Growth, Degrowth, and Irreversible Cell Differentiation inAurelia aurita

Growth patterns of the Scyphomedusa Aurelia aurita from Tomales Bay. California, were examined in the field and in the laboratory. Manipulation of growth patterns demonstrated that degrowth and regrowth are not constrained by initial ievelopmental stage. Although initial degrowth of certain tissues is allometric (e.g., gonads regress in 5 to 8 days; bell diameter decreases more rapidly at first than do the oral arms), thereafter regression appears identical to, but reversed from normal growth. Regrowth patterns are normal. Sexual maturation in the sea does not always alter subsequent capacity for degrowth or regrowth to sexual maturity in the laboratory, because reproductive and somatic tissues do not always degenerate after spawning. Gonadal tissue can be renewed and maintained in a ripe condition in the laboratory apparently indefinitely. Sexual maturation is a size-dependent phenomenon, not an agespecific developmental event. Spermatogenesis, once initiated, proceeds irrespective of outside events. Labeled spermatogonial cells can continue to differentiate to form sperm even though the gonad containing those cells, and the animal itself, show rapid degrowth. The importance of this decoupling of developmental events is discussed. The experimental importance of animals with flexible life cycles is emphasized.