Quick Facts

Species Type



Prefers clear, cold freshwater streams. Attaches to rocks, plants and other submerged surfaces by stalks that the algae cells secrete. Stalks weave together to form dense mats. Populations appear to peak in winter.


Found in Maryland’s Gunpowder Falls between Prettyboy and Loch Raven reservoirs in Baltimore County in 2008. Discovered in the Savage River below Savage River Reservoir in 2009. Native to Scotland and extreme northern Europe and Asia.

Conservation Status



Didymo can be white, yellow or light brown and looks slimy, but actually has a rough texture similar to wet wool. It forms thick, heavy mats and can be tough to pull apart or remove from rocks.

Reproduction and Life Cycle

Didymo reproduces asexually when cells divide. As the algae cells divide, the stalks divide as well, forming a mass of stalks. Thick mats of didymo can remain for up to two months after the cells die.

Did You Know?

  • Didymo is also known as rock snot. It is a type of single-celled algae called a diatom.
  • It is not known to pose a health risk to humans.
  • Anglers can help prevent didymo from spreading by replacing felt-soled waders with rubber-soled ones. Also, always thoroughly scrub, wash and dry waders and other fishing equipment after use.
  • You can report didymo sightings by calling the Maryland Department of Natural Resources at (410) 260-8287.

Sources and Additional Information