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.