Morphological Abnormalities in Chironomidae (Diptera) Larvae as Measures of Toxic Stress in Freshwater Ecosystems: Indexing Antennal Deformities in Chironomus Meigen

Abstract

An indexing scheme for classifying antennal deformations in the genus Chironomus is presented to illustrate the potential for using deformities in chironomid larvae as a biological screening technique for the detection and assessment of contaminants in aquatic ecosystems. The scheme utilizes six basic steps to determine an index of morphological response (IMR) for each individual larvae; these steps evaluate (1) the loss of genuine segments and/or the presence of questionable segments, (2) the reduction in length of the antenna, (3) the displacement or loss of the ring organ, (4) the fusion of the apex of the basal segment, (5) the displacement of accessory organs, and (6) the presence of structures of unknown homology. The values for IMR are summed and divided by the total number of larvae examined to calculate an index of severity of antennal deformation (ISAD) for the sample population. Comparison of the incidence (percent) and severity of antennal deformations (ISAD) in Chironomus populations from Tobin Lake (contaminated) and Last Mountain Lake (comparatively uncontaminated) ecosystems in south-central Saskatchewan show that both indices are higher in Tobin Lake. Improved mounting procedures are presented in which specimens are fully flattened to improve visibility of fine detail and enhance analytical consistency. Some results from preliminary dose–response experiments using 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (DDE) are presented. These results suggest that larval antennae are very sensitive to low-level, chronic exposure to toxic chemicals. New receptor organs are also described which make the morphology of the antennae of Chironomini and Tanytarsini directly comparable.