cyclomorphosis Seasonal change in body shape found in rotifers (phylum Rotifera), and in cladoceran Crustacea (phylum Arthropoda). In cladocerans, e.g. Daphnia species, the changes in shape involve the head, which is rounded from midsummer to spring and then progressively becomes helmet-shaped from spring to summer, reverting to the rounded shape by midsummer. The process is poorly understood and may be the result of genetic factors interacting with external conditions, e.g. temperature or day length, or, as in rotifers, the result of internal factors alone.

cyclomorphosis (seasonal polyphenism) 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.

cyclomorphosis Seasonal changes in body shape found in rotifers (phylum Rotifera), and in cladoceran Crustacea. In cladocerans (e.g. Daphnia species) the changes in shape involve the head, which is rounded from midsummer to spring and then progressively becomes helmet-shaped from spring to summer, reverting to the rounded shape by midsummer. The process is poorly understood and may be the result of genetic factors interacting with external conditions, e.g. temperature or day length, or, as in rotifers, the result of internal factors alone.