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The aim of this study was to explore whether the widespread zooplankter Daphnia magna is able to differentiate between the kairomones of planktivorous common bleak (Alburnus alburnus) and omnivorous black bullhead (Ameiurus melas). We expected different responses in some plastic defensive traits because the presence of a planktivorous and an omnivorous fish pose a different degree of threat to daphnids. Our mesocosm study showed that the size distribution of D. magna in the bleak treatment tended to differ from both black bullhead and control treatments, while there was no apparent difference between the latter two treatments. The eye diameter/body length ratio and tail spine/body length ratio of D. magna were highest in the bleak treatment, differing from both black bullhead and control treatments, and without any significant difference between the latter two treatments. We found that only bleak triggered detectable responses in D. magna, suggesting either that kairomones of the studied fish species could differ in their chemical composition, or that black bullhead may exude less kairomone than bleak. [ABSTRACT FROM AUTHOR]

Calcium decline is an emerging environmental concern for thousands of soft-water lakes. Daphnia, an important freshwater herbivore, has reduced reproduction, a weaker, less-rigid carapace, and decreased survival under low aqueous calcium concentrations, but there has been little attention to study how calcium decline will influence predator-prey relationships. We conducted a mesocosm experiment to determine the joint effects of calcium concentration (0.3, 1.2, and 2.3 mg/l) and kairomones from Chaoborus americanus, a ubiquitous invertebrate predator, on Daphnia minnehaha populations. In the presence of Chaoborus kairomones, chemical signals of Chaoborus presence, Daphnia populations had delayed peak abundance and grew antipredator neck spine defenses. However, aqueous calcium concentration influenced the strength of the response. Daphnia in both 0.3 and 2.3 mg Ca/l had stronger neck spine responses and larger reproductive delays than Daphnia in 1.2 mg Ca/l. Our results suggest that as lake calcium concentrations continue to fall, Daphnia populations will respond in complex, nonlinear ways. [ABSTRACT FROM AUTHOR]

Water hyacinth ( Eichhornia crassipes) is one of the most problematic invasive macrophyte in the world. Although it has become a key element of the systems where it was introduced several decades ago, there is insufficient information to determine its role on biological interactions. To elucidate water hyacinth-littoral zooplankton-planktivorous fish interactions, we conducted habitat choice experiments in the absence and presence of fish chemical cues (kairomones) for two tropical littoral cladocerans, Chydorus brevilabris and Simocephalus vetulus. We classified habitat selection as a habitat (preference for the plant without predation risk) and/or as a refuge (preference for the plant with predation risk). Our results showed that E. crassipes is used as a habitat by C. brevilabris and as a refuge by S. vetulus against fish predation. Although C. brevilabris did not use the water hyacinth as a refuge in the presence of kairomones, the results suggested that both C. brevilabris and S. vetulus exhibited behavioral responses that further reduced their predation risk. To our knowledge, this study is the first experimental evidence for littoral cladocerans using free-floating macrophytes as a refuge against fish predation in tropical systems. Our results support the assertion that habitat association is not necessarily an evidence of habitat preference. [ABSTRACT FROM AUTHOR]

Dormant propagule pools (egg banks) of zooplankton populations that accumulate in sediments provide biological archives of past conditions and enable the investigation of evolutionary changes in populations over relatively long periods of time (many decades). This study examined the egg bank of a Daphnia pulicaria population in a lake that has been stocked annually with rainbow trout (a zooplanktivore) since 1961. Resting eggs from sediments from the 1920s to 2001 (determined by Pb dating) were hatched and established as isofemale clonal lines. The phototactic behavior (a proxy for vertical migration behavior) of clones was assessed in the presence and absence of fish kairomones to evaluate the hypothesis that the heavy and consistent level of predation imposed by the stocked trout would have selected for clones that are more negatively phototactic. In addition, exposure to kairomones was expected to induce stronger negative phototaxis for all clones relative to control conditions. The behavior of clones derived from resting eggs from the trout era (after 1961) was significantly more negatively phototactic than it was for pre-trout era clones. Kairomone exposure induced a more negative phototactic response in clones from both eras, but the response was much greater for the pre-trout era clones. These results suggest that the consistently high level of predation by trout over several decades has selected for a conservative (less plastic) vertical migration strategy in which Daphnia maintain a deep daytime distribution with or without the presence of chemical cues from fish predators. [ABSTRACT FROM AUTHOR]

