Didymo (Rock Snot)

Didymo thumbnail

(Didymosphenia geminata)
*Detected in Michigan*

Report this species:

Use the Midwest Invasive Species Information Network (MISIN) online reporting tool 

- Or - download the MISIN smartphone app and report from your phone - http://www.misin.msu.edu/tools/apps/#home



  • Looks and feels like white or tan/brown wet wool
  • Ranges from small cotton ball sized patches to thick blankets and long ropy strings that flow in currents
  • Although often referred to as “rock snot”, Didymo is not slimy

Didymo infestation


Didymo infestation

Habitat: Didymo thrives in low nutrient cold water systems. It is a microscopic single-celled freshwater diatom whose cells can form stalks that attach to hard substrates such as rocks. Under the right conditions, prolific growth patterns result in thick mats that can cover river and stream bottoms.

Native Range: Likely native to Lake Superior and parts of Canada.

Local Concern: 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.

Distribution: Didymo cells haves been documented in the Great Lakes Basin and Michigan waters in low abundance, but nuisance blooms have not been documented in Michigan until recently in the St. Marys River.

Means of Introduction/spread: Didymo can incidentally be transported on boats, anchors and fishing gear such as waders, felt soled boots and nets.

Didymo Invasive Species Alert (printable PDF)