While it is generally assumed that jellyfishes (Cnidaria: Scyphozoa) are stably tetramerous, variation in symmetry (i.e. unimerous to octamerous) can be observed in most populations at a rate of approximately 2%, but sometimes as high as 10%. This type of variation has been observed among clonemates during strobilation in five taxa, namely Aurelia aurita, A. labiata, Chrysaora fuscescens, Pelagia colorata, and Phacellophora camtschatica. It is currently unclear whether the symmetry variation is caused by genetic, environmental, or developmental factors, or some combination. Although hexamerous lineages were not bred successfully, lineages were raised with rates of variation higher than normal. Thus, there may be some genetic component to the variation. In one lineage observed over 4.5 months, the rate of non-tetramery declined substantially from an initial high of 88.9% to a final cumulative low of 29%. Apparently this lineage was able to stabilize tetramery over time, possibly indicating some self-correcting developmental mechanism. Furthermore, no difference was found in variation rates between stressed and unstressed polyps, indicating that environmental factors may not play an important role in symmetry determination in these animals. These results indicate stabilizing selection in controlling the expression of variable symmetry.