Cells are dynamic systems capable of switching from an isotropic growth to a polarized growth even in the absence of any pre-existing external asymmetry. Here, we study this process of symmetry breaking in the acellular slime mold Physarum polycephalum. In the experiments slime molds could grow on two identical opposed sources of calcium. We highlighted a positive correlation between growth dynamic, level of symmetry breaking and calcium concentration. We identified three populations of slime molds within our clonal lineage with similar symmetry breaking behaviours but different motility characteristics. These behavioural differences between slime molds emerged in absence of any environmental differences. Such behavioural plasticity could generate cellular diversity, which might be critical for survival.