Piotr Maszczyk

Threat or treat: the role of fish exudates in the growth and life history of Daphnia

In kairomone studies, it is often implicitly assumed that the only effect of predator exudates on their prey is to trigger anti-predator defenses. However, chemicals originating from fish activity may also fertilize the environment and enhance the growth of zooplankton prey by increasing bacterial food availability. It is necessary to separate these two effects in order to examine the adaptive significance of zooplankton anti-predation defenses and the ability of the prey to benefit from fish-related food. Here, we have employed differential filtration of media to permit assessment of these two effects on growth rate and life history adjustments in two Daphnia species that differ in body size and the ability to collect small particles (greater in small-bodied D. cucullata than in larger D. hyalina). Filters of three mesh sizes were employed: 2 μm (most bacteria pass, but detritus is retained), 0.45 μm (standard) and 0.20 μm (most bacteria and detritus retained). The concentration of kairomones in the fish...

Interference competition in a planktivorous fish (Rutilus rutilus) at different prey densities and temperatures

The effect of interference competition can be assessed by comparing the capture rate of a predator foraging alone with that of the predator within a group. Since such an effect could be prey density dependent, a constant density of prey must be maintained while assessing this effect, irrespective of the elimination of prey by predation. However, when studying a predator-harvester, such as a planktivorous fish, which collects zooplankton at a rate of up to 1 prey s−1, instantaneous replacement of each consumed prey item is not feasible. This problem was solved in short-lasting mesocosm experiments by minute-by-minute supplementation to replace eliminated Daphnia and maintain a constant average prey density. Such experiments were performed with different numbers of foraging roach (Rutilus rutilus) at three prey densities and in two ranges of ambient temperature. The number of Daphnia required at the start of each experiment to establish the initial prey density and the number that it was necessary to add per minute were determined in experiments conducted without prey supplementation and in preliminary experiments with prey supplementation. The results of this study revealed that fish foraging in a group eat less, due to both exploitation and non-aggressive competition for space. Moreover, the effect of interference competition was stronger at higher temperatures, irrespective of the prey density, indicating that natural populations of roach foraging in shoals may suffer more from competitive interactions in warmer waters.

The reaction distance of a planktivorous fish (Scardinius erythrophthalmus) and the evasiveness of its prey (Daphnia pulex × pulicaria) under different artificial light spectra

Artificial light at night may affect mortality risk in prey from visually oriented predators because the effect of the artificial light spectrum may differ for a predator’s visual prey detection and for prey evasiveness. To test this, we conducted two types of experiment. First, we assessed the reaction distance and swimming speed of juvenile rudd (Scardinius erythrophthalmus) allowed to forage on juvenile Daphnia pulex × pulicaria under three artificial light sources: halogen, high pressure sodium (HPS), and metal halide bulbs, at the same light intensity. Second, we assessed the evasiveness of D. pulex × pulicaria under the same artificial light sources and in darkness (as a control), in the presence and absence of chemical information on predation risk (kairomones) of juvenile rudd. We found that while both reaction distance and swimming speed of fish was greater under halogen compared to HPS, and similar under metal halide light compared to halogen and HPS, the evasiveness of Daphnia was greater under halogen and HPS-generated light than under metal halide light. The results suggest a possible mismatch of Daphnia’s behavioural response under metal halide light to predicted predation risk, and thus a possible threat to predator–prey balance in a lake ecosystem.

Ideal free distribution of Daphnia under predation risk—model predictions and experimental verification

Abstract The vertical distribution of planktonic animals, such as Daphnia, in overlapping gradients of food concentration and risk of visual predation should depend on Daphnia population density and should be the result of the group effect of optimizing decisions taken by each individual (juvenile or adult), trading-off a high growth rate to low mortality risk. We tested this hypothesis by comparing the theoretical distributions from simulations based on an experimentally parameterized, optimizing individual-based model (consistent with the assumptions of the concept of the interference ideal free distribution with costs) with distributions observed in laboratory experiments. The simulations were generated for two scenarios, where the shape of the functional response of fish is consistent with either type II or III. The results confirmed the hypothesis. The greatest similarity of the distributions obtained in the experiments and simulations was found for the simulations based on the scenario assuming the type III rather than type II for both age classes of Daphnia. This was consistent with the results of the experiments for the model parameterization, which revealed the type III functional response of fish. Therefore, the results suggest that aggregating may be maladaptive as an anti-vertebrate-predation defense in the case of zooplankton.