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Prokaryotes and eukaryotes are unicellular, made of one cell. Great. Eukaryotes are unicellular or multicellular. But the typical examples of multicellular eukaryotes we have are made of, often, trillions of cells, like us humans. Ants must still be made of many millions of cells. Are there known eukaryotes with very few cells that make them up? Like, 5, or something? Or maybe a dozen cells making up the whole organism in its fully developed state?

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  • $\begingroup$ Well, there are multicellular prokaryotes too but they are not obligately multicellular. You may find the case of Caulobacter cresentus interesting. You can describe it as a bicellular bacteria however it exists as a single cell in one of the stages of its life cycle. $\endgroup$ – WYSIWYG Nov 17 '20 at 12:06
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    $\begingroup$ Does this answer your question? Are there oligocellular organisms in nature and, if so, what are they like? $\endgroup$ – Maximilian Press Nov 17 '20 at 19:34
  • $\begingroup$ It seems like there's overlap, but they are not the same. That question is about what oligocellular organisms are "like", whereas this thread seems to simply involve listing them out, as many as we can, and focusing on just how small, really, they can get. I think it's valuable to have both of these up. $\endgroup$ – Korvexius Nov 18 '20 at 19:47
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There's Trichoplax adhaerens, a Placozoa, made of a few thousand cells. Then there is Dicyema japonicum, a simple mesozoan, made up of 9 to 41 cells. Arguably, the simplest multicellular organism is the algae Tetrabaena socialis, whose body consists of 4 cells. Then, there's the parasitic Myxozoa which have 7 cells.

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    $\begingroup$ An interesting follow up question would be: at this scale, how do we draw the line between unicellular and multicellular? E.g. can you kill a Myxozoa by splitting up the cells? What would that tell us? $\endgroup$ – Kal Nov 18 '20 at 4:41
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This nematode always has either 959 or 1031 cells.

https://en.wikipedia.org/wiki/Caenorhabditis_elegans

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  • $\begingroup$ An interesting note is that the nematode is the smallest multicellular organism with a nervous system. $\endgroup$ – MOON Dec 5 '20 at 11:27
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Species from Gonium genus are typically 4-16 celled. Gonium pectorale is 16 celled.

Arakaki, Yoko et al. “The simplest integrated multicellular organism unveiled.”, vol. 8,12, e81641. 11 Dec. 2013, doi:10.1371/journal.pone.0081641

Also, there is four celled Basichlamys sacculifera. See this picture for reference

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There is a whole class of organisms called "oligocellular" organisms, see also here on SE.

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