The microsporidian parasite Hamiltosporidium tvaerminnensis can infect Daphnia magna both horizontally (through environmental spores) and vertically (through parthenogenetic and sexually produced eggs). The spores of H. tvaerminnensis come in three distinguishable morphologies, which are thought to have different roles in the transmission of the parasite. In this study, we examined the role of the two most common spore morphologies (i.e. oval-shaped spores and pear-shaped spores) in horizontal transmission of H. tvaerminnensis. To this end, we infected hosts with solutions consisting of either mostly oval- or mostly pear-shaped spores, and quantified infection rates, parasite-induced host mortality and mean number of parasite spores produced per host. We found that spore morphology by itself did not influence infection rates and parasite-induced host mortality. Instead, host clone and parasite isolate interacted with spore morphology in shaping infection outcome and mortality. Thus, there appear to be strong genotype-by-genotype (G × G) interactions in this system. While there is no dispute that H. tvaerminnensis can transmit both vertically and horizontally, our findings do not support theoretical predictions that different spore morphologies hold different roles in horizontal transmission of H. tvaerminnensis.

Many parasites survive harsh periods together with their hosts. Without the possibility of horizontal transmission during host diapause, parasite persistence depends entirely on host survival. We therefore hypothesize that a parasite should be avirulent during its host's diapausing stage. In contrast, the parasite may express higher virulence, i.e. parasite-induced fitness reduction of the host, during host life stages with good opportunities for horizontal transmission. Here we study the effects of a vertically and horizontally transmitted microsporidium parasite, Hamiltosporidium tvaerminnensis, on the quantity and survival of resting eggs of its host Daphnia magna. We find that the parasite did not affect egg volume, hatching success and time to hatching of the Daphnia's resting eggs, although it did strongly reduce the number of resting eggs produced by infected females, revealing high virulence during the non-diapause phase of the host's life cycle. These results also explain another aspect of this system – namely the strong decline in natural population prevalence across diapause. This decline is not caused by mortality in infected resting stages, as was previously hypothesized, but because infected female hosts produce lower rates of resting eggs. Together, these results help explain the epidemiological dynamics of a microsporidian disease and highlight the adaptive nature of life stage-dependent parasite virulence.

In this study, we investigated variation in offspring size (OS) of Daphnia magna in relation to multiple maternal traits and environmental variables. Data originated from laboratory experiments conducted at different feeding scenarios. The mother daphnids had different life-history traits and were reared under various feeding and density conditions. OS showed linear relationships with maternal traits, varying positively with maternal body size, age and brood number, and negatively with brood size and with the amount of ingested carbon. OS increased exponentially with crowding. Using stepwise multiple regression analysis, we developed an empirical model for the OS variation with the relevant maternal and environmental variables. Density dependence was considered by multiplying the resulting model by a density-effect function. We found that the ingested carbon and the maternal body size were the strongest determinants of the observed variation in the OS, whereas the brood size had the least impact on OS. Additionally, the brood number had no significant effect in determining the variability in the OS. The validity of the multivariate model was tested against an independent dataset. The model accurately predicted the OS despite several genetic and environmental differences compared with the data used for parameterization.

The effects of cyanobacterial toxins on herbivorous zooplankton depend on cyanobacterial strains, zooplankton species and environmental conditions. To explore the relationship between zooplankton and cyanobacteria, we investigated the effects of Planktothrix agardhii extracts on Daphnia magna population dynamics. We designed an experiment where individuals were grown in the presence of extracts of two P. agardhii strains. We monitored daily life-history parameters of D. magna individuals subjected to microcystin-RR (MC-RR), intracellular and extracellular extracts of a microcystin-producing strain (MC-strain, PMC 75.02) and a microcystin-free strain (MC-free strain, PMC 87.02) of P. agardhii. Measured life-history parameters of D. magna were used to build population dynamics models and compute expected population growth rate, replacement rate, generation time and proportion of adult and juveniles at demographic equilibrium. Results show that MC-RR tends to slow the life history (reduced growth rate and larger proportion of adults). In contrast, intracellular extracts of the two strains tend to accelerate the life history (increased growth rate, decreased generation time and lower proportion of adults). Extracellular extracts produce the same trends as the intracellular extracts but to a lesser extent. However, the MC-strain has stronger effects than the MC-free strain. Interestingly, extracellular extracts of the MC-free strain may have effects comparable to pure MC-RR. Moreover, in the presence of MC-RR and both cyanobacterial extracts, the daily fecundities present a cyclic pattern. These results suggest that MC-RR and unknown metabolites of cyanobacterial extracts have negative effects on D. magna reproduction processes such as those observed with endocrine-disruptive molecules.

