INTRAPOPULATION VARIABILITY IN SUBLETHAL RESPONSE TO HEAVY-METAL STRESS IN SEXUAL AND ASEXUAL GASTROPOD POPULATIONS

1. Asexual species have been described by some to be more broadly tole rant of environmental stresses (i.e. to have general purpose genotypes ) and by others to be more responsive to environmental stresses (i.e. to exhibit narrower tolerance distributions to environmental perturbat ions) in comparison to their sexual relatives. 2. We examined the subl ethal response of asexual and sexual gastropod populations of the fami ly Hydrobiidae to cadmium exposure to determine: whether populations r ank consistently in mean and variance of growth rate when reared in di fferent environments; whether the variance in growth rate within popul ations alters in response to stress; and whether growth rates within g enetically uniform clones are less variable than growth rates within s exually outcrossed populations. 3. We found that, in addition to reduc ing mean growth rate in all populations, exposure to cadmium increased the variability in growth rate within populations and altered the ran king of both means and variances among populations. 4. The average int raclonal variability in growth in the parthenogenetic species Potamopy rgus antipodarum was comparable to the average intrapopulation variabi lity in growth within the closely related but sexually reproducing Hyd robia species. Our results indicate that environmental toxicants, such as cadmium, can alter the relative performance of these potentially c ompeting species. 5. Our results are of broad relevance to the examina tion of phenotype variance and its partitioning between genetic and en vironmental sources of variation. 6. Non-genetic variation was the mos t important component of the total phenotypic variance in growth rate within these gastropod populations. 7. In addition, the assumption fre quently applied in the development of toxicity test protocols, that un iformity in phenotypic traits (i.e. response to toxicant exposure) is tightly coupled to genotypic uniformity, is not supported by our resul ts.