Chemical ecology of Chaoborus as predator and prey: Effects of infochemicals and food quality on inducible defences and gene expression in Daphnia

The crustacean grazer and model organism Daphnia is prey to both vertebrate and invertebrate predators and thus in a keystone position for the energy transfer in standing freshwater ecosystems. It exhibits a high degree of phenotypic plasticity in a vast variety of traits, including inducible defences against predators such as changes in life-history, morphology or behavior. These defences are likely to impose metabolic costs, as they are not continually expressed but only in the presence of a reliable chemical stimulus, a so called kairomone, released by the respective predator indicating its presence and thus the imminent danger of predation. This present study is focussed on the elucidation of different mechanisms connected with or underlying the reciprocal predator-prey interactions of Daphnia and the aquatic larvae of the phantom midge Chaoborus, an important gape-limited invertebrate predator, both under the impact of one lower and one higher trophic level. As Daphnia is known to feed unselectively on phytoplankton of a given size range, they are highly depended on the overall food quality of the available phytoplankton. I investigated the influence of three different food algae on the capability of Daphnia pulex to induce a morphological defence called neck-teeth and related the strength of the induction to the juvenile growth rate of the first two juvenile instars with was conditional on the quality of the food. The quality of the respective algae as a food for Daphnia was determined by observing the time juveniles took to complete two moulds. As low quality food leads to slower growth, a higher neck-teeth induction was observed, which was presumably to compensate for a longer time needed to outgrow the optimal prey size range of Chaoborus. A poly-unsaturated fatty acid (eicosapentaenoic acid - EPA) was identified as molecule responsible for the suppression of neck-teeth induction in high quality food, which has previously been shown to lead to an increased juvenile growth rate when provided to Daphnia reared on low quality food.Additionally, I investigated relative gene expression of candidate genes putatively involved in the regulation of neck-teeth induction. No evidence for an involvement of the presumably involved juvenile hormone / insulin pathway could be found, as Chaoborus kairomone did not induce any differential gene expression of candidate genes as reported by previously published studies. I applied a stricter method for both RNA sampling and kairomone enrichment in order to avoid potential flaws of preceding studies on this topic and to produce more substantial and reliable results. Further, the relative expression of two new candidate genes, i.e. chitin deacetylases, were found to be significantly correlated with the neck-teeth induction in three different D. pulex clones, all exhibiting different neck-teeth inductions with corresponding gene expression levels. Finally this study presents for the first time data on a foraging kairomone released by Daphnia, which was shown to induce a change in the vertical position of Chaoborus larvae in the water column towards the strata containing the kairomone. Additionally, the simultaneous presence of fish kairomone induced different behavioural responses from Chaoborus. If applied in the top strata of the water column where visibility was high, the effect of the predator kairomone overruled the effect of the foraging kairomone. If applied to the lower strata of the water column where visibility was lower, the effect of foraging kairomone overruled the effect of the predator kairomone. These results clarify, how Chaoborus can assess potential costs and benefits resulting from the presence of different kairomones and the respective surrounding light conditions

Prey-induced vertical migration in Chaoborus larvae under different predator and light regimes

For freshwater prey like Daphnia and larvae of Chaoborus, it is well documented that they adjust their residence depth in the water column in response to predator kairomones in order to decrease encounter probability with the respective predator. Despite the importance of infochemicals in predator-prey interactions, it has not been tested if predators adjust their residence depth in response to infochemicals released by prey. Here, we use an indoor system with a stratified water column and show that the predatory phantom midge Chaoborus prefers strata with Daphnia incubation water over strata with control water. Further, the chemically mediated effect of a top predator (fish) on this system was shown to be light dependent with Chaoborus avoiding prey-conditioned water when it also contained fish kairomone in brighter surface water, but not in deeper and thus darker water layers. The foraging kairomone released by Daphnia can be extracted from incubation water via C-18-based solid-phase extraction. These results add another dimension to the steering role of infochemicals in predator-prey interactions in zooplankton