Distribution and Sex Ratio of Ethmolaimus Pratensis De Man, 1880 (Nematoda, Chromadorida) in an Oligotrophic Lake

The distribution of Ethmolaimus pratensis (n = 1627 individuals), a common nematode species of Konigssee, was studied throughout 1986. E. pratensis comprised 3.5% of all nematodes in the littoral, 3.1% in the littoriprofundal and 5.5% in the profundal. The species contributed to a considerable portion of nematode biomass in Konigssee with mean annual biomass being highest at 1 m depth (0.14 mg/100 cm 2 ww; 17% of total nematode biomass) and 2 m depth (0.13 mg/100 cm 2 ww; 22% of total nematode biomass). At 190 m depth it was 40% of total nematode biomass. Population density of E. pratensis peaked three times a year at 1 m and 2 m water depth (spring, summer, winter) and twice in deeper regions. There was considerable water depth and time dependent variation in sex ratios of E. pratensis. In littoral and littoriprofundal zones the sex ratio was about 1 : 1, while in the profundal males were very rare. About 48% of E. pratensis individuals were found in the top sediment layer and the percentage increased with increasing water depth. Juveniles and adults of E. pratensis were partially segregated in the sediment column. Morphometric data revealed negligible sexual dimorphism in size, body shape and weight.