Didymo (Rock Snot)

Didymo thumbnail

(Didymosphenia geminata)
*Detected in Michigan*


WHAT TO LOOK FOR

  • Didymo is a microscopic alga (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.

Didymo infestation

Didymo mats can grow to 12" deep with stalks reaching 3' in length.

REPORT DIDYMO

ABOUT THIS SPECIES

Habitat: Didymo thrives in low-nutrient cold water rivers and streams. 

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 St. Marys River.

Michigan Status:  Didymo cells haves been documented in the Great Lakes Basin and Michigan waters in low abundance. Nuisance blooms have been documented in the St. Marys River and more recently in the Upper Manistee River.

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.

MORE INFORMATION

Didymo Invasive Species Alert (printable PDF)