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Alaska Department of Fish and Game

Invasive Species — Didymo (Didymosphenia geminata)
Biological Characteristics


Didymosphenia geminata Didymosphenia geminata (D. geminata or Didymo) sometimes commonly called rock snot because of its appearance, is a freshwater alga that can form massive blooms on the bottom of streams, and rivers, yet is rarely found in lakes. It strongly attaches to river stones, plants and other substrate. D. geminata forms thick mats with flowing tails that turn white at the ends.

Distinguishing characteristics are the color, feel, and strength of the alga, as well as its growth patterns. Didymo may be beige, brown, or white but is never green. Although it appears slimy, the feel is more like wet cotton balls, spongy and somewhat scratchy. Unlike some algae, it does not have a distinctive odor. When rubbed between the fingers it does not break up or fall apart easily, nor does it easily degrade.

Person holding a rock covered in Didymosphenia geminataPerson holding a rock covered in Didymosphenia geminata


Didymo is a species of freshwater diatom, which is a type of single-celled algae. Diatoms are unique for their silica cell walls, and for being found in nearly every freshwater and marine habitat. Individual D. geminata cells cannot be seen with the naked eye.

Similar to other diatoms, D. geminata populations primarily grow by vegetative cell division, though like most diatoms, it also undergo sexual reproduction at some stage. The extensive mats are the result of a stalk secreted by individual cells. The stalk attaches to rocks, plants and other submerged substrate. When the cell undergoes vegetative reproduction, the stalk also divides. The massive production of extracellular stalk is responsible for extensive mats. Nuisance blooms consisting of thick mats resembling wet tissue paper, fiberglass insulation or “rock snot” can cover 30% – 100% of the river bottom.

Importance: Diatoms are considered primary producers because they are part of the foundation of all food webs. They contribute to the global carbon cycle through photosynthesis and serve as a valuable food source.

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