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You are here: Home / Aquatic Species / Species Profiles / Didymo
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Rock snot (didymo)

Scientific name: Didymosphenia geminata (Lyngb.) M. Schmidt (ITIS)

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

Common names: Didymo, Rock snot

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Selected Resources

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.



Selected Resources:


Invasive Species Compendium - Didymosphenia geminata
CAB International.

New York Invasive Species Information - Rock Snot, Didymo
New York Invasive Species Clearinghouse.

NEANS Panel Online Guide - Didymo (a.k.a. rock snot)
Northeast Aquatic Nuisance Species Panel.

Global Invasive Species Database - Didymosphenia geminata (algae)
IUCN. Species Survival Commission. Invasive Species Specialist Group.

Federal Government

Yosemite National Park - Invasive Species: Didymo a.k.a. "Rock Snot" (Mar 2017; PDF | 192 KB)
DOI. National Park Service.
See also: Environmental Factors - Invasive Species for more information

International Government

Biodiversity and Pest Control - Didymo
Otago Regional Council (New Zealand).

State Government

Aquatic Invasive Species: Rock Snot (Didymo/Cymbella) Risk Analysis for Arizona (Jul 2010; PDF | 178 KB)
Arizona Game and Fish Department.
See also: Aquatic Invasive Species for additional risk analyses and related species information

New Aquatic Nuisance Species Threatens Maine Streams and Rivers: Didymo or "Rock Snot"
Maine Department of Environmental Protection.

Nuisance & Invasive Species - Didymo (Rock Snot)
New York Department of Environmental Conservation.

Invasive Plants in Pennsylvania: Didymo (PDF | 153 KB)
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

AIS Action Plan: Didymo (Jun 2011; PDF | 193 KB)
Pennsylvania Fish and Boat Commission.
See also: Aquatic Invasive Species for more action plans

Didymo (Invasive Freshwater Algae) in Virginia
Virginia Department of Game and Inland Fisheries.

Didymo (Rock Snot) Fact Sheet
West Virginia Department of Natural Resources.


Didymo (or Rock Snot)
University of California - Riverside. Center for Invasive Species Research.

How Didymo Became Rock Snot (Dec 2009)
University of Minnesota. Minnesota Sea Grant. Seiche Newsletter.

Factsheet: Didymo
Pennsylvania State University. Pennsylvania Sea Grant.
See also: Aquatic Invasive Species: Resources for additional species information


Bergey, E.A., J.T. Cooper, and C.R. Tackett. 2009. Occurrence of the invasive diatom Didymosphenia geminata in southeast Oklahoma. Publications of the Oklahoma Biological Survey, 2nd Series 9:13-15.

Integrated Taxonomic Information System. Didymosphenia geminata. [Accessed Sep 10, 2014].

Kilroy, C. and M. Unwin. 2011. The arrival and spread of the bloom-forming, freshwater diatom, Didymosphenia geminata, in New Zealand. Aquatic Invasions 6(3):249-262.

Spaulding, S.A. and L. Elwell. 2007. Increase in nuisance blooms and geographic expansion of the freshwater diatom Didymosphenia geminata (PDF | 1.08 MB) (Open-File Report 2007-1425) Reston, Va.: U.S. Geological Survey.

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Last Modified: Sep 03, 2018
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