Correcting systematic error in PO2 measurement to improve measures of oxygen supply capacity (α).

An organism's oxygen supply capacity, measured as a ratio of a metabolic rate to its critical oxygen partial pressure, describes the efficacy of oxygen uptake and transport. This metric is sensitive to errors in oxygen measurement, especially near anoxia where the magnitude of instrument error as a proportion of total signal is magnified. Here, we present a conceptual and mathematical method that uses this sensitivity to identify, quantify, and therefore correct oxygen measurements collected using inaccurately calibrated sensors. When appropriate, adding a small correction value to each oxygen measurement counteracts the effects of this error and provides results that are comparable to data from accurately calibrated oxygen probes. We demonstrate, using simulated, laboratory, and literature datasets, how this method can be used post hoc to diagnose error in, correct the magnitude of, and reduce the variability in repeat measures of traits relevant to oxygen tolerance.

Metabolic physiology of the Mayan cichlid fish (Mayaheros uropthalmus): Re-examination of classification as an oxyconformer.

The Mayan cichlid (Mayaheros uropthalmus) is a freshwater fish inhabiting warm, potentially hypoxic and/or brackish waters, in Mexico and Central America. Despite its description as highly hypoxia tolerant, M. uropthalmus has been classified physiologically as an 'oxyconformer', which would place it in a very small (and shrinking) category of fishes that purportedly cannot maintain oxygen consumption (MO2) as ambient PO2 falls. However, hypoxia tolerance is often associated with strong oxyregulation, not oxyconformation as described for M. uropthalmus. To resolve these inconsistencies, we measured MO2, the ambient PO2 at which MO2 begins to decline as PO2 falls (PCrit), and gill ventilation rate (fG) in the Mayan cichlid. Variables were measured at 23o, 28 o and 33 °C and temperature sensitivity (Q10) calculated for each function. MO2 at air saturation was 2.9 ±â€¯0.2, 4.3 ±â€¯0.4, and 5.9 ±â€¯0.3 µmol O2/g/h at 23o, 28o and 33 °C, respectively. PCrits were low at 2.6 ±â€¯0.8 kPa, 3.2 ±â€¯0.8 kPa and 4.7 ±â€¯0.9 kPa at 23o, 28o and 33 °C, respectively. Q10 values for MO2 were 2.56 ±â€¯0.21 (23-28 °C), 1.89 ±â€¯0.15 (28-33 °C) and 2.2 ±â€¯0.1 (full temperature range of 23-33 °C), suggesting overall Q10s typical for tropical freshwater fish. fG was 39 ±â€¯3, 45 ±â€¯4, and 53 ±â€¯6 breaths/min at 23o, 28o and 33 °C, respectively, and increase 2-3 fold in severe hypoxia at each temperature. Experiments employing hyperoxia up to 35 kPa indicate a strong 'hypoxic drive' for gill ventilation. Collectively, these data show that, in contrast to a previous characterization, the Mayan cichlid is a strong oxyregulator exhibiting attributes (e.g. very low PCrit) typical of very hypoxia-tolerant fishes.

Investigating links between thermal tolerance and oxygen supply capacity in shark neonates from a hyperoxic tropical environment.

Temperature and oxygen limit the distribution of marine ectotherms. Haematological traits underlying blood-oxygen carrying capacity are thought to be correlated with thermal tolerance in certain fishes, and this relationship is hypothesised to be explained by oxygen supply capacity. We tested this hypothesis using reef shark neonates as experimental models because they live near their upper thermal limits and are physiologically sensitive to low oxygen conditions. We first described in situ associations between temperature and oxygen at the study site (Moorea, French Polynesia) and found that the habitats for reef shark neonates (Carcharhinus melanopterus and Negaprion acutidens) were hyperoxic at the maximum recorded temperatures. Next, we tested for in situ associations between thermal habitat characteristics and haematological traits of neonates. Contrary to predictions, we only demonstrated a negative association between haemoglobin concentration and maximum habitat temperatures in C. melanopterus. Next, we tested for ex situ associations between critical thermal maximum (CTMax) and haematological traits, but only demonstrated a negative association between haematocrit and CTMax in C. melanopterus. Finally, we measured critical oxygen tension (pcrit) ex situ and estimated its temperature sensitivity to predict oxygen-dependent values of CTMax. Estimated temperature sensitivity of pcrit was similar to reported values for sharks and skates, and predicted values for CTMax equalled maximum habitat temperatures. These data demonstrate unique associations between haematological traits and thermal tolerance in a reef shark that are likely not explained by oxygen supply capacity. However, a relationship between oxygen supply capacity and thermal tolerance remains to be demonstrated empirically.

