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Didymo growing on rock

Didymo or Rock Snot

Fact Sheet
Scientific Name (Didymosphenia geminata)
Classification and Permitting Level 1 - Prohibited Aquatic Nuisance Species
Species Description Didymo is a microscopic algae (known as a diatom) that appears as a mat of brown or tan material. It has earned the nickname "rock snot" from its appearance, though actual texture is like wet wool rather than being slick.
Preferred Habitat Didymo is typically found forming mats on rocky surfaces in cool, clear streams. Often these streams are poor in nutrients and are considered "pristine" or relatively undisturbed.
Reason for ANS Designation This diatom forms thick mats on rocks that can alter the available habitat for invertebrates and the trout that rely on them. Once established, there is no known control.
Place of Origin Didymo is thought to be native to at least parts of North America. It can easily be spread on equipment that comes in contact with some of the microscopic organisms or in water. Felt-soled waders have traditionally been thought to spread this diatom.
Stopping the Spread If wading in streams, avoid felt-soled waders. Inspect for and remove any vegetation, mud, or plants from equipment before leaving a stream or river. Allow the equipment to dry completely, rinse with 140°F water for 10 seconds of contact, or disinfect using an approved method such as a .02% bleach solution for 10 minutes of contact time or freezing for 72 hours.
Additional Information New York Department of Environmental Conservation Fact Sheet