Dynamics and adaptive significance of inducible defenses and offenses in phenotypically plastic ciliates

Inducible trophic polymorphisms (ITPs) allow organisms to respond to environmental variation through changes in morphology and trophic position.  While ITPs occur in a diversity of taxa, the costs and benefits of their expression, and their consequences for long-term population and community dynamics, remain incompletely understood.  We studied Tetrahymena vorax, where individual cells within isogenic populations occur as one of three distinct morphs: a pyriform bacterivorous microstome morph; a larger carnivorous inducible macrostome morph; and an elongate inducible tailed microstome morph, which may represent an inducible defense against cannibalism.  We monitored morph dynamics in the presence and absence of a transformation-inducing ciliate (Colpidium striatum) across a gradient of productivities.  We also evaluated prey selection by macrostomes to assess the adaptive significance of the tailed microstome morph.  Our results indicate potential tradeoffs in the success of different morphs at different productivity levels.  At low levels of productivity, T. vorax feeding on Colpidium attain higher densities than T. vorax feeding only on bacteria.  At higher levels of productivity, the pattern is reversed.  Macrostomes do not appear to discriminate between pyriform microstomes and heterospecific prey.  However, tailed microstomes suffer much less cannibalism than undefended pyriform microstomes.  Reduced consumption rates, together with the simultaneous induction of tailed microstomes and cannibalistic macrostomes, suggest that tailed microstomes constitute an effective, adaptive defense against cannibalism.