoids, not true roots [1][2]
4. Usually lacks the gritty lime crust and musky smell of native muskgrasses [2]

Leaves:

Starry stonewort is an alga and has no true leaves; the structures below are its leaf-like branchlets.

1. Arrangement: in whorls (rings) of usually 5?8 branchlets at each node along the central stalk [2][3]
2. Form: long, soft, smooth, mostly unbranched branchlets, with small bracts that can give an uneven, forked look [2]
3. Spacing: widely spaced whorls (large gaps between nodes), giving the alga an open, airy appearance [3]
4. Color/texture: bright to translucent green and soft ? not gritty or lime-crusted like native muskgrasses [2]

Stems:

An alga has no true stem; the structure below is its central axis (thallus) and anchoring rhizoids.

1. Habit: a rooted, submersed green macroalga growing up from the lake bottom, sometimes tall enough to reach the surface and form dense mats [1][2]
2. Axis: a slender, translucent green central stalk bearing the whorls of branchlets; smooth, without the lime crust or musky smell of native muskgrasses [2]
3. Anchoring: held to the bottom by colorless, thread-like rhizoids (not true roots) [1][2]

Flowers:

Starry stonewort is an alga and does not flower; the structures below are its reproductive organs.

1. Sexes: separate male and female plants (dioecious) ? but in North America only male plants have been found [2]
2. Male organs: small orange reproductive structures (antheridia) borne on the branchlets [2]
3. Female organs: egg-bearing structures (oogonia) are rare or absent in North America, so fertile sexual reproduction generally does not occur here [2]
4. No petals, sepals, or true flowers of any kind [2]

Fruit and seeds:

An alga produces no fruit or seeds; the structures below are its bulbils and spores.

1. Bulbils: distinctive white, star-shaped (five-pointed) bulbils about 2?5 mm across, borne on the rhizoids and lower plant ? the diagnostic feature and the main means of spread [2]
2. Oospores: orange, seed-like sexual spores can form where both sexes occur, but since only males are known in North America these are generally not produced here [2]
3. Dispersal: bulbils and broken fragments cling to boats, gear, and wildlife and start new infestations; bulbils also build a long-lasting bank in the sediment [1][2]

Habitat:

Starry stonewort grows rooted to the bottom of still or slow-moving fresh water ? lakes, ponds, and quiet rivers ? and can grow in fairly deep water, often forming tall mats that reach toward the surface [1][2]. It tolerates a range of conditions and can dominate both shallow and deeper areas of a lake [2].

The alga is native to Europe and Asia, where it is actually in decline in some regions, but in North America it is invasive [1][2]. It was first found on this continent in the St. Lawrence River in Ontario in 1978, very likely arriving in ship ballast water, and has since spread through the Great Lakes basin and into many inland lakes [1][2]. It is not native anywhere on this continent [1].

Reproduction:

Starry stonewort spreads by pieces of itself and by its tiny star-shaped bulbils ? not by seed. Broken fragments and the small white bulbils cling to boats, trailers, motors, anchors, and fishing gear and can also catch on the fur and feathers of animals, carrying the alga from lake to lake [1][2]. In North America only male plants have been found, so it does not make fertile seed-like spores here and reproduces almost entirely by fragments and bulbils [2].

The bulbils are the heart of the problem: they collect in the bottom sediment and form a long-lasting "bank" that can resprout the alga even after the visible growth is removed, so a cleared lake bed can grow back [2]. This is why preventing spread between waters is the main defense and why control is so difficult ? removal must avoid scattering fragments and bulbils, and a site needs watching for years (see Management) [2].

Impact and Damage:

Starry stonewort is not a danger to people, but it is a serious nuisance and ecological problem in lakes. It grows into dense, tall, single-species mats that crowd out native underwater plants and lower the variety of plant life a lake supports [1][2]. The thick mats also tangle boat propellers and make swimming and fishing difficult, lowering a lake's recreational value [2].

The mats are especially hard on fish. By blanketing the bottom and filling the water column, starry stonewort can degrade the open, plant-and-bottom habitat that many fish need for spawning and feeding [1][2]. Because it can reshape whole areas of a lake bottom this way and is extremely difficult to remove once its bulbil bank is established, it is considered an aggressive "ecosystem engineer" and a high-priority aquatic invader [1][2].

Similar species:

Starry stonewort is easily confused with native stoneworts and some underwater plants; the white star-shaped bulbils are the best way to tell it apart.

Muskgrasses (Chara spp.) are native stoneworts that look similar, but they feel gritty and lime-crusted, give off a skunky, musky smell, and lack the white star-shaped bulbils of starry stonewort.

Nitella stoneworts (Nitella spp.) are native algae with repeatedly forked branchlets and no lime crust, but they too lack the star-shaped bulbils and the long, widely spaced branchlets of starry stonewort.

Coontail (Ceratophyllum demersum) is a native flowering underwater plant with whorled, forked leaves that can look superficially similar, but it is a true plant with firmer, toothed leaf segments rather than the soft, smooth branchlets of an alga.

Monitoring and rapid response:

Signs to watch for:

1. Bright, translucent green, grass-like alga in whorls of long branchlets, forming dense underwater mats in lakes [2][3]
2. Small white, star-shaped bulbils on the lower plant and rhizoids [2]
3. Fragments fouling boat propellers, trailers, and gear pulled from a lake [2]

What you can do:

1. Clean, Drain, and Dry every time: before leaving a launch, remove all plant material ? and the tiny white bulbils ? from your boat, trailer, motor, anchor, and gear, and drain all water, since fragments and bulbils start new infestations [1][2].
2. Never move water, plants, or bait from one lake to another, and never dump aquarium or water-garden contents into a waterway [2].
3. For a small patch at your own shoreline, remove the alga carefully and collect every fragment and bulbil you can ? but be aware that bulbils left in the sediment can regrow, so this rarely eradicates it [2].
4. Natural area crews and lake associations should treat starry stonewort as a difficult, long-term problem, coordinate with the state aquatic invasive species program, avoid removal methods that scatter fragments, and monitor for years because of the bulbil bank [2].
5. Report sightings (below), since starry stonewort is easily confused with native stoneworts ? a sample and an expert help confirm it [1][2].

When to call a professional:

1. Established mats or a whole-lake infestation ? starry stonewort is one of the hardest aquatic invaders to control, so contact your state aquatic invasive species program or a lake-management professional for a coordinated plan [1][2].
2. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. U.S. Geological Survey, Nonindigenous Aquatic Species Database. Nitellopsis obtusa (Desvaux) J. Groves ? Starry Stonewort (Species Profile). nas.er.usgs.gov
2. Minnesota Department of Natural Resources. Starry Stonewort (Nitellopsis obtusa). dnr.state.mn.us
3. Wisconsin Department of Natural Resources. Starry Stonewort (Nitellopsis obtusa). dnr.wisconsin.gov

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