Distribution, Seasonal Occurrence and Vertical Pattern of Tobrilus Gracilis (Bastian, 1865) and T. Medius (Schneider, 1916)

The autecology of Tobrilus gracilis (n = 1705 individuals) and T. medius (n = 298 individuals), two common nematode species of Lake Konigssee, was studied throughout the year 1986. T. gracilis comprised 3.5% of all nematodes in the littoral, 6.3% in the littoriprofundal and 2.7% in the profundal. T. medius comprised 0.3% of all nematodes in the littoral, 1.4% in the littoriprofundal and 3.3% in the profundal. The species contributed to a considerable portion of nematode biomass in Lake Konigssee. Mean yearly biomass of T. gracilis was greatest at 1 m depth (0.52 mg/100 cm 2 ww; 65% of total nematode biomass) and of T. medius at 60 m depth (0.2 mg/100 cm 2 ww; 49% of total nematode biomass). Density of T. gracilis peaked twice a year in 1 m water depth (winter and summer) and once in deeper regions (winter/spring). T. medius peaked once a year in 10 m depth (spring), showed no distinct maximum in 30 m depth and reached higher numbers in 60 m depth in winter. There was considerable water depth- and time-dependent variation in sex ratios of T. gracilis. The main reproductive period seemed to be different for the investigated depths. The proportion of individuals of T. gracilis in the top sediment layer was about 57% and of T. medius about 72%. Juveniles and adults of T. gracilis were partially segregated in the sediment column. Morphometric data of both species revealed sexual dimorphism in size, body shape and weight.