Can Fluctuating Asymmetry in Adult Burrowing Mayflies ( Hexagenia rigida, Ephemeroptera) be used as a Measure of Contaminant Stress?

Abstract

Fluctuating asymmetry (FA) in bilateral organisms may provide an early warning for detrimental effects of environmental stress in organisms well before populations are affected. Adult Hexagenia rigida were collected (17 June to 1 July 1996) at 6 sites around Lake Erie, representing areas with high (Monroe, MI; Colchester, ON; Middle Sister Island, ON) and low (Windsor, ON; Kingsville, ON; Catawba, OH) exposure to PCB contaminants. Balsam Lake, ON served as a reference site. Initially 27 structures were examined on the left and right sides of 30 males from both the most contaminated (Monroe) and least contaminated (Balsam Lake) sites using image analysis. Traits were autocorrelated; therefore, 7 independent traits were identified using principal component analysis. It was anticipated that there would be greater variation (i.e., greater FA) in organisms collected from sites known to exhibit high PCB body burdens than in organisms having low levels of contaminants. However, there were no significant differences in FA for male or female adults of H. rigida among the sites for the morphometric traits measured. There were significant differences in FA for two traits in H. rigida males among years (1991, 1994 to 96) at Windsor, but trends were not consistent. Although measurement error was low (1 to 6 %), results suggest that contaminant effects may be difficult to detect in the field because either the stress (PCBs) in the field was not strong enough to elicit a response by Hexagenia or PCB body burdens did not alter development in the insects.