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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 (Didymosphenia geminata) is a species of diatom, a type of algae, that occasionally suddenly becomes very abundant in rivers in many parts of the world. It forms large, thick, slimy mats on underwater rocks. It is yellowish-brown and gelatinous, thus the nickname ‘Rock Snot’. 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 decrease the oxygen level of the water, thus harming fish.

"Invasion" History - Didymo isn’t actually an "alien" invader!

Our ideas about the invasive nature of this alga have changed. Initially it was thought that Didymo was an invasive species that had first appeared in British Columbia in 1989 in central Vancouver Island rivers. Then it was detected in the Bulkley, South Thompson, Kettle, Columbia and Kootenay Rivers. Blooms were noted in the rivers of many other countries as well. Was it spreading and how? Some investigators thought that maybe it was transported by felt-soled waders, introduced to the fishing market at the same time. 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. More recently, closer examination of historical records indicate that it has been present on Vancouver Island since as early as 1894. Thus although it does on occasion form ‘blooms’, it is in fact a native species that occasionally becomes very abundant. Similarly, in other parts of the world where it occurs, closer examination of historical and fossil records indicates that it has been present in those areas for many decades if not longer. Laboratory experiments and observations in the field have yielded the surprising result that low concentrations of phosphorus in the water stimulate its growth. One likely explanation, is that the conditions, some perhaps linked to changes in the environment, that result in low concentrations of phosphorus in streams, have become more common in recent years.