FLUCTUATING ASYMMETRY AND MENTUM GAPS IN POPULATIONS OF THE MIDGE CHIRONOMUS-RIPARIUS (DIPTERA, CHIRONOMIDAE) FROM A METAL-CONTAMINATED RIVER

The developmental stability of both metal-exposed and nonexposed Chiro nomus riparius populations from the lowland River Dommel was investiga ted using fluctuating asymmetry (FA) and the incidence of mentum gaps. It was hypothesized that larval development was affected by the influ x of metals, directly by chemical stress, as well as through inbreedin g of metal-adapted and nonadapted specimens. Morphological parameters were therefore assessed in field-collected larvae and in clean, labora tory-cultured, first-generation (F1) larvae. Fluctuating asymmetry val ues and mentum gap incidence at contaminated field sites were signific antly higher than at clean, upstream locations. Furthermore, FA values of clean, laboratory-cultured F1 larvae generally fell to reference v alues, indicating the direct effect of metal pollution on developmenta l aberrations. Mentum gaps were not observed in clean F1 cultures. Sli ghtly elevated FA values were, however, still observed in clean F1 lar vae from polluted locations downstream from the metal input. This resi dual disturbance was thought to reflect genetic stress emerging from i nterbreeding between metal-adapted and nonadapted specimens. Fluctuati ng asymmetry and mentum gaps together serve as a useful ecotoxicologic al marker for metal stress and, when combined with in situ studies and F1 cultures, allow for analysis of the response of animal populations to spatial and temporal gradients in metal exposure.