When I looked for the largest protozoan I've found the name Syringammina fragilissima. According to wikipedia " .. Syringammina fragilissima, is among the largest known coenocytes, reaching up to 20 centimetres (7.9 in) in diameter." The catch is it is multi-nucleated organism, not exactly what I had in mind. The reason I am interested in the upper limit of the size of the free living unicellular organism is because I want to have a sense of the limitation of single center has in emitting and receiving information across a large span of space. My interest is not about non-compartmentalization (as in the case of multi-nucleated unicellularity) rather about single center.

  • $\begingroup$ There are individual neurons in multicellular organisms that are meters long with a single nucleus. Certainly they are nothing near spherical, but just something to think about. $\endgroup$ – Bryan Krause Aug 28 '18 at 19:01
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    $\begingroup$ I have edited the title to only include free living organism. Apparently a too specialised and dependent cell can become very large. :) $\endgroup$ – Ahmed Abdullah Aug 29 '18 at 6:00
  • $\begingroup$ Some bacteria form > 100 micrometers long unicellular filaments (e.g., Caldilinea tarbellica). cz.pinterest.com/pin/518125132116454830 $\endgroup$ – user37894 Aug 29 '18 at 12:19

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