title: Oxygen dependence of upper thermal limits in fishes.
abstract: Temperature-induced limitations on the capacity of the cardiorespiratory system to transport oxygen from the environment to the tissues, manifested as a reduced aerobic scope (maximum minus standard metabolic rate), have been proposed as the principal determinant of the upper thermal limits of fishes and other water-breathing ectotherms. Consequently, the upper thermal niche boundaries of these animals are expected to be highly sensitive to aquatic hypoxia and other environmental stressors that constrain their cardiorespiratory performance. However, the generality of this dogma has recently been questioned, as some species have been shown to maintain aerobic scope at thermal extremes. Here, we experimentally tested whether reduced oxygen availability due to aquatic hypoxia would decrease the upper thermal limits (i.e. the critical thermal maximum, CTmax) of the estuarine red drum (Sciaenops ocellatus) and the marine lumpfish (Cyclopterus lumpus). In both species, CTmaxwas independent of oxygen availability over a wide range of oxygen levels despite substantial (>72%) reductions in aerobic scope. These data show that the upper thermal limits of water-breathing ectotherms are not always linked to the capacity for oxygen transport. Consequently, we propose a novel metric for classifying the oxygen dependence of thermal tolerance; the oxygen limit for thermal tolerance (PCTmax), which is the water oxygen tension (PwO2) where an organism's CTmaxstarts to decline. We suggest that this metric can be used for assessing the oxygen sensitivity of upper thermal limits in water-breathing ectotherms, and the susceptibility of their upper thermal niche boundaries to environmental hypoxia.
authors: Department of Marine Science, The University of Texas at Austin, Marine Science Institute, Port Aransas, TX 78373, USA rasmus@ern.dk.
authors: Rasmus
authors: R
authors: Ern
authors: Department of Ocean Sciences, Memorial University of Newfoundland, St John's, NL, Canada A1C 5S7.
authors: Tommy
authors: T
authors: Norin
authors: Department of Ocean Sciences, Memorial University of Newfoundland, St John's, NL, Canada A1C 5S7.
authors: A Kurt
authors: AK
authors: Gamperl
authors: Department of Marine Science, The University of Texas at Austin, Marine Science Institute, Port Aransas, TX 78373, USA.
authors: Andrew J
authors: AJ
authors: Esbaugh
mesh_terms: D000332:Aerobiosis; D000818:Animals; D001481:Basal Metabolism / Q000502:physiology; D000860:Hypoxia / Q000503:physiopathology; D010100:Oxygen / Q000378:metabolism*; D010101:Oxygen Consumption; D010473:Perciformes / Q000502:physiology*; D013696:Temperature; D000071436:Thermotolerance / Q000502:physiology
chemical_list: D010100:Oxygen
keywords: Aerobic scope; CTmax; Critical oxygen tension; Critical thermal maximum; Hypoxia; Oxygen limit for thermal tolerance; PCTmax; Pcrit; Temperature