Cyclomorphosis in Daphnia: an adaptation to avoid invertebrate predation

Seasonal morphological changes in three Daphnia species were followed over a two-year period in two lakes that differ in invertebrate and fish pressure. Whereas the morphology of D. hyalina, the biggest of the three species, varied little from season to season, D. cucullata, the smallest, exhibited the most pronounced seasonal changes in head height/carapace length ratio. The pattern of seasonal changes of body proportions was similar in all size classes and isometric growth of the head was reported for D. cucullata. Unlike the head, tail spine length/carapace length ratio almost did not vary seasonally. Strong negative allometry of tail spine growth was observed. These results are consistent with the hypothesis that helmets and tail spines provide protection against invertebrates in the two smallest, thus most endangered species.