Preface: Patterns and processes of meiofauna in freshwater ecosystems.

Never heard of harpacticoids, ostracods, gastrotrichs or microturbellarians? This is no surprise, they are so tiny! Yet these taxa and many others more famous (nematodes, rotifers, or tardigrades) show complex behaviours and extraordinary physiologies that allow them to colonize inland waters worldwide. This exuberant fauna is better known as the meiofauna (or meiobenthos). Meiofaunal organisms have been fascinating study objects for zoologists since the seventeenth century and recent research has demonstrated their intermediate role in benthic food webs. This special issue highlights how meiofauna can help freshwater ecologists to describe and predict species distribution patterns, to assess production of biomass and trait functions relationships, as well as to examine the trophic links between microscopic and macroscopic worlds and to better understand species' resilience to environmental extremes. Overall, meiofaunal organisms are bridging scales, and as such they deserve better integration to develop more comprehensive concepts and theories in ecology.

Selective grazing behaviour of chironomids on microalgae under pesticide pressure.

The herbicide diuron and the insecticide imidacloprid are amongst the most frequently detected pesticides in French rivers, and each is known to affect many aquatic organisms. However, the question of whether and how both pesticides together might induce multi-stress conditions, which could induce indirect effects such as the modification of biological interactions within freshwater microbial communities has not received much attention. This study was undertaken to determine the effect of diuron and imidacloprid alone and in combination on the feeding behaviour of chironomid larvae. An initial experiment measured the impact of the different contamination conditions at environmental concentrations (5 µg L-1 for each pesticide) on the grazing rate of chironomids on three microalgae species, independently. Two diatom species, Gomphonema gracile (two different morphotypes: normal and teratogen) and Planothidium lanceolatum, and one green alga Desmodesmus sp. were offered as food, during 24 h. Chironomids grazing rates varied according to the pesticide and algae species. Indeed, diuron impacted algae more strongly and probably affected their palatability, leading chironomids to increase grazing pressure on less nutritionally interesting algae. Imidacloprid, by targeting insect larvae, increased or inhibited their grazing capacity depending on the food source. In a second experiment (cafeteria design), the food selectivity of chironomids on previous algae was determined under similar contamination conditions during 4 h: under diuron, larvae switched equally between the microalgae and were as mobile as in the control without pesticide. However, imidacloprid and the pesticide mixture condition altered chironomid movements and grazing behaviour. By investigating the impact of an herbicide and an insecticide, alone and in combination, on the responses of food (algae growth rate) and biological (mortality) and behavioural (mobility, food selection) responses of chironomid larvae, this study provided new insights on the direct and indirect effects of pesticide contamination on a simplified trophic web.

Flatworm mucus as the base of a food web.

BACKGROUND: By altering their habitats, engineering species can improve their own fitness. However, the effect of this strategy on the fitness of coexisting species or on the structure of the respective food web is poorly understood. In this study, bacteria and bacterivorous nematodes with short (Caenorhabditis elegans) and long (Plectus acuminatus) life cycles were exposed to the mucus secreted by the freshwater flatworm Polycelis tenuis. The growth, reproduction, and feeding preferences of the nematodes in the presence/absence of the mucus were then determined. In addition, confocal laser scanning microscopy (CLSM) was used to examine the structural footprint of the mucus and the mucus colonization dynamics of bacteria and protozoans. RESULTS: Mucus exposure resulted in a greater reproductive output in P. acuminatus than in C. elegans. In a cafeteria experiment, both nematode species were attracted by bacteria-rich patches and were not deterred by mucus. CLSM showed that the flatworms spread a layer of polysaccharide-rich mucus ca. 15 µm thick from their tails. Subsequent colonization of the mucus by bacteria and protozoans resulted in an architecture that progressively resembled a complex biofilm. The presence of protozoans reduced nematode reproduction, presumably due to competition for their bacterial food supply. CONCLUSION: Animal secretions such as mucus may have broader, community-level consequences and contribute to fueling microbial food webs.

Effects of a broad range of experimental temperatures on the population growth and body-size of five species of free-living nematodes.

