Divergent developmental patterns of induced morphological defenses in rotifers and Daphnia: Ecological and evolutionary context

Induced morphological defenses commonly develop in some loricate rotifers as greatly increased spine development and in many species of the cladoceran genus Daphnia as an alteration in the shape and size of its head, producing helmets, crests, and neck spines. This restructuring of the shape of these animals during development, which reduces their vulnerability to a variety of planktonic predators, involves a series of developmental challenges and potential evolutionary constraints for each specific type of defense. In this review, we examine these challenges and the divergent developmental patterns that evolved in rotifers and Daphnia in response to the particular forms of predation pressure they face in their lake and pond environments. These patterns involve differences in the sensitivity of various developmental stages to predator kairomones, the timing of when the defense is present during postembryonic development, the ability to reverse the induced defense if the predator disappears from the community, and the effect of water temperature on the induction process. The type of developmental pattern associated with each induced defense appears to have been shaped by natural selection to effectively protect the animal, at minimal cost, from the specific kind of predation pressure to which it is commonly subjected.