Daphnids adaptive strategies to UV radiation
Abstract: This thesis focuses on the adaptive strategies of daphnids that allows them to maintain stable populations while deal with UV radiation, highlighting the differences in the responses that populations with different evolutionary histories may have.Although exposition to lethal doses in nature is unlikely, sub-lethal UV-B radiation doses may have strong impacts on zooplankton. This work initiates with a study aimed at determining the effect of sub-lethal doses UV-B radiation on filtration rates of two cladoceran species (Daphnia pulicaria and Ceriodaphnia dubia) and showed that filtration rates of D. pulicaria and C. dubia decreased by 50% and more than 80%, respectively, enhancing the importance of avoidance behaviour and recovery strategies for freshwater zooplankton species. This study was followed by a second one that assessed the ability of Daphnia pulex to develop tolerance to UV radiation, while disentangling, at the same time, the relative importance of local adaptations and ontogenetic processes behind such tolerance. Two populations from environments strongly differing in UV radiation conditions (Bolivian Andes and southern of Sweden) were UV-induced to produce photo-protective compounds and changes in behavioural responses that were monitored by 3D Nano-tracking. Although changes in tolerance capacity of both populations were evident in the results, differences between populations were more related to local evolution than to short-time ontogenetic processes, showing that tolerance to UV radiation is dependent on the evolutionary history of each species population.Such evolutionary differences should be reflected in the life-histories of both populations so that the next study, through structural equation modelling, shows clear differences in the life-history structures of both populations of Daphnia pulex. Main differences involve indirect effects of UV radiation on offspring production as well as the age at first reproduction. In addition to known tolerance strategies to UV radiation, a population strategy that includes early reproduction and high fertility to compensate for the fitness loss imposed by UV radiation stress, may be developed by populations from high-UV environments. A further study was conducted to explore how these high fertility populations may maintain stable populations in a gradient of resource availability and different sources of light. The results suggest that Daphnia may use resting eggs as self-regulating strategy, which can work as a dissipative structure which slows entropy production in the system, or as a mechanism of population-density control that allows re-population on the long term.
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