Host-parasite interactions are shaped by the co-evolutionary arms race of parasite virulence, transmission success as well as host resistance and recovery. The virulence and fitness of parasites may depend on host condition, which is mediated, for instance, by host energy constraints. Here, we investigated to what extent stress imposed by predation threat and environmental pollutants influences host-parasite interactions. We challenged the crustacean host Daphnia magna with the sterilizing bacterial endoparasite Pasteuria ramosa and simultaneously exposed the host to fish kairomones, the pesticide carbaryl or both stressors. While parasite virulence, measured as impact on host mortality and sterilization, increased markedly after short-term pesticide exposure, it was not influenced by predation threat. Parasite fitness, measured in terms of produced transmission stages, decreased both in fish and pesticide treatments. This effect was much stronger under predation threat than carbaryl exposure, and was attributable to reduced somatic growth of the host, presumably resulting in fewer resources for parasite development. While the indirect impact of both stressors on spore loads provides evidence for host condition-dependent parasite fitness, the finding of increased virulence only under carbaryl exposure indicates a stronger physiological impact of the neurotoxic chemical compared with the effect of a non-toxic fish kairomone.

To understand the relationship between mode of transmission and virulence, we investigated 2 microsporidian parasites in Daphnia magna laboratory populations. Pleistophora intestinalis is only transmitted horizontally, while Tuzetia sp. is transmitted vertically with high efficiency from mothers to parthenogenetic male and female offspring. We were not able to transmit Tuzetia horizontally in the laboratory. Tuzetia reduces host life-time reproductive success and host survival to a much greater extent than does P. intestinalis. Tuzetia-infected hosts were rapidly outcompeted by uninfected hosts. We suspect that Tuzetia infections may persist in natural populations by an as yet undiscovered horizontal transmission. It is possible that an alternate host species may be involved. We present a mathematical model to analyse the conditions for the persistence of a parasite with perfect vertical and an additional degree of horizontal transmission. We show that horizontal and vertical transmission contribute additively to a parasite's ability to invade and persist. Since the fitness contribution of horizontal transmission increases with population size, only very low transmission probabilities per host-to-host contact are necessary for the persistence of parasites occurring in large populations such as those commonly found for Daphnia. The detection of such low rates of horizontal transmission appears unlikely under laboratory conditions since the necessary number of host-to-host (or host-to-spore) contacts is not feasible. We review mechanisms that can maintain vertically transmitted parasites in nature.

Phytoplankton dynamics in freshwater ecosystems can be faced as a meaningful natural stressor for its main grazers, and henceconstrain bottom-up interactions within the trophic structure of the food-web. Filter-feeding zooplankters, such as Daphnia, copewith resources fluctuations by adjusting their life-history as a function of balances in the energy allocation rules. Additionally,Daphnia populations often withstand interspecific competition within coexisting related species, and it is generally assumedthat body size is a strategic trait conditioning exploitative ability and the related competitive advantage of species. The lifehistoryresponses of one Daphnia magna population and three populations of Daphnia cf longispina to a discrete gradient of algaeconcentration were assessed. This experiment was focused on (i) how these distinct populations regulate life-history when feedingover low, intermediate and high food-levels; and on (ii) whether the large-bodied D. magna can be better in exploiting resourcesover the gradient than can the smaller D. cf longispina populations. We found evidences that body size might not be the singletrait influencing the exploitative ability of related species. The D. cf longispina populations were often better than D. magna inexploiting different levels of resources, and remarkable differences in fitness were found between these similar-sized individualswithin food-level; indeed, these differences were frequently of higher scale than those found between the larger D. magna and anyof the D. cf longispina populations.

