Ecological implications of parasites in natural <Emphasis Type="Italic">Daphnia</Emphasis> populations

In natural host populations, parasitism is considered to be omnipresent and to play an important role in shaping host life history and population dynamics. Here, we study parasitism in natural populations of the zooplankton host Daphnia magna investigating their individual and population level effects during a 2-year field study. Our results revealed a rich and highly prevalent community of parasites, with eight endoparasite species (four microsporidia, one amoeba, two bacteria and one nematode) and six epibionts (belonging to five different taxa: Chlorophyta, Bacillariophyceae, Ciliata, Fungi and Rotifera). Several of the endoparasites were associated with a severe overall fecundity reduction of the hosts, while such effects were not seen for epibionts. In particular, infections by Pasteuria ramosa, White Fat Cell Disease and Flabelliforma magnivora were strongly associated with a reduction in overall D. magna fecundity. Across the sampling period, average population fecundity of D. magna was negatively associated with overall infection intensity and total endoparasite richness. Population density of D. magna was negatively correlated to overall endoparasite prevalence and positively correlated with epibiont richness. Finally, the reduction in host fecundity caused by different parasite species was negatively correlated to both parasite prevalence and the length of the time period during which the parasite persisted in the host population. Consistent with epidemiological models, these results indicate that parasite mediated host damages influence the population dynamics of both hosts and parasites.     

<Emphasis Type="Italic">Daphnia</Emphasis>species diversity in Kenya,and a key to the identification of their ephippia

The distribution of Daphniaspecies in tropical Africa is poorly known and understood. Daphniaare assumed rare in tropical regions, but systematic studies covering large areas are sparse. We sampled the active community (live zooplankton) and/or the dormant community (diapausing egg banks in the sediment) of 41 standing water bodies in Kenya in search for Daphnia.Overall the dormant communities yielded 11 species of Daphnia, a species richness more than twice the species richness found in the active communities. Dormant community species diversity better reflects the spatial, and particularly the temporal (multi-annual) variation in environmental conditions available to Daphniain these tropical standing waters. Hence, we suggest that the dormant community be taken into account when assessing local zooplankton diversity, especially in fluctuating tropical lake ecosystems, where the presence of each local Daphniaspecies in the active community may be strongly seasonal or erratic. Geographic distribution data from this study are supplemented with previous records of Daphniain East Africa to provide an overview of the known distribution of Daphniain Kenya and neighbouring countries. We also present a detailed key for morphological identification of the ephippia of the 11 Daphniaspecies encountered, complemented with photographs and drawings of diagnostic characters.     

Morphological Variation in Daphnia Cucullata Sars with Progressive Eutrophication of a Polymictic Lowland Reservoir

Morphological variation in Daphnia cucullata Sars was studied from 1994 to 2001 when fast increasing water trophy of the Siemianówka Reservoir was related to high TP and chlorophyll concentrations. During this period, D. cucullata biomass, as well as its contribution to the total cladocerans biomass clearly decreased. We found that among the six morphological parameters analyzed in D. cucullata, body length, height of the head and length of the tail spine had a tendency to decrease. The maximum values of these traits and carapace length also decreased. The morphology of D. cucullata appeared to be influenced by factors such as food availability, fish predation and an exclusive presence of D. cucullata among Daphniidae species. Increased water mixing in the reservoir may have also affected the changes in the head inclination of D. cucullata.     

Genetic variation in the cellular response of Daphnia magna (Crustacea: Cladocera) to its bacterial parasite

Linking measures of immune function with infection, and ultimately, host and parasite fitness is a major goal in the field of ecological immunology. In this study, we tested for the presence and timing of a cellular immune response in the crustacean Daphnia magna following exposure to its sterilizing endoparasite Pasteuria ramosa. We found that D. magna possesses two cell types circulating in the haemolymph: a spherical one, which we call a granulocyte and an irregular-shaped amoeboid cell first described by Metchnikoff over 125 years ago. Daphnia magna mounts a strong cellular response (of the amoeboid cells) just a few hours after parasite exposure. We further tested for, and found, considerable genetic variation for the magnitude of this cellular response. These data fostered a heuristic model of resistance in this naturally coevolving host–parasite interaction. Specifically, the strongest cellular responses were found in the most susceptible hosts, indicating resistance is not always borne from a response that destroys invading parasites, but rather stems from mechanisms that prevent their initial entry. Thus, D. magna may have a two-stage defence—a genetically determined barrier to parasite establishment and a cellular response once establishment has begun.     