Intertidal triplefin fishes have a lower critical oxygen tension (Pcrit), higher maximal aerobic capacity, and higher tissue glycogen stores than their subtidal counterparts.

Decreased oxygen (O2) availability (hypoxia) is common in rock pools and challenges the aerobic metabolism of fishes living in these habitats. In this study, the critical O2 tension (Pcrit), a whole animal measure of the aerobic contribution to hypoxia tolerance, was compared between four New Zealand triplefin fishes including an intertidal specialist (Bellapiscis medius), an occasional intertidal inhabitant (Forsterygion lapillum) and two exclusively subtidal species (F. varium and F. malcolmi). The intertidal species had lower Pcrit values than the subtidal species indicating traits to meet resting O2 demands at lower O2 tensions. While resting O2 demand (standard metabolic rate; SMR) did not show a major difference between species, the intertidal species had higher maximal rates of O2 consumption ([Formula: see text]) and higher aerobic metabolic scope (MS). The high O2 extractive capacity of the intertidal species was associated with increased blood O2 carrying capacity (i.e., higher Hb concentration), in addition to higher mass-specific gill surface area and thinner gill secondary lamellae that collectively conveyed a higher capacity for O2 flux across the gills. The specialist intertidal species B. medius also had higher glycogen stores in both white muscle and brain tissues, suggesting a greater potential to generate ATP anaerobically and survive in rock pools with O2 tensions less than Pcrit. Overall, this study shows that the superior Pcrit of intertidal triplefin species is not linked to a minimisation of SMR, but is instead associated with an increased O2 extractive capacity of the cardiorespiratory system (i.e., [Formula: see text], MS, Hb and gill O2 flux).

Physiological impacts and bioaccumulation of dietary Cu and Cd in a model teleost: The Amazonian tambaqui (Colossoma macropomum).

Increasing anthropogenic activities in the Amazon have led to elevated metals in the aquatic environment. Since fish are the main source of animal protein for the Amazonian population, understanding metal bioaccumulation patterns and physiological impacts is of critical importance. Juvenile tambaqui, a local model species, were exposed to chronic dietary Cu (essential, 500 µg Cu/g food) and Cd (non-essential, 500 µg Cd/g food). Fish were sampled at 10-14, 18-20 and 33-36 days of exposure and the following parameters were analyzed: growth, voluntary food consumption, conversion efficiency, tissue-specific metal bioaccumulation, ammonia and urea-N excretion, O2 consumption, Pcrit, hypoxia tolerance, nitrogen quotient, major blood plasma ions and metabolites, gill and gut enzyme activities, and in vitro gut fluid transport. The results indicate no ionoregulatory impacts of either of the metal-contaminated diets at gill, gut, or plasma levels, and no differences in plasma cortisol or lactate. The Cd diet appeared to have suppressed feeding, though overall tank growth was not affected. Bioaccumulation of both metals was observed. Distinct tissue-specific and time-specific patterns were seen. Metal burdens in the edible white muscle remained low. Overall, physiological impacts of the Cu diet were minimal. However dietary Cd increased hypoxia tolerance, as evidenced by decreased Pcrit, increased time to loss of equilibrium, a lack of plasma glucose elevation, decreased plasma ethanol, and decreased NQ during hypoxia. Blood O2 transport characteristics (P50, Bohr coefficient, hemoglobin, hematocrit) were unaffected, suggesting that tissue level changes in metabolism accounted for the greater hypoxia tolerance in tambaqui fed with a Cd-contaminated diet.