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Synopsis “Polar gigantism” describes a biogeographic pattern in which many ectotherms in polar seas are larger than their warmer-water relatives. Although many mechanisms have been proposed, one idea—the oxygen–temperature hypothesis—has received significant attention because it emerges from basic biophysical principles and is appealingly straightforward and testable. Low temperatures depress metabolic demand for oxygen more than supply of oxygen from the environment to the organism. This creates a greater ratio of oxygen supply to demand, releasing polar organisms from oxygen-based constraints on body size. Here we review evidence for and against the oxygen–temperature hypothesis. Some data suggest that larger-bodied taxa live closer to an oxygen limit, or that rising temperatures can challenge oxygen delivery systems; other data provide no evidence for interactions between body size, temperature, and oxygen sufficiency. We propose that these findings can be partially reconciled by recognizing that the oxygen–temperature hypothesis focuses primarily on passive movement of oxygen, implicitly ignoring other important processes including ventilation of respiratory surfaces or internal transport of oxygen by distribution systems. Thus, the hypothesis may apply most meaningfully to organisms with poorly developed physiological systems (eggs, embryos, egg masses, juveniles, or adults without mechanisms for ventilating internal or external surfaces). Finally, most tests of the oxygen–temperature hypothesis have involved short-term experiments. Many organisms can mount effective responses to physiological challenges over short time periods; however, the energetic cost of doing so may have impacts that appear only in the longer term. We therefore advocate a renewed focus on long-term studies of oxygen–temperature interactions. 

ABSTRACT Aquatic insects cope with hypoxia and anoxia using a variety of behavioral and physiological responses. Most stoneflies (Plecoptera) occur in highly oxygenated surface waters, but some species live underground in alluvial aquifers containing heterogeneous oxygen concentrations. Aquifer stoneflies appear to be supported by methane-derived food resources, which they may exploit using anoxia-resistant behaviors. We documented dissolved oxygen dynamics and collected stoneflies over 5 years in floodplain wells of the Flathead River, Montana. Hypoxia regularly occurred in two wells, and nymphs of Paraperla frontalis were collected during hypoxic periods. We measured mass-specific metabolic rates (MSMRs) at different oxygen concentrations (12, 8, 6, 4, 2, 0.5 mg l −1 , and during recovery) for 111 stonefly nymphs to determine whether aquifer and benthic taxa differed in hypoxia tolerance. Metabolic rates of aquifer taxa were similar across oxygen concentrations spanning 2 to 12 mg l −1 ( P >0.437), but the MSMRs of benthic taxa dropped significantly with declining oxygen ( P <0.0001; 2.9-times lower at 2 vs. 12 mg l −1 ). Aquifer taxa tolerated short-term repeated exposure to extreme hypoxia surprisingly well (100% survival), but repeated longer-term (>12 h) exposures resulted in lower survival (38–91%) and lower MSMRs during recovery. Our work suggests that aquifer stoneflies have evolved a remarkable set of behavioral and physiological adaptations that allow them to exploit the unique food resources available in hypoxic zones. These adaptations help to explain how large-bodied consumers might thrive in the underground aquifers of diverse and productive river floodplains. 

Oxygen bioavailability is declining in aquatic systems worldwide as a result of climate change and other anthropogenic stressors. For aquatic organisms, the consequences are poorly known but are likely to reflect both direct effects of declining oxygen bioavailability and interactions between oxygen and other stressors, including two—warming and acidification— that have received substantial attention in recent decades and that typically accompany oxygen changes. Drawing on the collected papers in this symposium volume (“An Oxygen Perspective on Climate Change”), we outline the causes and consequences of declining oxygen bioavailability. First, we discuss the scope of natural and predicted anthropogenic changes in aquatic oxygen levels. Although modern organisms are the result of long evolutionary histories during which they were exposed to natural oxygen regimes, anthropogenic change is now exposing them to more extreme conditions and novel combinations of low oxygen with other stressors. Second, we identify behavioral and physiological mechanisms that underlie the interactive effects of oxygen with other stressors, and we assess the range of potential organismal responses to oxygen limitation that occur across levels of biological organization and over multiple timescales. We argue that metabolism and energetics provide a powerful and unifying framework for understanding organism-oxygen interactions. Third,we conclude by outlining a set of approaches for maximizing the effectiveness of future work, including focusing on long-term experiments using biologically realistic variation in experimental factors and taking truly cross disciplinary and integrative approaches to understanding and predicting future effects.