Exposure to heavy metal stress does not increase fluctuating asymmetry in populations of isopod and hardwood trees

Fluctuating asymmetry (FA) refers to random, small and non-directional deviations from ideal bilateral symmetry is proposed as a bio-indicator of abiotic stress in both animals and plants. We investigated the effect of heavy metal stress on FA levels of morphological traits in a terrestrial isopod (Philoscia muscorum) as well as in the leaves of two hardwood tree species: Gray birch (Betula populifolia) and eastern cottonwood (Populus deltoides), in an urban brownfield in New Jersey. FA levels measured for five traits (length of two segments of antennae and three segments of the seventh legs) were compared in male and female populations of P. muscorum sampled from three low and three high soil metal load sites within the brownfield. FA levels measured for leaf width (perpendicular distance from a midpoint on midrib to the widest point of the lamina in right and left half in a leaf) were compared for both gray birch and eastern cottonwood leaves collected from the same low and high soil metal load sites. Contrary to the hypothesis that FA increases with higher heavy metal stress in isopods and trees, our results revealed that true asymmetry in gray birch and for some isopod traits (2nd antenna article, 3rd antenna article and merus of males and females, carpus and prodopus in females) did not differ between low and high metal contaminated sites. Furthermore, FAs measured in eastern cottonwood leaves and other isopod traits (carpus and prodopus in males) were found to be even lower at high metal contaminated sites than the low metal load sites. The overall effect of metal stress was shown as reduction in growth (measured as body size for a given head width in an individual) of isopods at high metal load sites as compared to the low metal load sites. Various hypotheses including induction of detoxification mechanisms in response to metal stress, selection against individuals with presumably lower fitness (high FA), difference in sensitivity of traits to stress, and plasticity are discussed to explain the observed lack of a significant association between FA and heavy metal stress in isopods and trees.