We quantified the indirect effects of different densities (1, 4, 16 individuals per jar) of the predator Asplanchna brightwellii on the morphometry and demography of their prey Brachionus calyciflorus and Plationus macracanthus. Population growth and life table demography experiments were separately conducted for each of the two prey rotifer species while keeping A. brightwellii in indirect contact. The brachionids were fed the green alga Chlorella vulgaris at a density of 1 × 10 cells ml. As compared to those cultured in the absence of the predator, in the presence of A. brightwellii the postero-lateral spines of P. macracanthus increased by about 15 μm, while in B. calyciflorus the increase was more (>80 μm). For both the brachionid species, the peak population density significantly decreased in the presence of A. brightwellii. The reproductive variables viz., net reproductive rate, generation time, and the rate of population increase of B. calyciflorus and P. macracanthus were negatively affected in the presence of kairomones from A. brightwellii. [ABSTRACT FROM AUTHOR]

The combined effect of temperature, food level and the presence of an invertebrate predator on the body size of the rotifer Brachionus havanaensis were tested in this study. B. havanaensis was cultured at 15, 20, and 25°C under three different Chlorella vulgaris levels (0.5 × 106, 1.0 × 106 and 2.0 × 106 cells ml−1) in the presence and in the absence of Asplanchna girodi. For each treatment we maintained three replicates and constant (0.4 ind ml−1) population density of B. havanaensis. In treatments containing A. girodi, the predator was separated from the prey by a mesh (pore size 50 μm). On the last day of the experiment, a portion of the B. havanaensis population was sampled for several morphometric measurements (adult lorica length, width, posterior spine length, body volume, and the egg volume). Size measurements were done by drawing the specimens using a calibrated camera lucida. Statistically significant impact of temperature as well as the predator’s presence was observed on the lorica length, posterior spine, and egg volume of B. havanaensis. The interactions of food × temperature, or predator´s presence × food × temperature were non-significant ( P > 0.05) for lorica length, spine length, body volume, and egg volume. Regardless of the type of treatment, there was a direct positive correlation between lorica length and width. Egg volume was linearly related to the adult size. Notably long posterior spines were observed in treatments containing the presence of A. girodi. [ABSTRACT FROM AUTHOR]

In aquatic environments, predator kairomones have been shown to affect morphology of prey species. Past work on the interaction between zooplankton and phytoplankton was based mainly on the Daphnia–Scenedesmus model. Algae of the genus Staurastrum can produce mucilage, causing cell clumping and settling out of the water column. These clumps are too large to be eaten by daphniids. Thus, we hypothesised that this may be a grazer defence. We investigated whether Daphnia magna induces the formation of mucus globules in Staurastrum, how this occurs, and if the formation of clumps inhibits growth in juvenile Daphnia. Eight strains of Staurastrum were used to check whether mucus extrusion is induced by the presence of Daphnia or possibly by a chemical excreted by Daphnia magna. None of the strains reacted to the presence of Daphnia water alone, animals had to be present to induce clumping. Mechanical action (gentle stirring) caused the same strains to clump. The ecological relevance of clumping was then investigated. The different Staurastrum strains were used as food in a growth experiment with ecologically relevant densities of neonates of Daphnia hyalina. These small daphniids did not cause the same clumping observed for Daphnia magna when present in experiments at high densities. We observed that juvenile daphniids grew less well on strains with larger cell size. [ABSTRACT FROM AUTHOR]