SEXUAL DIMORPHISM IN CHIRONOMUS-RIPARIUS (MEIGEN) - IMPACT ON INTERPRETATION OF GROWTH IN WHOLE-SEDIMENT TOXICITY TESTS

Sexual dimorphism in fourth-instar larvae and adults was studied for C hironomus riparius. Wet weight of fourth-instar male chironomids was o n average 29.4% lower than that of fourth-instar female chironomids at day 10 post-hatch of eggs, when organisms were reared individually in 250-ml beakers with 60 g sediment. This weight differential continued to the adult stage and was enhanced with males weighing 39.8% less th an females (dry weight). When animals were reared in groups of 15 larv ae per beaker, differences between the sexes in larval wet weight were not statistically significant, but males were still 7.4019 smaller th an females; however, adult males weighed an average of 42.7% less than females upon emergence. Stage of development (fourth instar) was conf irmed by head capsule measurement; no significant differences in head capsule widths were detected between the sexes. Animals reared alone, regardless of sex, weighed more than animals reared as a group, indica ting that initial larval densities and size of bioassay container can significantly affect larval growth. The probability of making a type I error in sediment toxicity tests due to sexual dimorphism in weight w as estimated to be only 3% when dimorphism was most enhanced, that is, for animals reared individually. The effect of dimorphism on data int erpretation when animals are reared in groups of 15 to 50 animals per container is thought to be minimal. It is recommended that both larval weight and head capsule width (millimeters) be measured as end points in sediment toxicity tests to differentiate reduced growth from retar dation of instar development.