Didymo is a type of algae that forms dense mats at the bottom of freshwater streams. It is an invasive species.
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.
Didymo prefers clear, cold freshwater streams. It attaches to rocks, plants and other submerged surfaces by stalks that the algae cells secrete. The 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.
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.