We studied the Cladocera in four shallow brackish lakes at El Hondo and Salinas de Santa Pola in Mediterranean Spain fromFebruary 1999 to June 2000, sampling each month. We recorded five species : Daphnia magna, D. pulex, D. longispina,Simocephalus exspinosus and Moina brachiata. D. magna was the most abundant species and the only one present all yearround. Total abundance of Cladocera varied significantly between wetlands and months, and was highest in the largest, deepest,least saline wetlands. For a given wetland and month, abundance increased at points of greater depth or lower salinity. Speciesrichness of Cladocera was highest in those seasons and lakes where salinity was lowest, and in those seasons and lakes wherecladoceran abundance was highest. These wetlands hold a particularly high number of waterbird species, but a very low numberof Cladocera species. Ready dispersal via waterbirds does not appear to promote cladoceran diversity in this system, owingto local conditions such as high salinity. We suggest that the conservation value of wetlands for birds is not a good predictor ofvalue for Cladocera.

Periods of low host density impose a constraint on parasites with direct transmission, challenging their permanence in the system. The microsporidium Octosporea bayeri faces such constraint in a metapopulation of its host, the cladoceran Daphnia magna, where ponds frequently lose their host population due to ponds drying out in summer and freezing in winter. We conducted experiments aimed to investigate the mechanisms of transmission of O. bayeri, and discuss how these mechanisms could contribute to the parasite's permanence in the system. Spores accumulate in the fat cells and the ovaries of the host, and vary in morphology, possibly corresponding to 3 different spore types. Horizontal transmission occurred through the release of spores from dead hosts, with the proportion of infected hosts depending on the spore dose. Further, spores are able to persist outside the host both in dry and wet conditions. Vertical transmission occurred to both parthenogenetic and sexual offspring. The former were invariably infected, while the sexually produced resting eggs (=ephippia) had a less efficient transmission. The parasite may be carried by the ephippia, and thus disperse to new ponds together with the host. Together, these mechanisms may allow the parasite to endure periods of harsh environmental conditions both outside and inside the host.

The rules governing the allocation of available resources to varying physiological processes are evaluated in three physiologically-based models of individual Daphnia. In laboratory experiments using a single clone, subjected to varying regimes of food deprivation, growth was found to vary inversely with food level, ceasing in the absence of food, implying that growth allocation is derived directly from food. Reproductive investment was reduced under food deprivation, ceasing at low food levels. By exposing juvenile and adult daphnia to varying food deprivation regimens during different periods within their intermoult interval, it was shown that instar durations vary as a function of body size and food availability independent of the age of the animal. For the first time, the role of variable instar duration, which critically influences physiological processes such as growth, moulting and reproduction in adult females, that has been neglected by existing Daphnia models, is explicitly incorporated in a physiological allocation model. The consequent model provides a simplified framework for modelling the consequences of food deprivation in cladocerans which has important application in population modelling and environmental risk assessment.

Ecotoxicological studies with cladocerans frequently assess the individual growth of the organisms. Measures upon living organisms should be avoided once they can lead to their death or affectation, therefore constraining the results. In this way, and considering that cladocerans species go through series of moults along their life cycle, measures upon the moult are preferred in almost all the studies. Thus, the establishment of allometric relations is required to enable feasible estimations of diverse growth parameters. Exopodite and total body length of two autochthonous (Daphnia longispina; Ceriodaphnia pulchella) and two standard (Daphnia magna; Ceriodaphnia dubia) living daphnids were registered. Regression analysis was applied in order to establish a relation between the two allometric measurements in each species. As high R-squared values were reached in all species, the obtained equations can be used as reference for estimations of body length in face of the «moult» exopodite length measure. Consequently, this work provides a support reference for ecotoxicological studies performed in lab with these species.

The fresh-water crustacean Daphnia magna may acquire an infection with the microsporidium Octosporea bayeri either by ingesting spores from the water (horizontally), or directly from its mother (vertically). Due to differences in the time and mechanisms of transmission, horizontal and vertical infections may lead to differences in the growth of the parasite within the host. This may influence parasite virulence, transmission to new hosts, and, consequently, epidemiology and evolution. Here we describe the within-host dynamics of 3 spore-types of O.bayeri from infections that were acquired either horizontally or vertically. In all treatments the number of spores increased exponentially until spore density reached a plateau, suggesting density-dependent within-host growth. The spore types seen differ in their growth dynamics, suggesting different roles in the parasite life-cycle. Horizontally-infected hosts harboured significantly fewer spores than vertically-infected hosts. Further, host survival was affected by infection route, with mortality being higher in horizontal infections than in vertical infections. Our results suggest that different routes of infection have an immediate effect on within-host parasite growth and thus on parasite fitness and epidemiology.

