Novel resilience in response to revitalisation after exposure to lethal salinity causes differential reproductive success in an extremely plastic organism.

Phenotypic plasticity is central to an organism's ability to adapt to variable environmental conditions. For aquatic organisms, exposure to elevated salt levels poses a challenge and organisms may fail to tolerate or survive much higher levels short-term. Here we demonstrate, for the first time, in a laboratory study of Daphnia magna that exposure to levels of salinity higher than those previously shown to lead to apparent death (paralysis) can be reversed following a transfer to optimal conditions. We established experimental populations from one clone of D. magna, each with five replicates, that were exposed to different short periods of three different lethal levels of salinity (12.27 PSU [45, 60, 90 and 120 min], 18.24 PSU [45, 60 and 90 min] and 24.22 PSU [45, 60 and 90 min]). In all populations, all individuals were paralysed at the end of their exposure, usually classified in the literature as dead. Subsequently, all individuals were transferred to optimal conditions. However, after the transfer, a proportion of the individuals not only came back from the verge of death (i.e. were revitalised), but also showed afterwards differential reproductive success over a period of 20 days, depending on the level and the length of exposure before revitalisation. Both exposure level and time had an overall negative effect on population size that differed across all treatments. Revitalisation occurred within an hour after the transfer to optimal conditions for 18.24 PSU but took 14-16 h for 12.27 PSU. There was no instantaneous revitalisation nor was there any revitalisation after 16 h no matter how long the paralysed Daphnia individuals were left in the optimal conditions. Our findings cast new light on resilience in cladocerans and suggest that abrupt environmental change can reveal novel plastic responses to extreme conditions.