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

Abstract

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.

Combined salinity and acidity stressors alterDaphnia magnapopulation growth and structure under severe absence of photoperiod

“…Finally, we observed no egg production under any conditions although daphnids are known to produce diapausing eggs during periods of environmental stress induced by natural and anthropogenic factors ( e.g ., [104,119]). This further suggests that our results may be reflective of plasticity found in Daphnia magna [47,120]; Supporting Information ( Note 1 ) for further information.…”

“…This might explain why D. magna in Salinity2 & Acidity2 obviously outperformed the reproductive success in Salinity1 & Acidity1 and Salinity3 & Acidity3 . This could be associated with the effects of the following factors: First, the ability of D. magna to regulate osmolarity in the face of external ionic challenge is a result of the remarkable capacity of this organism to counterbalance the effects of salinity beyond 5 gL −1 by switching from osmoregulation to osmoconformance which enabled daphnids to cope with increased salinisation [12,47,104]. Our results clearly demonstrate that the largest population sizes were generally under osmoregulation in the short run; that was the case, as well, in the long run for combined stress Salinity2 & Acidity2 ; but then there was a shift to osmoconformance in Salinity3 , which increased survival and reproduction in the longer term (Day 30).…”

“…Under these conditions, daphnids reproduce asexually by parthenogenesis resulting in a female-only population. The ability to endure a range of abiotic stressors or sensitivity to contaminants made daphnids a key model to study tolerance [2,46], phenotypic plasticity [47], and toxicity in aquatic environments [48].…”

“…Daphnia magna population size (total number of daphnids in the beaker at each census, as a proxy of reproductive success) and the proportion of juveniles relative to the total population in the beaker per census (age structure, hereafter) were examined. The experimental conditions were based on extensive pilot studies during which we established different levels of pH and salinity that were lethal and sub-lethal to our clone of D. magna [47]. The pH treatment levels were 5, 5.5 and 6 and the salinity treatment levels (plus the above-mentioned ADaM salinity) were 1.33, 3.33, and 6.33 gL −1 .…”