Neckteeth formation in Daphnia pulex as an example of continuous phenotypic plasticity: morphological effects of Chaoborus kairomone concentration and their quantification

Many clones of Daphnia pulex develop morphological changes as antipredator devices in the presence of chemicals released by Chaoborus larvae. The D.pulex clone used in this experiment develops neckteeth until the end of the fourth juvenile instar when exposed to chemical cues from Chaoborus flavicans. This clone develops small neckteeth in the first instar even when it has not been exposed to the chemicals. Neckteeth formation is a continuous process that involves changes in the whole neck region. On the basis of these changes, a scoring method is presented that allows easy classification of the morphological changes. Neckteeth formation was strongest in the second juvenile instar, followed by a slightly weaker response in the third juvenile instar. Neckteeth formation within one instar is concentration dependent and exhibits a saturation curve. The maintenance of neckteeth over several instars is also concentration dependent. At low kairomone concentrations, neckteeth were restricted to the first two instars. This study indicates that neckteeth formation in successive instars is different, probably to optimize costs and benefits by equalizing or reducing vulnerability in successive juvenile instars at different Chaoborus densities