Amphibious fishes can survive extreme changes, often occupying aquatic habitats with variable and unfavourable conditions. This group also makes the challenging transition from aquatic to terrestrial habitats, sometimes for prolonged durations. Here, I investigated the adaptive, behavioural, and plastic responses used by amphibious killifishes to cope with extreme environmental changes. First, I collaborated to investigate whether amphibious behaviour, in addition to physiological tolerance, provides mangrove fishes with a strategy to cope with high external levels of the naturally occurring toxicant, hydrogen sulfide. We found that tolerance alone was insufficient to cope with high, but ecologically relevant, levels of hydrogen sulfide and that all three mangrove fishes examined demonstrated avoidance behaviour through emersion from water. Next, I hypothesized that amphibious fishes retain larval skin traits as a developmental strategy through either a continuous expression or re-expression pattern. I found support for the continuous expression hypothesis regarding skin ion-transporting cells. I also found that the density of these skin ‘ionocytes’ was highest in more emersion tolerant species, suggesting that ionocytes facilitate survival in terrestrial habitats. In my final thesis chapter, I tested two related hypotheses that address different evolutionary mechanisms. First, that the skin ionocyte density in amphibious fishes evolved to be constitutively different from exclusively water-breathing species to promote ionoregulation via the skin in terrestrial environments. I also tested the alternate hypothesis that skin ionocyte density is instead primarily determined by plasticity in response to air-exposure. By rearing nine species of killifish in water with no access to air, I first showed that skin ionocytes are a constitutive trait. I also found that in two highly amphibious species, skin ionocyte density increased in response to one week of air-exposure, whereas this was not the case in four moderately amphibious species. Overall, my thesis documents critical strategies amphibious killifishes use to cope with aquatic and terrestrial homeostatic stressors as well as the value of cyprinodontiform killifishes as a model study group.