Azolla (common called mosquito fern, water fern, and fairy moss) is a genus of seven species of aquatic ferns in the family Salviniaceae. They are extremely reduced in form and specialized, having a significantly different appearance to other ferns and more resembling some mosses or even duckweeds. Azolla filiculoides is one of two fern species for which a reference genome has been published. It is believed that this genus grew so prolifically during the Eocene (and thus absorbed such a large amount of carbon) that it triggered a global cooling event that has lasted to the present.

Azolla may establish as an invasive plant in areas where it is not native. In such a situation, it can alter aquatic ecosystems and biodiversity substantially by exhausting oxygen and covering water surface making underwater plants unable to photosynthesise.

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