Tubifex tubifex (Müller, 1774) (Oligochaeta, Tubificidae) in the Profundal of Estonian Lakes

Abstract Tubifex tubifex (Müller, 1774) was found in the profundal of 23 Estonian lakes out of the 187 lakes where oligochaetes were met in the deeper zone: in 4 cases as the single tubificid species but usually together with Potamothrix hammoniensis (Michaelsen) dominating in almost all lakes. T. tubifex was abundant and large in size in the profundal of a few lakes with mesotrophic features but scarce and dwarfish (if not lacking) in the common eutrophic ones. Observations in aquaria demonstrate that its occurrence in eutrophic lakes is not limited by the bacterial food. Some kind of competition with P. hammoniensis can be supposed but no direct evidence exists. Recent colonization of the profundal of an eutrophic lake by T. tubifex, probably from the littoral of the same lake, was observed.