More and smaller resting eggs along a gradient for pollution by metals: dispersal, dormancy and detoxification strategies in Daphnia?

Abstract

Trace metals are bioavailable, persistent and potentially harmful chemicals commonly found in cities. In metal-polluted habitats, natural populations may evolve adaptations and may be composed of individuals exhibiting detoxification mechanisms, in particular through melanization, dispersal or dormancy abilities. Interestingly, Daphnia cyclically produce chitinous melanized envelopes called 'ephippia' encasing the resting eggs, which allow dispersal in space and in time (dormancy). Moreover, the success of dispersal decreases with increasing ephippial size. We hypothesized that populations living in polluted ponds produce more, darker and smaller ephippia than populations from unpolluted ponds. We sampled 51 ponds distributed in the Paris region and investigated the link between concentrations of seven trace metals and the presence of Daphnia and ephippia, and the size and pigmentation of ephippia. First, the presence of Daphnia was not linked to local metal concentrations, which ranged gradually from low to high values. Second, the probability of the presence of ephippia in sediments increased with metal concentrations, suggesting a selective advantage of Daphnia in producing dormancy stages in polluted habitats. Third, although ephippial pigmentation was not linked to metal concentrations, ephippial size decreased with increasing metal concentrations, suggesting a selection for increased dispersal in polluted habitats. Overall, our results show that anthropogenic pollution may have important microevolutionary consequences in urban populations, which are generally overlooked.