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Didymo

Didymosphenia geminata · Algae

Didymo: Underwater

Underwater · Photo: Steven Bodzin / CC BY

NativeBenthic algae (on rocks and the bottom)Diatom

Also called Didymosphenia, Didymosphenia geminata, Rock snot

How to identify Didymo

  • Thick tan, beige or white mats on stream rocks
  • Feels like wet wool or cotton, not slimy
  • Strands look like wet toilet paper when out of the water
  • Mats can be inches thick and cover large areas of streambed

Similar species

Cladophora is green and coarse; Hydrurus is slimy and smells foul; benthic diatom films are thin and slick.

Growth profile

SeasonMats build through late winter and spring and can persist into summer, while the alga survives year round on rocks.
StartsCold water with low phosphorus
LightBright light; clear water
DepthAttached to rocks on the streambed
How it spreads or reproducesCells carried in water and on wet waders, boots and gear
HabitatCold, clear rivers and streams with stable flows and low phosphorus, especially below dams.

Why it matters in a lake

Native to the region but heavy mats form in some cold, low nutrient waters. Heavy mats can smother streambed insects and spread on wet fishing gear.

Where it is found

  • Idaho: Occasional
  • Oregon: Occasional
  • Wyoming: Occasional

Classification

KingdomChromista
PhylumBacillariophyta
ClassBacillariophyceae
OrderCymbellales
Scientific nameDidymosphenia geminata

More benthic algae

Frog spawn algaHydrurusLemaneaCompsopogonRed crust algaSpirogyra

Browse all algae in the Species Database

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