There are approximately 20 species described in the genus, separated from each other usually on the basis of habitat (freshwater versus marine), presence/length or absence of stipe, and average cell size. Boundaries among these species, and between species of Monosiga and those now placed in other genera of Codosigidae and Salpingoecidae, are uncertain and require investigation at the microscopical and molecular levels.
See also the Classification page.
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