Dynamics of an inducible defence in the protist Euplotes

Abstract

Inducible antipredator defences are expressed according to current predation risk. They enhance survival in the presence of predators but have associated fitness costs. Theory predicts that food webs in which members express inducible defences can be either more or less stable than food webs in which inducible defences are absent. This effect depends, to a large degree, on the ability of the prey defence to track predation risk. We investigated the dynamics of an inducible defence in hypotrich ciliates of the genus Euplotes in replicated experimental microcosms. In response to predators or their cues (live or frozen Stenostomum sp.), Euplotes develop lateral pro- jections that significantly increase their width and effectively protect them from being eaten by this gape limited predator. Paramecium aurelia shares resources with Euplo- tes, but does not alter its morphology in response to predators. We exposed Euplotes, alone or together with Paramecium, to either live, frozen or no predators and followed the dynamics over thirty generations (cell cycles). The morphological defence of Euplotes varied over time and loosely tracked predation risk. The dynamics of the de- fence were affected by predator densities and conspecific densities to a similar degree. Loose coupling between defence level and predation risk is expected to lead to an overall weak impact of the defence on community dynamics. The dynamics of our ex- perimental Euplotes populations, characterised by the coefficient of variation in pop- ulation size, were not noticeably affected by how closely the defence matched risk.