The temperature-size rule postulates that the growth rates of ectotherms increase under rising temperatures, while the sizes of these organisms at maturity decrease. However, the upper temperature-tolerance range is also typically represented by a metabolic tipping point after which growth suddenly ceases. Free-living nematodes are important members of ecosystems, but little is known about their thermal tolerance. In the present study we measured the population growth rates and body-size distributions of five species of free-living bacterivorous nematodes exposed in the laboratory to a broad range of temperatures. This allowed a determination of their different thermal tolerance ranges, even of closely related species, including Plectus acuminatus (thermal optimum of 20-25 °C) and P. cf. velox (10-15 °C). With the exception of Acrobeloides nanus, which had the broadest thermal tolerance range, the population growth of the other species declined between 25 and 30 °C. Our results were consistent with the temperature-size rule, as the body-size of the tested species at maturity decreased with increasing temperature. This reduction was accompanied by a smaller number of eggs carried by mature females. Although our study was purely experimental, it suggests that heat waves or other alterations in the thermal regime affect the population dynamics and body-size structure of nematode communities in the field.

Analyzing life-history traits and lipid storage using CARS microscopy for assessing effects of copper on the fitness of Caenorhabditis elegans.

Lipid storage provides energy for cell survival, growth, and reproduction and is closely related to the organismal response to stress imposed by toxic chemicals. However, the effects of toxicants on energy storage as it impacts certain life-history traits have rarely been investigated. Here, we used the nematode Caenorhabditis elegans as a test species for a chronic exposure to copper (Cu) at EC20 (0.50 mg Cu/l). Effects on the fatty acid distribution in C. elegans body were determined using coherent anti-Stokes Raman spectroscopy (CARS) to link population fitness responses with individual ecophysiological responses. Cu inhibited nematode reproductive capacity and offspring growth in addition to shortening the lifespan of exposed individuals. In adult nematodes, Cu exposure led to significant reduction of lipid storage compared to the Cu-free control: Under Cu, lipids filled only 0.5% of the nematode body volume vs. 7.5% in control nematodes, lipid droplets were on average 74% smaller and the number of tiny lipids (0-10 µm2) was increased. These results suggest that (1) Cu has an important effect on the life-history traits of nematodes; (2) the quantification of lipid storage can provide important information on the response of organisms to toxic stress; and (3) CARS microscopy is a promising tool for non-invasive quantitative and qualitative analyses of lipids as a measure of nematode fitness.

Depth-Related Effects on a Meiofaunal Community Dwelling in the Periphyton of a Mesotrophic Lake.

Periphyton is a complex assemblage of micro- and meiofauna embedded in the organic matrix that coats most submerged substrate in the littoral of lakes. The aim of this study was to better understand the consequences of depth-level fluctuation on a periphytic community. The effects of light and wave disturbance on the development of littoral periphyton were evaluated in Lake Erken (Sweden) using an experimental design that combined in situ shading with periphyton depth transfers. Free-living nematodes were a major contributor to the meiofaunal community. Their species composition was therefore used as a proxy to distinguish the contributions of light- and wave-related effects. The periphyton layer was much thicker at a depth of 30 cm than at 200 cm, as indicated by differences in the amounts of organic and phototrophic biomass and meiofaunal and nematode densities. A reduction of the depth-level of periphyton via a transfer from a deep to a shallow location induced rapid positive responses by its algal, meiofaunal, and nematode communities. The slower and weaker negative responses to the reverse transfer were attributed to the potentially higher resilience of periphytic communities to increases in the water level. In the shallow littoral of the lake, shading magnified the effects of phototrophic biomass erosion by waves, as the increased exposure to wave shear stress was not compensated for by an increase in photosynthesis. This finding suggests that benthic primary production will be strongly impeded in the shallow littoral zones of lakes artificially shaded by construction or embankments. However, regardless of the light constraints, an increased exposure to wave action had a generally positive short-term effect on meiofaunal density, by favoring the predominance of species able to anchor themselves to the substrate, especially the Chromadorid nematode Punctodora ratzeburgensis.

Life-History Traits of the Model Organism Pristionchus pacificus Recorded Using the Hanging Drop Method: Comparison with Caenorhabditis elegans.

The nematode Pristionchus pacificus is of growing interest as a model organism in evolutionary biology. However, despite multiple studies of its genetics, developmental cues, and ecology, the basic life-history traits (LHTs) of P. pacificus remain unknown. In this study, we used the hanging drop method to follow P. pacificus at the individual level and thereby quantify its LHTs. This approach allowed direct comparisons with the LHTs of Caenorhabditis elegans recently determined using this method. When provided with 5×10(9) Escherichia coli cells ml(-1) at 20°C, the intrinsic rate of natural increase of P. pacificus was 1.125 (individually, per day); mean net production was 115 juveniles produced during the life-time of each individual, and each nematode laid an average of 270 eggs (both fertile and unfertile). The mean age of P. pacificus individuals at first reproduction was 65 h, and the average life span was 22 days. The life cycle of P. pacificus is therefore slightly longer than that of C. elegans, with a longer average life span and hatching time and the production of fewer progeny.

