The occurrence of seasonal changes in the phenotype of an organism through successive generations. It occurs in small aquatic invertebrates that reproduce by parthenogenesis and give rise to several generations annually, such as rotifers and cladoceran crustaceans. Cyclomorphic species of Daphnia, for example, undergo changes in the shape of the head during the year; it is rounded from about midsummer to spring, and thereafter becomes helmet-shaped, reverting to the rounded shape at midsummer. Also, summer generations tend to be smaller and more transparent than at other times. Such changes are thought to be caused by the interaction of environmental cues with the organism’s genes, thereby altering the course of development. The modifications are associated with improved survival of the organism, for instance by reducing the likelihood of predation.