Light intensity controls anti-predator defences in Daphnia: the suppression of life-history changes.

A huge variety of organisms respond to the presence of predators with inducible defences, each of which is associated with costs. Many genotypes have the potential to respond with more than one defence, and it has been argued that it would be maladaptive to exhibit all possible responses at the same time. Here, we test how a well-known anti-fish defence in Daphnia, life-history changes (LHC), is controlled by light. We show that the kairomone-mediated reduction in size at first reproduction is inversely coupled to the light intensity. A similar effect was found for the kairomone-mediated expression of candidate genes in Daphnia. We argue that the light intensity an individual is exposed to determines the degree of LHC, which allows for plastic adjustment to fluctuating environments and simultaneously minimizes the associated costs of multiple alternately deployable defences. It is hypothesized that this allows for a coupling of multiple defences, i.e. LHC and diel vertical migration.