Free-living nematodes in the freshwater food web: a review.

Free-living nematodes are well-recognized as an abundant and ubiquitous component of benthic communities in inland waters. Compelling evidence from soil and marine ecosystems has highlighted the importance of nematodes as trophic intermediaries between microbial production and higher trophic levels. However, the paucity of empirical evidence of their role in freshwater ecosystems has hampered their inclusion in our understanding of freshwater food web functioning. This literature survey provides an overview of research efforts in the field of freshwater nematode ecology and of the complex trophic interactions between free-living nematodes and microbes, other meiofauna, macro-invertebrates, and fishes. Based on an analysis of the relevant literature and an appreciation of the potential of emerging approaches for the evaluation of nematode trophic ecology, we point out research gaps and recommend relevant directions for further research. The latter include (i) interactions of nematodes with protozoans and fungi; (ii) nonconsumptive effects of nematodes on microbial activity and the effects of nematodes on associated key ecosystem processes (decomposition, primary production); and (iii) the feeding selectivity and intraspecific feeding variability of nematodes and their potential impacts on the structure of benthic communities.

Predator effects on a detritus-based food web are primarily mediated by non-trophic interactions.

Predator effects on ecosystems can extend far beyond their prey and are often not solely lethally transmitted. Change in prey traits in response to predation risk can have important repercussions on community assembly and key ecosystem processes (i.e. trait-mediated indirect effects). In addition, some predators themselves alter habitat structure or nutrient cycling through ecological engineering effects. Tracking these non-trophic pathways is thus an important, yet challenging task to gain a better grasp of the functional role of predators. Multiple lines of evidence suggest that, in detritus-based food webs, non-trophic interactions may prevail over purely trophic interactions in determining predator effects on plant litter decomposition. This hypothesis was tested in a headwater stream by modulating the density of a flatworm predator (Polycelis felina) in enclosures containing oak (Quercus robur) leaf litter exposed to natural colonization by small invertebrates and microbial decomposers. Causal path modelling was used to infer how predator effects propagated through the food web. Flatworms accelerated litter decomposition through positive effects on microbial decomposers. The biomass of prey and non-prey invertebrates was not negatively affected by flatworms, suggesting that net predator effect on litter decomposition was primarily determined by non-trophic interactions. Flatworms enhanced the deposition and retention of fine sediments on leaf surface, thereby improving leaf colonization by invertebrates - most of which having strong affinities with interstitial habitats. This predator-induced improvement of habitat availability was attributed to the sticky nature of the mucus that flatworms secrete in copious amount while foraging. Results of path analyses further indicated that this bottom-up ecological engineering effect was as powerful as the top-down effect on invertebrate prey. Our findings suggest that predators have the potential to affect substantially carbon flow and nutrient cycling in detritus-based ecosystems and that this impact cannot be fully appreciated without considering non-trophic effects.

Selective feeding of bdelloid rotifers in river biofilms.

In situ pigment contents of biofilm-dwelling bdelloid rotifers of the Garonne River (France) were measured by high performance liquid chromatography (HPLC) and compared with pigment composition of surrounding biofilm microphytobenthic communities. Among pigments that were detected in rotifers, the presence of carotenoids fucoxanthin and myxoxanthophyll showed that the rotifers fed on diatoms and cyanobacteria. Unexpectedly, while diatoms strongly dominated microphytobenthic communities in terms of biomass, HPLC results hinted that rotifers selectively ingested benthic filamentous cyanobacteria. In doing so, rotifers could daily remove a substantial fraction (up to 28%) of this cyanobacterial biomass. The possibility that the rotifers hosted symbiotic myxoxanthophyll-containing cyanobacteria was examined by localisation of chlorophyll fluorescence within rotifers using confocal laser scanning microscopy (CLSM). CLSM results showed an even distribution of quasi-circular fluorescent objects (FO) throughout rotifer bodies, whereas myxoxanthophyll is a biomarker pigment of filamentous cyanobacteria, so the hypothesis was rejected. Our results also suggest that rotifers converted ß-carotene (provided by ingested algae) into echinenone, a photoprotective pigment. This study, which is the first one to detail in situ pigment contents of rotifers, clearly shows that the role of cyanobacteria as a food source for meiobenthic invertebrates has been underestimated so far, and deserves urgent consideration.