The ghost of herbivory past: slow defence relaxation in the chlorophyte Scenedesmus obliquus

The freshwater chlorophyte Scenedesmus obliquus (Turpin) Kützing produces colonies as an inducible defence against herbivores. We investigated the dynamics of Scenedesmus colony formation and disintegration in response to the density of the herbivorous rotifer Brachionus calyciflorus in large-scale mesocosms. Additional bioassays were performed to investigate Scenedesmus colony disintegration under different light regimes. In the mesocosm experiment, colony formation took place rapidly, but relaxation towards the initial size took relatively long (>10 d) after cessation of herbivory. In the bioassays, in the absence of infochemicals, colonies disintegrated almost immediately in the dark (1-1.5 d), within 4 d under a photoperiod of 16:8 (L:D) and between 8 and 12 days under full light. Colony disintegration times in the mesocosm experiment were substantially longer as compared to treatments with the same photoperiod (L:D 16:8) in the bioassays. So after a peak of herbivory, the 'ghost of herbivory past', i.e. the remaining infochemicals, may continue to induce colony formation, causing an additional lengthening of colony disintegration times and associated fitness costs (higher sedimentation loss rates). This indicates that costs of colony formation are not only important during the induction phase, but may be even more important during the relaxation phase. We compared these sedimentation costs to the costs of herbivory for differently sized Scenedesmus, and found a clear trade-off pattern for these costs.