This is due to the doughnut shape that cells of Thecamoeba homeri occasionally adopt.

[1] Thecamoeba homeri is a species of amoeba belonging to the genus Thecamoeba, characterized by a central ovoid cell nucleus and prominent dorsal folds that run longitudinally across each cell.

During its life cycle, the amoeba forms a transient doughnut-shaped or ring-like cell morphology, a phenomenon that has never been documented before.

[5] Through sequencing of the SSU rDNA gene, the microbes were assessed as members of the genus Thecamoeba but phylogenetically distinct from all other known species.

[1] A strain sister to T. homeri was described, named SK13-4H, but was lost shortly after the isolation and molecular sequencing.