Abstract-We present a method using the free-living nematode Caenorhabditis elegans (Maupas, 1899) to assess toxicity in liquid medium and whole-sediment setups. Test duration is 72 h; endpoints are body length, number of eggs inside worms, percentage of gravid worms, and number of offspring per worm. The effect of CdCl 2 on C. elegans in liquid-phase exposures is described as an example. Results from a field study with polluted sediments from the River Elbe (Germany) suggest that nematodes may be useful organisms in assessing toxicity of sediments in the whole phase.

Summary 1. Three groups of laboratory experiments clarified the role of nematodes as a potential food resource for the triclad Dugesia gonocephala. The first group measured the functional response of adult D. gonocephala feeding on juvenile or adult Caenorhabditis elegans. The feeding rates of D. gonocephala on adult and juvenile C. elegans followed a type II functional response. The maximum number of adult nematodes and juvenile nematodes eaten by a single D. gonocephala individual within 3 h was 94 and 197 nematodes, respectively. 2. A second group of microcosm experiments investigated the effect of D. gonocephala on the density and the vertical distribution of a nematode community in fine sand. The following treatments were performed: (i) microcosms with 400 nematodes and (ii) microcosms with 400 nematodes and one D. gonocephala. After 5 days, nematodes as a group, as well as the dominant species Tobrilus pellucidus and Trischistoma monohystera, showed no significant difference in vertical patterns between the treatments with and without D. gonocephala. 3. The third group of experiments determined whether grain size of the sediment (sand, fine gravel and coarse gravel) altered the ability of D. gonocephala to consume adult C. elegans. Sand and fine gravel reduced the predation effectiveness of D. gonocephala by 100%, whereas the predator consumed nematodes in coarse gravel (19 nematodes within 3 h).

The temporal distribution of the nematode fauna in the upper 5 cm sediment of the streambed of a small submountain carbonate stream (Krähenbach, Germany) was investigated over a 1-year period. The total nematode density was low during autumn and winter and reached a maximum value of 84 individuals=10 cm 2 in summer. The fresh biomass of nematodes ranged from 3.6 to 44.2 ¹g=10 cm 2 . Deposit-feeders (65% of total fauna) and chewers (21%) were the two dominant feeding types. Of the 1027 nematodes examined, 9% were gravid females, 35% females, 2% males and 54% juveniles. At least 71 nematode species were identi ed. The relative low value of the Shannon Wiener index of diversity (2.61) is not the consequence of a low number of species, but of the presence of three species with a much higher frequency (57% of total fauna) than the others. The dominant species are Eumonhystera liformis (Bastian, 1865)/vulgaris (de Man, 1880), Tobrilus cf. pellucidus (Bastian, 1865) and E. simplex (de Man, 1880). The seasonal pattern of E. liformis/vulgaris showed a minimum density in summer and a maximum density in spring and is similar to the abundance variation of all deposit-feeders. The seasonal pattern of T. cf. pellucidus showed a minimum density in winter and a maximum density in summer and is similar to the abundance variation of all chewers. Throughout the year, nematodes were found in both layers (analysed depths: 0-2 cm, 2-5 cm), but higher mean annual densities occurred in the upper layer (68% of all nematodes). The preference for the upper cm of sediment was independent of feeding-type, age structure or species. The calculation of the Maturity Index and of the ratio of Secernentea/Adenophorea indicate a 'high status' of water quality at the analysed sampling site.

We present a method using the free‐living nematode Caenorhabditis elegans (Maupas, 1899) to assess toxicity in liquid medium and whole‐sediment setups. Test duration is 72 h; endpoints are body length, number of eggs inside worms, percentage of gravid worms, and number of offspring per worm. The effect of CdCl2 on C. elegans in liquid‐phase exposures is described as an example. Results from a field study with polluted sediments from the River Elbe (Germany) suggest that nematodes may be useful organisms in assessing toxicity of sediments in the whole phase.

No abstract