Onset of kairomone sensitivity and the development of inducible morphological defenses in Daphnia pulex

The micro-crustacean Daphnia pulex is a model species for studying predator-induced defenses. When exposed to chemical cues released by its predator, the phantom midge larvae Chaoborus (Diptera), it develops protective neckteeth that reduce the predator's success of predation in the juvenile instars. Defensive traits need to be expressed as soon as possible, which requires an early sensitivity to predator cues. We investigated the exact kairomone-sensitive period in three D. pulex strains and the timeline of neckteeth expression in early juvenile instars. We divided embryonic development into five major stages based on successive morphological landmarks. We exposed animals in these developmental stages to kairomones in order to determine the sensitive periods for neckteeth expression in the 1st and 2nd juvenile instar. Our results indicate that kairomone sensitivity starts during embryogenesis when compound eye spots begin to fuse and egg membranes are shed. Neckteeth develop with a stage-dependent time lag, being shorter when exposed in the first kairomone-sensitive stage and longer when exposed in the following developmental stages. Evolution of early kairomone sensitivity and fast defense development is a crucial step in D. pulex's defenses against Chaoborus as it allows for protection of the most vulnerable juvenile stages.