Dear Raja,

for me some resemblance with Mycetozoa, Classis Cercomonadea. But they are free-living in saprobic conditions. Attached Vol in Russian but with pictures.

Andrey

Yes, it ispossible if it all protozoans. But due to explanation I thought that this are cells of carapax of crayfish  infected by some semi-parasitic protozoan (inside). Rhizopods cannot be any problem for crayfish as they all are free-living. So, some Arcellids can be like periphytic on dead crayfish but cannot be on living crustacea. May be some better clarification from Raja?

Andrey

The protist appears to be a cyst.  The size is not stated, but if the "cysts" are between 15 and 30 micrometers, they might be a species of Acanthamoeba (Amoebozoa, Discosea, Longamoeba, Centramoebida).  Acanthamoeba  lives in all possible environments.  There is a space between the cell and the membrane (wall). What is the refreactle (prominent) circular object in most the cells?  

Stating the size of these cells or cysts is necessary for your colleagues here to offer more satisfactory explanations.  You might place these cells(cysts) on bacteria covered agar plates to coax the protists "to show them selves."

Happy New Year colleagues,

I think we should respect the opinion of Stuart as he is famous specialist in this and related groups of protists (I upvoted his explanation). Moreover the fact of such infection is enough interesting and I want to propose short communication of the fact in Vestnik Zoologii in Kiev (this is SJR and Scopus J). But need more information from Raja still keeping silence...

Kind regards,

Andrey

Stuart, there is a size bar at the bottom of both photographs.  The "cysts" are roughly 60microns in diameter.  They are not anything that I am familiar with.

Paul

These could well be 'detached' peritrich cilates (vorticellids) of which many taxa live epizoic on carapaces of aquatic organisms. The central spot you see could be the attachment site of the stalk (which was not present anymore apparently). The genus Vorticella seems the most probable candidate... however impossible to be certain, just a resemblance...

And what about being eggs of some invertebrate without having been fertilised? Are they strongly attached to the carapace?