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Rock snot (didymo)
Scientific Name:

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


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

Common Name:

Didymo, Rock snot

Rock snot (didymo) - Photo by T. Thorney
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)


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.



Selected Resources

The section below contains highly relevant resources for this species, organized by source. Or, to display all related content view all resources for Didymo.


IUCN. Species Survival Commission. Invasive Species Specialist Group.
Northeast Aquatic Nuisance Species Panel.

New York Invasive Species Clearinghouse.

Federal Government

DOCNOAA. Great Lakes Environmental Research Laboratory; DOIUSGS. Wetland and Aquatic Research Center.

International Government

Otago Regional Council (New Zealand).

State and Local Government

Pennsylvania Fish and Boat Commission.

See also: Aquatic Invasive Species for more action plans

Virginia Department of Game and Inland Fisheries.
West Virginia Department of Natural Resources.
Pennsylvania Department of Conservation and Natural Resources.
See also: Invasive Plant Fact Sheets for plant species (trees, shrubs, vines, herbs and aquatic plants) that have impacted the state's natural lands

Maryland Department of Natural Resources.

New York Department of Environmental Conservation.


University of California - Riverside. Center for Invasive Species Research.

Pennsylvania State University. Pennsylvania Sea Grant.
See also: Aquatic Invasive Species: Resources for additional species information
University of Minnesota. Minnesota Sea Grant. Seiche Newsletter.