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Colonization of freshwater habitats from marine environments exposes organisms to novel combinations of temperature and salinity, but little is known about physiological responses to the interactive effects of these stressors. Here, we examined the effects of temperature (14 versus 4 °C) and salinity (11 versus 0.3 ppt) on gill gene expression in marine, anadromous, and freshwater populations of threespine stickleback (Gasterosteus aculeatus). Expression of the epithelial calcium channel was not affected by temperature or salinity, but had significantly higher expression in the freshwater ecotype. The combination of low temperature and low salinity had non-additive effects on the expression of the Na + /H + exchanger. Fish exposed to the combination of low temperature and low salinity had expression levels similar to fish exposed to either factor in isolation. Expression of Na + ,K + -ATPase α-subunit was greater in fish exposed to low temperature and low salinity than in fish exposed to the factors separately, and this effect was the most pronounced in the marine ecotype. We also examined the interactive effects of salinity and temperature on gill morphology in the marine ecotype, and observed non-additive effects. Low temperature increased the size of the interlamellar cell mass in fish held at 11 ppt, but not at 0.3 ppt, and the effect of low salinity was in the opposite direction in fish at high and low temperatures. These data demonstrate interactive effects of temperature and salinity and highlight that overwintering in cold freshwater was likely a physiological challenge for marine stickleback as they colonized freshwater following the last glaciation., (Copyright © 2018 Elsevier Inc. All rights reserved.)

Human activities are increasing both the frequency of hypoxic episodes and the mean temperature of aquatic ecosystems, but few studies have considered the possibility that acclimation to one of these stressors could improve the ability to cope with the other stressor. Here, we used Atlantic killifish, Fundulus heteroclitus, to test this hypothesis. Hypoxia tolerance was measured as time to loss of equilibrium in hypoxia (LOEhyp) at 0.4 kPa oxygen. Time to LOEhyp declined from 73.3 ± 6.9 min at 15 °C to 2.6 ± 3.8 min at 23 °C, and at 30 °C no fish could withstand this level of hypoxia. Prior acclimation to warm temperatures significantly increased time to LOEhyp. Hypoxia tolerance of the southern subspecies of killifish, F. heteroclitus heteroclitus, was greater than that of the northern subspecies, F. heteroclitus macrolepidotus, measured both as critical oxygen tension (Pcrit) and as time to LOEhyp. Warm acclimation offset the negative effects of temperature on time to LOEhyp to a similar extent in the two subspecies. Warm acclimation increased total lamellar surface area of the gill in both subspecies as a result of regression of an interlamellar cell mass (ILCM). However, differences in total lamellar surface area could not explain differences in time to LOEhyp between the subspecies, suggesting that the lower time to LOEhyp of northern fish is related to their higher routine metabolic rate. These data suggest that thermal plasticity in gill morphology can improve the capacity of this species to tolerate hypoxia, and shows how existing plasticity may help organisms to cope with the complex interacting stressors that they will encounter with increasing frequency as our climate changes., (© 2016. Published by The Company of Biologists Ltd.)