Cyclomorphosis of Daphnia pulex spined morphs 1

Chaoborus ‐induced spined phenotypes of Daphnia pulex were present at frequencies > 80% in two Wisconsin ponds during summer 1983 but were rare earlier in the year, even though predaceous instars of Chaoborus americanus were common. Laboratory induction experiments with one clone revealed that the proportion of Daphnia juveniles which bore the Chaoborus ‐induced spines (% SM) was a positive function of both temperature and Chaoborus density, and C. americanus from different times of year could induce high percentages of spined morphs. For both ponds, clonal descendants of Daphnia isolated during the spring were less sensitive to a standard Chaoborus treatment than descendants of summer Daphnia. The results suggest that the changing frequency of spined rnorphs in the field was due in part to a shift of Daphnia populations, one unresponsive and another responsive to the Chaoborus factor. Phenotypic plasticity to environmental factors may also influence the distribution of morphs in the field.