An official website of the United States government.

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

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)

Rock snot (didymo)

Credit

Photo by T. Thorney

Find more images

Spotlights

  • Dreaded Didymo - or 'Rock Snot' - Found in Upper Manistee River, Michigan

    • Dec 6, 2021
    • Michigan Department of Natural Resources.

    • The Michigan departments of Environment, Great Lakes, and Energy and Natural Resources confirmed a report of didymo, a nuisance freshwater alga, in a stretch of the Upper Manistee River in Kalkaska County. Also known as rock snot despite its coarse, woolly texture, didymo can grow into thick mats that cover the river bottom. The Manistee River finding marks the first detection of didymo blooms in the Lower Peninsula. In 2015, extensive mats of didymo were found on the Michigan side of the St. Marys River near Sault Ste. Marie in the Upper Peninsula.

      If you observe didymo in the water, either as small, cotton ball-sized patches or thick blankets with rope-like strings that flow in currents, take photos, note the location and report it by using the Midwest Invasive Species Information Network, available online at MISIN.MSU.edu or as a downloadable smartphone app. The MISIN smartphone app will take a GPS location point if a report is made at the site; it also will allow you to upload photos with a report. Find more information on didymo and how to identify it at Michigan.gov/Invasives.

Videos

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
Federal Government
International Government
State and Local Government
Academic
Citations