  Zooplankton have evolved several mechanisms to deal with environmental
threats, such as ultraviolet radiation (UVR), and in order to identify
strategies inherent to organisms exposed to different UVR environments, we here
examine life-history traits of two lineages of Daphnia pulex. The lineages
differed in the UVR dose they had received at their place of origin from
extremely high UVR stress at high-altitude Bolivian lakes to low UVR stress
near the sea level in temperate Sweden. Nine life-history variables of each
lineage were analyzed in laboratory experiments in the presence and the absence
of sub-lethal doses of UVR (UV-A band), and we identified trade-offs among
variables through structural equation modeling (SEM). The UVR treatment was
detrimental to almost all life-history variables of both lineages; however, the
Daphnia historically exposed to higher doses of UVR (HighUV) showed a higher
overall fecundity than those historically exposed to lower doses of UVR
(LowUV). The total offspring and ephippia production, as well as the number of
clutches and number of offspring at first reproduction, was directly affected
by UVR in both lineages. Main differences between lineages involved indirect
effects that affected offspring production as the age at first reproduction. We
here show that organisms within the same species have developed different
strategies as responses to UVR, although no increased physiological tolerance
or plasticity was shown by the HighUV lineage. In addition to known tolerance
strategies to UVR, including avoidance, prevention, or repairing of damages, we
here propose a population strategy that includes early reproduction and high
fertility, which we show compensated for the fitness loss imposed by UVR
stress.
