Defensive responses in invertebrates: Evolutionary and neural aspects.

Lower invertebrates exhibit both morphological and behavioral defensive responses to aversive stimuli, characterized by withdrawal. Typical immobility responses are "sinking" in Rotifers and "crumpling" in Cnidaria. They also display individual adaptation and phenotypic plasticity but not tonic immobility (TI). The higher phyla with a more organized nervous system have developed morphological and behavioral defensive strategies including TI, occurring both in natural and laboratory conditions. There are general but also specific prey-predator mechanisms, that have coevolved leading to reciprocal phenotypic plasticity. The evolution of traits differentiated in subpopulations has been described in many species (animal personality). In insects the variability in TI is heritable and inversely related to boldness. In two genetic lines of beetles with long and short TI duration, the long duration line has higher survival rate but lower mating success (behavioral syndromes). TI may have an adaptive significance also in intraspecific interactions in the context of sexual selection.