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Invasive Species: Didymo (Rock Snot)

An open hand, extended over a riverbed, holding a clump of didymo.

Invasive Species: Didymo (Rock Snot)

Detected in Michigan

This species has been detected in Michigan.

What is didymo?

(Didymosphenia geminata)

Didymo (also known as "rock snot") is a microscopic algae (diatom) that produces stalks that form thick mats on hard surfaces like rocks in stream beds. It looks and feels like white or tan/brown wet wool. It ranges from small cotton ball-sized patches to thick blankets and long ropy strings that flow in currents. Although it is often referred to as "rock snot," didymo is not slimy.


What to look for

  • 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 photos

The following images can help identify didymo. Click on each photo for descriptions.


If you see suspect didymo, report it through the Eyes in the Field online reporting system. Note the location of the suspect didymo and be sure to include at least one photo of the algae to help in verification.

Species info


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: 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. 

In Michigan: 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.

How it spreads

Didymo cells can be transported on boats, anchors and fishing gear such as waders, felt-soled boots and nets.


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

What you can do:

Always Clean, Drain, and Dry your waders, boots, boats and other gear between trips or before moving to a new body of water. Take extra precaution in areas with known or suspected didymo or New Zealand mudsnail infestations. In addition to removing debris and mud, the State of Michigan recommends using a chemical disinfectant to achieve maximum decontamination for didymo and New Zealand mudsnail. Possible disinfectants with documented effectiveness for these species include:

  • Products such as Formula 409® Antibacterial All-Purpose Cleaner applied to waders and gear.
  • Bleach: Apply a solution of  ½ cup (4 fluid ounces) bleach to 5 gallons of water and let stand for 20 minutes.
  • Virkon Aquatic: Apply a solution of 20 grams per liter of water and let stand for 20 minutes (see manufacturer’s label for additional guidance).

Any chemical disinfectants should be applied to waders and gear on land, at a reasonable distance from the water, to avoid accidental discharge into surface waters. 

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.

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.