Osmoregulatory and demographic responses to salinity of the exotic cladoceran Daphnia exilis

Daphnia exilis is a halophylic species that was recently found in Chile, distant from its natural range. In this study, we analyze the osmoregulatory and life-history responses to salinity exhibited by Daphnia exilis, as a first step towards understanding the ecology of this exotic species whose invasion potential has been related to its ability to tolerate saline environments. A set of laboratory experiments were carried out, in which we exposed organisms to a salinity gradient, and measured internal and environmental osmolality, assessed the effect of acclimation time and measured life-history traits that were used to perform a demographic retrospective analysis. Our results showed that (i) D. exilis exhibited a hyper-osmoconformer response, (ii) acclimation time did not exert effects on the osmoregulatory response, (iii) salinity up to 6 g L did not alter the population growth rate, (iv) at 8 g L population growth rate declined, mainly due to a delayed age at maturity and reduced fertility. Based on this information, we suggest that the responses to salinity exhibited by the exotic D. exilis confer an advantage over its natural enemies and may facilitate colonization through exploiting disturbed places as chemical refuges.