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Spiny Water Flea

(Bythotrephes cederstroemi)

Photo of spiny water flea
Photo credit: USFWS

Spiny water fleas entered the Great Lakes in ship ballast water from Europe, arriving in the 1980s. With translucent bodies, only about ¼ to ½ inches in length, individual water fleas may go unnoticed. However, the species tend to gather in masses on fishing lines and downrigger cables, so anglers may be the first to discover a new infestation.


Other names for this animal include:
  • Common names: Spiny Water Flea
  • Scientific names: Bythotrephes cederstroemi
Ecological threat:
  • Spiny and fishhook water fleas are predators - they eat smaller zooplankton (planktonic animals), including Daphnia. This puts them in direct competition with juvenile fish for food.
  • Young fish have trouble eating these water fleas due to their long, spiny tails.
  • The spiny and fishhook water fleas produce rapidly through parthenogenesis, commonly known as asexual reproduction, which means that no males are required and populations can explode in number.
 Overview map of prohibited classification in WI
Prohibited (red) counties

Classification in Wisconsin: Prohibited

Species Assessment Groups (SAG) were assembled to recommend a legal classification for each species considered for NR 40. The recommendation for Spiny Water Fleas was based upon this literature review [PDF] developed by the department.


Body: The spiny water flea has a 1/4"-1/2" long, translucent body, making it hard to spot unless gathered in a large cluster. The species is also characterized by a long spine that extends from its abdomen, giving the reason for its name. In addition, the spiny water flea has a dark black eye that can easily be seen against its contrasting light body.


Spiny water fleas were found in the Gile Flowage (Iron County) in 2003, Stormy Lake (Vilas County) in 2007, and the Madison Chain of Lakes (Lake Mendota, Lake Monona, Lake Waubesa, and Lake Kegonsa in Dane County) in 2009. See all known locations of spiny water flea in Wisconsin

Do you know of additional populations? Send us a report.


Unfortunately, at this time no effective strategy is available to control the spiny water fleas once they are introduced to lakes.


Links for more information: