did·​y·​mo ˈdi-də-ˌmō How to pronounce didymo (audio)
: a freshwater, microscopic diatom (Didymosphenia geminata) typically of cool, nutrient-poor waters of Canada and the northern U.S. and Europe that has become invasive in warmer waters where it forms large, thick brown, yellow, or whitish mats attached to a submerged substrate (such as a rock or plant)
All visitors, especially boaters and fishermen, are being encouraged to wash their gear in an effort to avoid spreading didymo.Sean Sauro
… the unpleasant and unwanted freshwater algae, Didymophenia geminata, commonly called didymo, or, with absolutely no trace of affection, rock snot.James Gorman

called also rock snot

Examples of didymo in a Sentence

Recent Examples on the Web Find more information on didymo and how to identify it at michigan.gov/invasives. Keith Matheny, Detroit Free Press, 7 Dec. 2021 Extensive mats of didymo were found on the Michigan side of the St. Marys River near Sault Ste. Keith Matheny, Detroit Free Press, 7 Dec. 2021 According to Michigan State University Extension, didymo is thought to be native to Lake Superior, parts of Canada and Northern Europe. Keith Matheny, Detroit Free Press, 7 Dec. 2021 Mainly because there are so many four-legged and two-legged critters there that like to eat them. Anglers can be their own worst enemies by inadvertently spreading waterborne nuisances such as zebra mussels and didymo with their boats. Colin Moore, Outdoor Life, 7 Feb. 2020

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'didymo.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History


short for New Latin Didymosphenia, genus name (earlier, name for a subgenus of Gomphonema), from Greek dídymos "double," (in plural) "twins" + -o- -o- + sphḗn "wedge" (of obscure origin) + -ia -ia entry 1; so named for the symmetrical wedge-like nodes at each end of the raphe — more at didymium

First Known Use

2005, in the meaning defined above

Time Traveler
The first known use of didymo was in 2005

Dictionary Entries Near didymo

Cite this Entry

“Didymo.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/didymo. Accessed 21 Feb. 2024.

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