Role of planktonic bacteria in biodegradation of fish-exuded kairomone in laboratory bioassays of diel vertical migration

Daphnia, freshwater crustaceans that graze algae, often rely on predatory chemical cues termed kairomones as signals for predator-avoidance. Using laboratory bioassays, we studied how planktonic bacteria may modify kairomone activity. We measured the amplitude of diel vertical migration (DVM) of Daphnia pulex DE GEER among treatments with different amounts of bacteria. We used temperature incubation to increase bacterial densities and filtration to reduce abundance. Daphnids exposed to fish cue (F) and filtrate of fish cue (FF) (i. e. 3.0 fold decrease in the planktonic bacteria) exhibited a strong DVM response. In addition, the strength of the response remained the same for both treatments. However, daphnids exposed to an incubated fish cue (IF), which had higher bacterial densities, showed similar migration to daphnids in the control treatment. This IF treatment showed a 3-fold enrichment of bacteria. Besides observing a gradient in DVM response with bacterial density, we also found that DVM response varied seasonally in our experiments. DVM response to fish cue developed quickly in the experiments carried out in March and May compared to a delayed response that we observed in a similar experiment in January. A seasonal shift also occurred in the population sizes of cultivable planktonic bacteria. Responsiveness of D. pulex to predator cues may vary seasonally, possibly due to higher vulnerability during seasons with high fish production. Kairomone concentrations may also fluctuate due to varied release or degradation rates. Our results suggest that the in-situ, tight coupling between production and degradation of kairomone appears to be the cause of DVM response for daphnids during summer.