Predator-induced shape plasticity in D. pulex

All animals and plants respond to changes in the environment during their life cycle. This flexibility is known as phenotypic plasticity and allows organisms to cope with variable environments. A common source of environmental variation is predation risk, which describes the likelihood of being attacked and killed by a predator. Some species can respond to the level of predation risk by producing morphological defences against predation. A classic example is the production of so-called ‘neckteeth’ in the water flea, Daphnia pulex, which defend against predation from Chaoborus midge larvae. Previous studies of this defence have focussed on changes in pedestal size and the number of spikes along a gradient of predation risk. Although these studies have provided a model for continuous phenotypic plasticity, they do not capture the whole-organism shape response to predation risk. In contrast, studies in fish and amphibians focus on shape as a complex, multi-faceted trait made up of different variables. In this study, we analyse how multiple aspects of shape change in D. pulex along a gradient of predation risk from C. flavicans. These changes are dominated by the neckteeth defence, but there are also changes in the size and shape of the head and the body. We detected change in specific modules of the body plan and a level of integration among modules. These results are indicative of a complex, multi-faceted response to predation and provide insight into how predation risk drives variation in shape and size at the level of the whole organism.

Methods
The data from this study was collected by analysing photographs taken by Dennis et al., 2011. This included three Daphnia pulex clones which had been exposed to six levels of predation risk from their larval midge predator, Chaoborus flavicans. We used the geomorph package in R to digitise these images of D. pulex into sets of anatomical co-ordinates, called landmarks, to measure key aspects of shape. Principal component analysis was combined with phenotypic trajectories to measure how shape changed for each clone along the gradient of predation risk. Also, modularity and integration analysis was used to identify how shape was co-ordinated across different regions of the body plan.