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.