Acute tolerance tests with the oligochaetes Nais communis (Naididae) and Ilyodrilus frantzi (Tubificidae)

96-h LC50's were determined for Nais communis Piguet and Ilyodrilus frantzi Brinkhurst in relation to selected pollutants (Hg, NaPCP) and environmental factors (pH, temperature, salinity). The naidid N. communis was consistently less tolerant than the tubificid I. frantzi in all test treatments. Comparison with previously determined sensitivities for nine tubificids and one lumbriculid shows that I. frantzi is in the mid to upper tolerance range, while N. communis is almost consistently the least tolerant of all species tested.