The Problem of Cyclomorphosis in Daphnia

Abstract

Theories of the control of cyclomorphosis in Daphnia are reviewed. This phenomenon entails a seasonal cyclical change in the form of the head. No one theory accounts for it satisfactorily. One critical variant was found to be the temp.: if the young are incubated at temps. below 11[degree] C, only round heads are obtained; if at temps. above 15[degree] C, only pointed heads. Between these 2 temps. the population is mixed. This rule holds for embryos also. On the other hand, free-living young lose the point to become round-headed within a few molts, and this quite without respect to temp. Evidently several factors are at work in producing the changes.