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 assessed after chronic exposure (static with renewal) of eggs and subsequent instars to sublethal nominal cadmium concentrations of 0, 3 (NOEC), 9 (intermediate) and 27 (chronic (LC50)) mug Cd l(-1) during 7 to 10 generations. Deformities which could be associated with an indirect or direct cadmium effect 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 generations, to reach 40%. The unpredictability of fluctuations of deformity frequencies over the generations was associated with parental effects and experimental manipulation. The deformity percentages correlated positively with the mortalities and could be related to the induction of tolerance to cadmium, as was concluded on the basis of life cycle analysis in a previous paper. This experiment demonstrated a concentration-response relationship between deformities and sublethal levels of cadmium. However, the observed generation fluctuations caution for (1) the use of single-generation experiments for defining ecotoxicological threshold values, and (2) experimentally induced genetical drift in multi-generation experiments.