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National Invasive Species Information Center
U.S. Department of Agriculture

Didymo

Scientific Name

Didymosphenia geminata (Lyngb.) M. Schmidt (ITIS)

Common Name

Didymo, Rock snot

Synonym

Gomphonema geminata (Lyngb.) C. Agardh (ITIS)

Native To

Northern Europe and northern North America (Spaulding and Elwell 2007)

Date of U.S. Introduction

Was present in Canada in the late 1800s, but did not begin to cause problems until the early 1990s. It was first discovered east of the Mississippi River in 2005 in Tennessee. (Bergey et al. 2009; Spaulding and Elwell 2007)

Means of Introduction

Exact pathway unknown, but it spreads easily through contaminated fishing gear, particularly felt-soled waders (Kilroy and Unwin 2011)

Impact

Alters stream ecology by forming dense algal blooms that can cover up to 100 percent of stream bottoms (Spaulding and Elwell 2007)

Current U.S. Distribution

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

Rock snot (didymo)
Image use policy

Rock snot (didymo)

Credit

Photo by T. Thorney

Source
Invasive.org.

Find more images

Spotlights

  • Thick ‘rock snot’ continues to be found in Michigan rivers

    • May 26, 2022

    • Michigan State University Extension.

    • Anglers and paddlers are noticing a new phenomenon on the Upper Manistee River—a thick, brown snotty looking algae is clinging to rocks, downed trees, and anything else under the water. Didymo (Didymosphenia geminata), more often referred to as ‘rock snot’ due to its unappealing appearance, was reported in the Upper Manistee River last fall and it continues to plague popular fishing locations on the river. While non-toxic, didymo covers streambeds and reduces habitat for macroinvertebrates, an important food for fish such as trout.

      In addition to cleaning gear, one of the most important steps you can take is to report observations of didymo and any other suspected invasive species to conservation organizations. You can report to the Midwest Invasive Species Information Network (MISIN), a regional database used by invasive species specialists predominantly in Michigan and the Midwest. After creating an account, you can report your observation and biologists will be notified. Additionally, didymo can be reported by emailing the location and photos to EGLE-WRD-AIP@Michigan.gov. The United States Geological Survey also maintains a Nonindigenous Aquatic Species database that accepts reports from the public.

Distribution / Maps / Survey Status

Videos

All Resources

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Selected Resources

The section below contains highly relevant resources for this species, organized by source.

Council or Task Force

  • Fact Sheet: Didymo [PDF, 1.73 MB]

    • Feb 2018

    • Alberta Invasive Species Council (Canada).

    • See also: Fact Sheets for more information about individual invasive species, including those listed as "Prohibited Noxious" and "Noxious" under the Alberta Weed Control Act

Partnership
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Citations