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Didymo (Rock Snot)

Didymosphenia geminata

Identifying Characteristics

  • thick, gelatinous masses of algae that coat rocky-bottomed rivers
  • generally yellow/brown in colour
  • flowing tails can turn white at their ends and resemble tissue paper
  • feels like wet cotton wool
This is a photograph of Didymo in Little Qualicum River, BC. 3 images
This is a map of British Columbia showing the spread of invasive species.
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A large algae bloom that carpets riverbeds.

Didymo is a species of algae that, when overgrown, spreads in rivers, streams or lakes as a large, thick, slimy mat coating the underwater rocks. The Didymo masses are yellowish-brown and gelatinous, thus the nickname Rock Snot. The alga thrives in clear, warm, shallow and nutrient-poor flowing water. Didymo spreads quickly and the alga can be dispersed in a single drop of water. Didymo grows with the sun, appearing at the start of the warm summer weather and dying off as the sunlight decreases at the end of the season. Its growth is unpredictable and a river may be covered in Didymo one year and not the next.

Impact on Communities and Native Species

Didymo, although unattractive, is not toxic to humans. For fish, however, Didymo can be a problem: it can clog or irritate gills, push fish from their natural habitat, and restrict water flow, putting eggs and fry in gravel at risk. Decomposing algal mats may also affect the oxygen level of the water, thus harming fish.

Invasion History

Didymo first appeared in British Columbia in 1989 in central Vancouver Island rivers and quickly spread across the province to the Bulkley, South Thompson, Kettle, Columbia and Kootenay Rivers. Didymo may have been transported by felt-soled waders, which were introduced to the fishing market at the same time as the algae spread. Didymo can survive up to 30 days in these wet felt shoes and may have unwittingly been transported from river to river by fishermen. Migrating birds, other animals, boats and gear could also have spread the plant.