Plasticity in depth selection behavior and heat shock proteins in Daphnia

Habitat selection behavior by aquatic and terrestrial animals is influenced by both abiotic (e.g., temperature) and biotic (e.g., threat from predators) environmental factors. In this study, the mechanisms underlying the variability in behavior of habitat selection of Daphnia under environmental stress were examined. Experiments were conducted using five Daphnia clones with different environmental preferences and, consequently, with a different width of the reaction norm. These clones also showed variation in their constitutive levels of stress-related heat shock proteins (HSP60, HSP70 and HSP90), but none of the tested stress factors had any direct effect on their expression. However, behavioral plasticity was significantly positively correlated with the constitutive level of HSP70. It is likely that animals with a high constitutive HSP70 level can cope better with sudden changes in environment conditions that they experience, e.g., during vertical migrations. In contrast, non-migrating animals with low HSP levels do not allocate energy to the synthesis of stress proteins and have a narrow range of behavioral plasticity.