The biochemical composition of Daphnia magna Straus, the dominant planktonic crustacean of the waste stabilisation pond of Differdange (Grand-Duchy of Luxembourg), was quantitatively determined from October 1993 to July 1994. Over the sampling period, the average composition (mean ± S.D.) was 271 ± 64 mg proteins.g-1 dry weight (DW), 100 ± 28 mg lipids.g-1 (DW), 96 ± 58 μg carotenoids.g-1 (DW), 49 ± 14 mg chitin.g-1 (DW) and 125 ± 78 mg ash.g-1 (DW). The seasonal variations of the biochemical composition were related to several ecological variables (water temperature, dissolved oxygen concentration, pH, water transparency, chlorophyll a concentration and D. magna biomass). The chitin content was positively correlated to the water temperature as a result of the strong influence of this later variable on the moulting rate of the daphnids and, subsequently, on the chitin synthesis by these organisms. The carotenoid content was positively correlated to the water transparency as a result of their photoprotective role in daphnids. The fluctuations of the lipid, protein and ash levels in D. magna depended to the food availability. Despite a seasonal variation in the biochemical composition, D. magna appeared to have adequate lipid and protein levels to be used in aquaculture. Its carotenoid content is similar to fish meals used to color salmonid flesh and these organisms could be used for this purpose. The prospect of using D. magna for chitin extraction is worth considering with respect to its significant chitin content, especially if highly valuable applications are aimed.

Deux crustacés d'eau douce : Gammarus gauthieri et Daphnia magna sont utilisés pour évaluer la toxicité létale des extraits aqueux de sédiments. Quatorze échantillons de sédiments provenant de 4 cours d'eau marocains sont étudiés. Seuls les sédiments des stations situées en aval des rejets des eaux usées se sont révélés toxiques. Les CL50 24h montrent la grande sensibilité du test « G. gauthieri ». Ce dernier a permis de classer les cours d'eau selon leur degré de contamination : Oued Boufekrane > O. Tensift > O. Martil > O. Sebou. Le dosage de trois métaux (Cu, Mn et Zn) par Spectrophométrie d'Absorption Atomique dans les extraits de sédiments de l'O. Boufekrane montre une augmentation significative en Cu et en Zn dans les extraits toxiques. Dans les sédiments de ces stations, nous avons constaté également de fortes augmentations de la matière organique et de la conductivité. Le test G. gauthieri est une méthode utile pour comparer et classer la toxicité des extraits de sédiments contaminés.

L'étude des transferts direct et trophique du 106Ru chez Daphnia magna Strauss a été conduite dans le cadre d'un programme de recherches visant à expliquer et modéliser la distribution de ce radioélément au sein des écosystèmes d'eau douce. L'accumulation et l'élimination du 106Ru par des populations hétérogènes de daphnies, contaminées par un apport aigu de RuCl3 dans l'eau, se déroulent en deux phases pouvant être décrites à l'aide d'un modèle bicompartimental. Ce transfert est caractérisé par un facteur de concentration de 130, et une élimination très rapide se traduisant par deux périodes biologiques, la première (Tb1), de quelques minutes, et la seconde (Tb2), de trente heures. La contamination de Daphnia magna par voie alimentaire a été effectuée avec deux espèces d'algues unicellulaires planctoniques, Scenedesmus communis Hegewald et Chlorella pyrenoidosa Chick, distribuées quotidiennement sous forme de rations préalablement contaminées. Dans ce cas, la cinétique d'accumulation du 106Ru ne présente qu'une seule phase. L'analyse des facteurs de transfert trophique obtenus pour les deux espèces d'algues (0,107 et 0,013 respectivement avec Scenedesmus et Chlorella) montre que la digestion des cellules n'intervient pas dans l'accumulation du radionucléide. L'élimination du 106Ru ainsi accumulé est identique dans son intensité et sa rapidité à celle observée à la suite de la contamination par transfert direct. Les périodes biologiques calculées indiquent que quel que soit le vecteur de son transfert, la présence du 106Ru dans Daphnia magna n'est que transitoire.