Life-history traits of lake plankton species may govern their phenological response to climate warming

A prominent response of temperate aquatic ecosystems to climate warming is changes in phenology – advancements or delays in annually reoccurring events in an organism's life cycle. The exact seasonal timing of warming, in conjunction with species-specific life-history events such as emergence from resting stages, timing of spawning, generation times, or stage-specific prey requirements, may determine the nature of a species' response. We demonstrate that recent climate-induced shifts in the phenology of lake phytoplankton and zooplankton species in a temperate eutrophic lake (Müggelsee, Germany) differed according to differences in their characteristic life cycles. Fast-growing plankton in spring (diatoms, Daphnia ) showed significant and synchronous forward movements by about 1 month, induced by concurrent earlier ice break-up dates (diatoms) and higher spring water temperature ( Daphnia ). No such synchrony was observed for slow-growing summer zooplankton species with longer and more complex life cycles (copepods, larvae of the mussel Dreissena polymorpha ). Although coexisting, the summer plankton responded species specifically to seasonal warming trends, depending on whether the timing of warming matched their individual thermal requirements at decisive developmental stages such as emergence from diapause (copepods), or spawning ( Dreissena ). Others did not change their phenology significantly, but nevertheless, increased in abundances. We show that the detailed seasonal pattern of warming influences the response of phyto- and zooplankton species to climate change, and point to the diverse nature of responses for species exhibiting complex life-history traits.     

Persistence of host and parasite populations subject to experimental size-selective removal

Predators have the potential to limit the spread of pathogens not only by selecting infected prey but also by shaping prey demographics. We tested this idea with an epidemiological experiment in which we simulated variable levels of size-selective predation on zooplankton hosts and monitored the persistence of host and parasite populations. In the absence of simulated predation, the virulent protozoan Caullerya mesnili frequently drove its host Daphnia galeata to extinction. Uninfected control populations showed lower extinction rates and higher average densities than infected populations in the absence of simulated predation (all of the latter went extinct or remained infected). With a weak removal rate of the largest hosts, the proportion of populations in which the parasite drove the host to extinction decreased, while the number of populations in which the host persisted and the parasite went extinct increased. Host-parasite coexistence was also observed in some cases. With intermediate levels of removal, most of the parasite populations went extinct, while the host populations persisted. With an even higher removal rate, Daphnia were driven to extinction as well. Thus, variation in one factor, size-selective mortality, resulted in four different patterns of population dynamics. Our results highlight the potential role of predation in shaping the epidemiology and community structure of host-parasite systems.     

Automated Recording of Vertical Negative Phototactic Behaviour in Daphnia magna Straus (Crustacea)

Diurnal vertical migration is a well-known phenomenon in the circadian activity rhythms of zooplankton. Our goal was to test whether negative phototaxis in Daphnia magna clone BEAK (provoked by artificially induced light stress, alternating light and dark phases in 2 h intervals), and its interference with the endogenous rhythm of diurnal vertical migration, can be automatically registered with a biomonitor. For the first time the vertical swimming behaviour of D. magna was recorded quantitatively based on non-optical data recording in a fully automated biotest system, the Multispecies Freshwater Biomonitor in a new experimental setup consisting of a column of three recording units (3-level chambers). Circadian vertical migration was clearly recorded in the 3-level chambers and the rhythm was more clear with 5 than with 1 organism per chamber. The organisms clearly responded to induced light stress with negative phototaxis, however best in larger chambers. The artificially induced rhythm was influenced by the endogenous rhythm. This approach may facilitate long-term observations of vertical swimming activity of zooplankton in the future.     

