Phenotypic plasticity, intraguild predation and anti-cannibal defences in an enigmatic polymorphic ciliate

Inducible trophic polymorphisms are greatly underappreciated forms of phenotypic plasticity that allow organisms to respond dynamically to the environmental variation by enabling them to change the trophic level upon which they feed. Although inducible trophic polymorphisms occur in a diverse array of organisms, their costs, benefits and their consequences for long-term population and community dynamics are poorly understood. 2. We studied the inducible trophic polymorphism of the freshwater hymenostome ciliate Tetrahymena vorax, whose isogenic populations can contain three distinct morphs: pyriform, bacterivorous microstomes; larger, carnivorous macrostomes; and elongate, 'tailed' microstomes. We tested whether (i) the tailed microstome constitutes an inducible defence against macrostomes and (ii) the transformation of microstomes into macrostomes is size-dependent. We also describe the dynamics of the three morphs in the presence and absence of an intraguild prey (Colpidium) across a gradient of growth medium concentrations to infer potential trade-offs in the success of different morphs at different productivity levels. 3. Macrostomes do not discriminate between pyriform microstomes and readily consumed heterospecific prey (Colpidium). Tailed microstomes display greatly reduced susceptibility to consumption by macrostomes as compared with undefended, pyriform microstomes. Morph dynamics are consistent with the hypothesis that tailed microstomes function as an inducible defence against cannibalism; tailed microstomes and macrostomes appear simultaneously, in both the presence and absence of Colpidium. At low productivity, T. vorax achieves higher rates of growth when feeding on Colpidium instead of on bacteria. At higher productivity, this pattern is reversed, with growth rates maximized in the absence of Colpidium. 4. The reduced consumption rate of tailed microstomes by cannibalistic macrostomes, together with the simultaneous induction of tailed microstomes and macrostomes, suggests that both morphs comprise a coordinated adaptive response to the presence of intraguild prey.