Scale Bar = 10 micrometers.

This species has clavate valves which combined with the presence of stigmata gives it an appearance similar to Gomphonema. This makes sense given that they are both benthic biraphid diatoms that attach themselves to their substrate using a mucilage stalk. Didymo is easily distinguished by its prodigious size, frequently >100 micrometers in length and >35 micrometers in width. Striae are conspicuously punctate and radiate along the length of the valve becoming parallel at the apices. This diatom displays several stigmata on one side of the central area. The specimens from this sample displayed 3-4 per valve. This species is asymmetrical along the transapical axis with a headpole and footpole. At the bottom of the footpole is an apical porefield from which a mucilage stalk is secreted. The distal ends of the raphe deflect in the opposite direction of the side displaying the stigmata.

This species is well studied and notorious for its potential to be an invasive species forming monocultures in cold fast flowing water.