Predation by invertebrate predators on the colonial rotifer Sinantherina socialis

Abstract. Colonies of the freshwater colonial rotifer Sinantherina socialis (Monogononta, Flosculariidae) have been shown to be unpalatable to a variety of small-mouthed, zooplanktivorous fishes. To test whether invertebrate predators ingest the rotifer S. socialis , we conducted two types of experiments: (1) Microcosm experiments—in separate experiments, four invertebrate predators (i.e., dragonfly nymphs, damselfly nymphs, notonectids, and Hydra ) were offered prey either singly or in combination. Prey were comprised of S. socialis; Epiphanes senta , a solitary, free-swimming rotifer; and Daphnia magna , a microcrustacean. In each experiment, the percent of prey surviving after 12, 18, and 24 h was recorded. (2) Paired-feeding experiments—in separate experiments, predators were offered prey in a pairwise fashion, in which members of D. magna were alternated with a rotifer, either S. socialis or E. senta. The results of the microcosm experiments showed that, after 24 h, 60–100% prey items of S. socialis survived the predators, but significantly fewer individuals of E. senta (6–89%) and D. magna (<25%) survived. When offered rotifers and individuals of D. magna simultaneously, predators tested consistently consumed more specimens of Daphnia. However, predators significantly reduced percent survival in E. senta but not in S. socialis. Predators, given a choice between the two rotifer species, all consumed significantly more specimens of E. senta than S. socialis after 24 h. In the paired-feeding experiments, three of the four predators captured members of S. socialis , but these colonies were frequently released rather than ingested, although in some cases colony structure was seriously disrupted. Our results suggest that the unpalatable nature of members of S. socialis to certain fishes extends to several invertebrate predators, but the nature of the putative factor(s) responsible for this remains unknown.     

Resistance to a bacterial parasite in the crustacean Daphnia magna shows Mendelian segregation with dominance

The influence of host and parasite genetic background on infection outcome is a topic of great interest because of its pertinence to theoretical issues in evolutionary biology. In the present study, we use a classical genetics approach to examine the mode of inheritance of infection outcome in the crustacean Daphnia magna when exposed to the bacterial parasite Pasteuria ramosa. In contrast to previous studies in this system, we use a clone of P. ramosa, not field isolates, which allows for a more definitive interpretation of results. We test parental, F1, F2, backcross and selfed parental clones (total 284 genotypes) for susceptibility against a clone of P. ramosa using two different methods, infection trials and the recently developed attachment test. We find that D. magna clones reliably exhibit either complete resistance or complete susceptibility to P. ramosa clone C1 and that resistance is dominant, and inherited in a pattern consistent with Mendelian segregation of a single-locus with two alleles. The finding of a single host locus controlling susceptibility to P. ramosa suggests that the previously observed genotype-genotype interactions in this system have a simple genetic basis. This has important implications for the outcome of host-parasite co-evolution. Our results add to the growing body of evidence that resistance to parasites in invertebrates is mostly coded by one or few loci with dominance.     

Benthic resources are the key to Daphnia middendorffiana survival in a high arctic pond

1. Shallow arctic lakes and ponds have simple and short food webs, but large uncertainties remain about benthic–pelagic links in these systems. We tested whether organic matter of benthic origin supports zooplankton biomass in a pond in NE Greenland, using stable isotope analysis of carbon and nitrogen in the pond itself and in a ¹³C‐enrichment enclosure experiment. In the latter, we manipulated the carbon isotope signature of benthic algae to enhance its isotopic discrimination from other potential food sources for zooplankton. 2. The cladoceran Daphnia middendorffiana responded to the ¹³C‐enrichment of benthic mats with progressively increasing δ¹³C values, suggesting benthic feeding. Stable isotope analysis also pointed towards a negligible contribution of terrestrial carbon to the diet of D. middendorffiana. This agreed with the apparent dominance of autochthonous dissolved organic matter in the pond revealed by analysis of coloured dissolved organic matter. 3. Daily net production by phytoplankton in the pond (18 mg C m^?2 day^?1) could satisfy only up to half of the calculated minimum energy requirements of D. middendorffiana (35 mg C m^?2 day^?1), whereas benthic primary production alone (145 mg C m^?2 day^?1) was more than sufficient. 4. Our findings highlight benthic primary production as a major dietary source for D. middendorffiana in this system and suggest that benthic organic matter may play a key role in sustaining pelagic secondary production in such nutrient‐limited high arctic ponds.