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Note: question rewritten to prevent misunderstanding and make it more answerable

I know that some small animals like C. elegans display surprising sophistication with a very small number of cells. But I wonder, among the animals with known/estimated cell count, which has the fewest cells?

To make the question answerable, please note these constraints:

  • I'm only asking about members of the kingdom Animalia, so single cell organisms and colonials like Vovox are excluded. If there are single cell organisms which are classified under Animalia (I think there aren't, but not 100% sure), I still want to know about the smallest multicellular animal.
  • I'm only asking about species where we can distinguish individuals. If some kind of polyp or sponge is just a bunch of cells which can merge with or separate from other cells and remain a viable organism, it's not interesting.
  • Please only consider species where the adult form reaches some size and stops growing. (Macroscopic) examples like the tapeworm, which continues building more proglottides throughout life, or fish which continue getting larger as they age, are excluded.
  • If the species consists of distinctive, stable "subgroups" of different sizes, the smallest size subgroup counts. For dogs, the size of the average chihuahua counts. For C. elegans, the size of the hermaphrodite (959 cells vs 1031 in the male) counts.
  • What counts is the number of cells in a healthy adult of the species or the relevant subgroup. For non-eutelic animals, make that the average healthy adult. Not the smallest ever observed exemplar, and also not amputated or stunted exemplars.
  • I am aware that we haven't discovered all microscopic animals yet, and haven't counted the cells of all discovered ones. Still, I'd like info on the current level of knowledge, including current known cell count, evidence that something has less than the current lowest known-for-sure count, and other relevant information.
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    $\begingroup$ What is an "actual animal"? This will be hard to define. Take a worm like C.elegans which has around 1000 cells (there are different numbers available here). $\endgroup$ – Chris Jan 4 '15 at 11:40
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    $\begingroup$ @Chris one officially classified in Animalia, the taxonomic kingdom. I know of C. Elegans, but I wonder if there are smaller ones. $\endgroup$ – rumtscho Jan 4 '15 at 12:03
  • $\begingroup$ I doubt that the number of cells has been estimated for many organisms, especially since most small animals are also usually poorly known (with C. elegans as a model organism being the exception). I therefore think you will have a better chance of getting accurate answers if "smallest animal" is framed in terms of actual body size (which is better known) instead of number of cells. $\endgroup$ – fileunderwater Jan 4 '15 at 12:16
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    $\begingroup$ @fileunderwater but that would be boring :) and also an example of a tend which personally bugs me, as in "X, which interests us, is hard to measure, let's take the related Y and pretend we are talking about the same thing." I'm really interested in cell count, so I'd prefer to frame it as the smallest known, estimated or suspected cell count. I assumed that this is implicit, as we are unlikely to have discovered all microscopic animals anyway. $\endgroup$ – rumtscho Jan 4 '15 at 12:26
  • $\begingroup$ @rumtscho I'm not saying that its hard to measure, I'm saying that it is unknown for most animals. Therefore, any answer you get on cell count is most likely not the animal with the smallest cell count (just the smallest known cell count). $\endgroup$ – fileunderwater Jan 4 '15 at 12:38
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Rotifers are microscopic protostomes with around 1000 cells apiece. This website claims the smallest rotifers have less than 100 cells, but I couldn't confirm this. Most rotifers are eutelic — within a given species, each adult individual has the same number of cells (Encyclopeia of Life: Rotifers).

Bdelloid rotifer (Source)

Edit: Myxozoans might be even smaller. These are microcsopic parasites that have been classified as cnidarians based on molecular studies. They have complicated life cycles including a single-celled stage.

Myxobolus cerebralis (Source)

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    $\begingroup$ About the edit: the single cell stage is probably not relevant. The Q is asking to adult stages. You've also had a single-celled stage - just after fertilization. $\endgroup$ – fileunderwater Jan 6 '15 at 23:04
  • $\begingroup$ @fileunderwater Good point. But it's hard to point to a single "adult" stage for myxozoans, they have two different hosts so they alternate between spores and infectious forms. $\endgroup$ – augurar Jan 8 '15 at 3:13
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I don't know about all known animals, or actual cell count. However, males of the wasp Dicopomorpha echmepterygis are considered the smallest known insects with a body size of 139 μm. This should at least be a contender for smallest animal. As a reference point, C. elegans are usually about 1mm long.

However, given that we know only about 10% of the estimated number of species, and the ones we know are biased towards larger animal species, the smallest animal is most likely undescribed. In the case of parasitic wasps (or other insect parasites) these are extremely poorly known and new species are often described. Our knowledge of soil communities are also poorly known and includes many extremely small animals.

For the record, answers to a question like this depends alot on exactly what you include and how size is calculated. Could it be either sex or the average between sexes?

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    $\begingroup$ comments to downvotes are always appreciated... $\endgroup$ – fileunderwater Jan 7 '15 at 12:22
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Some of the smallest animals have only one cell. Paramecium Aurelia enter image description here

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    $\begingroup$ Can you please indicate the species name and its position in the eukaryote phylogeny? $\endgroup$ – Remi.b Jan 4 '15 at 16:07
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    $\begingroup$ As far as I know, there are no single cellular animals. All single cellular organisms belong to other kingdoms. I admit that my taxonomy knowledge is not very good, so I may be mistaken. If there are single cellular animals please expand the answer, I'd like to learn more about them. $\endgroup$ – rumtscho Jan 4 '15 at 17:28
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    $\begingroup$ @fileunderwater the way I learned it, Protozoa were always considered separate from animals taxonomically, even though some of them were considered "animal like" and others "plant like" on an informal level. $\endgroup$ – rumtscho Jan 4 '15 at 20:12
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    $\begingroup$ @fileunderwater Chromaveolata is closer to plantae than to animalia; Chromaveolata and Plants fall under bikonta. $\endgroup$ – WYSIWYG Jan 6 '15 at 5:36
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    $\begingroup$ The classification of protozoa as animals (note "protozoa" means "first animals") dates back to a very old taxonomic system in which all living things were classified as plants or animals. I learned in high school that bacteria were plants. Really, this is just wrong and should be contradicted at every opportunity. $\endgroup$ – Leon Avery Jan 27 '15 at 14:10

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