Morphological deformities in Chironomus riparius meigen larvae after exposure to cadmium over several generations

In order to test the potential of chironomid deformities for biomonitoring, induction of morphological deformities in Chironomus riparius larvae was a ssessed after chronic exposure (static with renewal) of eggs and subsequent instars to sublethal nominal cadmium concentrations of 0, 3 (NOEC), 9 (int ermediate) and 27 (chronic (LC50)) mug Cd l(-1) during 7 to 10 generations. Deformities which could be associated with an indirect or direct cadmium e ffect were split medial mentum teeth (more frequent in 9 and 27 mug Cd l(-1 )) and premandible deformities (especially in 3 mug Cd l(-1)). The control contained more larvae with additional teeth in mentum and mandible than the metal-exposed conditions. In the 9 mug Cd l(-1) condition the frequencies of larvae with split medial mentum teeth increased in the last four generat ions, to reach 40%. The unpredictability of fluctuations of deformity frequ encies over the generations was associated with parental effects and experi mental manipulation. The deformity percentages correlated positively with t he mortalities and could be related to the induction of tolerance to cadmiu m, as was concluded on the basis of life cycle analysis in a previous paper . This experiment demonstrated a concentration-response relationship betwee n deformities and sublethal levels of cadmium. However, the observed genera tion fluctuations caution for (1) the use of single-generation experiments for defining ecotoxicological threshold values, and (2) experimentally indu ced genetical drift in multi-generation experiments.