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Invasive Species: Didymo (Rock Snot)

Didymo (Rock Snot)

(Didymosphenia geminata)
*Detected in Michigan*


  • Didymo is a microscopic algae (diatom) that produces stalks that form thick mats on hard surfaces like rocks in stream beds. 
  • Looks and feels like white or brown wet wool.
  • Ranges from small, cotton ball-sized patches to thick blankets and long, rope-like strings that flow in currents.
  • Although often referred to as “rock snot,” didymo is not slimy.

Didymo infestation

 Didymo is thick and fibrous and may be difficult to pull apart or remove from rocks.

Looking through water at the bed of a river where rocks and stones are covered with didymo. 

Didymo mats can cover rocks and substrate in rivers and streams.


If possible, please take one or more photos of the invasive species you are reporting. Also make note of the location, date and time of the observation. This will aid in verification of your report. You may be asked to provide your name and contact information if follow-up is needed.


Habitat: Didymo thrives in low-nutrient cold water rivers and streams. Unlike blue-green algae, didymo’s presence does not indicate a decline in water quality. Researchers are still working to determine what triggers didymo’s nuisance blooms.

Native Range: Far northern regions of Europe, Asia and North America. Didymo is likely native to Lake Superior and parts of Canada.

U.S. Distribution: Scattered populations exist throughout the United States, including New England, the Mid-Atlantic Region, and the Western U.S.

Michigan Distribution: Didymo cells haves been documented in the Great Lakes Basin and Michigan waters in low abundance. Nuisance blooms have been documented in Michigan in the Boardman, Upper Manistee and St. Marys rivers. Didymo may be present but undetected in other rivers or streams. Follow the Clean, Drain, Dry instructions below after every visit to a water resource.

Local Concern: Under the right conditions, prolific growth patterns result in thick mats that can cover river and stream bottoms. Didymo mats alter habitat and food sources for fish and can make recreation difficult or unpleasant. Mats can also foul water intakes and fishing gear as well as impact fishing access and wading.

Means of Introduction or Spread: Didymo can be transported on boats, anchors and fishing gear such as waders, felt-soled boots and nets.

What you can do:

  • CLEAN boats and gear with HOT (140° F) water or chemical disinfectants and remove any visible plants, mud and aquatic life before transporting.
  • DRAIN the water from all equipment before transporting elsewhere.
  • DRY all your gear for at least 5 days before going to a new waterbody.

These methods work for didymo, New Zealand mudsnails and most other aquatic invasive species.

Helpful Tips:

  • Avoid visiting multiple rivers in a single day.
  • Plan time to decontaminate between trips.
  • Designate specific gear, especially porous items like felt soled waders, for use only in infested waters.

Control: Currently, there is no effective method to eradicate didymo from the environment.

What’s being done:

  • Since 2015, the Michigan Invasive Species Grant Program has supported researchers at Lake Superior State University’s Center for Freshwater Research and Education in an extensive study of occurrences of didymo in the St. Marys River and Upper Peninsula waters, the risk of spread and why nuisance blooms are increasing - a phenomenon being observed worldwide.
  • In spring 2022, the Michigan Department of Environment, Great Lakes, and Energy conducted targeted samplings high-use access sites on the Manistee, Little Manistee, Au Sable (Main, North, South, and East branches), Pine, Betsie, Platte, Boardman, Muskegon, Black, Pigeon, Pere Marquette,  Baldwin, and Rogue rivers and found no additional infested areas.
  • New signs are being installed at access sites on trout streams across Northern Michigan. If you see an access site in this region without a sign, contact and provide the location.
  • A recording of the June 9, 2022 NotMISpecies webinar, Didymo: What you need to know, is available at

A yellow and red sign on a wood post near a boat launch warning anglers about didymo and New Zealand mudsnails

Boating access signs notify boaters and anglers of best practices to prevent the spread of didymo and New Zealand mudsnail.


Didymo Invasive Species Alert (printable PDF)

Didymo & New Zealand mudsnail rack card (printable PDF)

Fly-anglers and Wader Users: Felt-soled Wader Issues and Alternatives

Didymo Control: Increasing the Effectiveness of Decontamination Strategies and Reducing Spread