Background: The free-living nematode Caenorhabditis elegans is the predominant model organism in biological research, being used by a huge number of laboratories worldwide. Many researchers have evaluated life-history traits of C. elegans in investigations covering quite different aspects such as ecotoxicology, inbreeding depression and heterosis, dietary restriction/supplement, mutations, and ageing. Such traits include juvenile growth rates, age at sexual maturity, adult body size, agespecific fecundity/mortality, total reproduction, mean and maximum lifespan, and intrinsic population growth rates. However, we found that in life-cycle experiments care is needed regarding protocol design. Here, we test a recently developed method that overcomes some problems associated with traditional cultivation techniques. In this fast and yet precise approach, single individuals are maintained within hanging drops of semi-fluid culture medium, allowing the simultaneous investigation of various life-history traits at any desired degree of accuracy. Here, the life cycles of wild-type C. elegans strains N2 (Bristol, UK) and MY6 (Münster, Germany) were compared at 20°C with 5 × 10 9 Escherichia coli ml -1 as food source.

Meiofaunal organisms in the periphyton of stony hard-substrates (epilithon) were studied in three Swedish lakes with different trophic states (oligo-, meso-and eutrophic)

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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.

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Temporal fluctuations in the diversity, sex ratio, age, and species composition of free-living nematodes inhabiting the periphyton of stony hard-substrates were determined in three Swedish lakes differing in trophic state. Over a two-year sampling period, different temporal patterns were observed. In the oligotrophic lake, nematode diversity showed moderate fluctuations and peaked in early summer. Diversity in meso-and eutrophic lakes showed the opposite pattern. Species composition changed along seasons, enabling to identify typical early and late season assemblages. This succession was linked to different reproduction periods of dominant species. The age structure and sex ratio of periphytic nematode communities varied both temporally and between lakes according to species traits. As an example, the male/female ratio increased with increasing lake trophic state. This trend reflects the decrease of partenogenetically reproducing species with increasing lake trophic state, and may be further considered as an indicator of lake trophic state.