Hypoxia is a pervasive stressor in aquatic environments, and both phenotypic plasticity and evolutionary adaptation could shape the ability to cope with hypoxia. We investigated evolved variation in hypoxia tolerance and the hypoxia acclimation response across fundulid killifishes that naturally experience different patterns of hypoxia exposure. We compared resting O2 consumption rate (MO2), and various indices of hypoxia tolerance (critical O2 tension [Pcrit], regulation index [RI], O2 tension [PO2] at loss of equilibrium [PLOE], and time to LOE [tLOE] at 0.6 kPa O2) in Fundulus confluentus, F. diaphanus, F. heteroclitus, F. rathbuni, Lucania goodei, and L. parva. We examined the effects of chronic (28 d) exposure to constant hypoxia (2 kPa) or nocturnal intermittent hypoxia (12 h normoxia: 12 h hypoxia) in a subset of species. Some species exhibited a two-breakpoint model in MO2 caused by early, modest declines in MO2 in moderate hypoxia. We found that hypoxia tolerance varied appreciably across species: F. confluentus was the most tolerant (lowest PLOE and Pcrit, longest tLOE), whereas F. rathbuni and F. diaphanus were the least tolerant. However, there was not a consistent pattern of interspecific variation for different indices of hypoxia tolerance, with or without taking phylogenetic relatedness into account, likely because these different indices are underlaid by partially distinct mechanisms. Hypoxia acclimation generally improved hypoxia tolerance, but the magnitude of plasticity and responsiveness to different hypoxia patterns varied interspecifically. Our results therefore suggest that hypoxia tolerance is a complex trait that is best appreciated by considering multiple indices of tolerance.

A key challenge in conservation biology is to identify natural populations with compromised health and identify causative agents. However, wildlife are exposed to a complex matrix of natural and anthropogenic stressors such that identifying particular agents of distress is difficult. Yet, establishing cause and effect between human-induced environmental changes and adverse health is necessary to guide conservation planning. Transcriptome profiling, with thoughtful experimental design and appropriate metadata, is useful for establishing cause and effect between exposures to environmental stressors and adverse health outcomes. Here we describe transcriptome profiling and associated paradigms that are useful for wildlife health assessment and conservation planning, with particular emphasis on pollution. We emphasize that these tools are important for testing hypotheses about causative agents of distress, but also for generating new hypotheses about causes and consequences. We outline two case studies that highlight attributes of transcriptomics tools and approaches that add value for conservation practitioners.