Blood oxygenation and cardiorespiratory function in steelhead trout (Oncorhynchus mykiss) challenged with an acute temperature increase and zatebradine-induced bradycardia

To explore whether temperature-dependent increases in cardiac output (Q) are mediated solely through heart rate (f_H) in fish to ensure adequate/efficient blood oxygenation, we injected steelhead trout with saline (control) or zatebradine hydrochloride (1.0 mg kg⁻¹), and measured blood oxygen status, cardiorespiratory variables and cardiorespiratory synchrony during a critical thermal maximum (CT_Max) test. The increasing temperature regimen itself (from 12 °C to CT_Max) resulted in large decreases in arterial oxygen partial pressure (P_aO₂) and content (C_aO₂) (by ∼35% and 25%, respectively). Further, there was little evidence of cardiorespiratory synchrony at 12 °C, and the number of fish that showed synchrony at high temperatures only increased marginally (to 3 out of 7) despite the large decrease in P_aO₂. These results: (1) indicate that in some situations (e.g. when ventilation is exclusively/predominantly dependent on buccal–opercular pumping) the upper thermal tolerance of fish may be constrained by both cardiovascular and ventilatory performance; and (2) question the importance of cardiorespiratory synchrony (ventilation–perfusion matching) for gas exchange in salmonids, and fishes, in general.     

Respiratory properties of the blood of the burrowing red band fish Cepola rubescens L.

The respiratory properties of the whole blood of the burrowing red band fish Cepola rubescens L. were investigated. Oxygen dissociation curves constructed at 15°C were found to be close to hyperbolic in shape with a mean value for the cooperativity coefficient at half-saturation (n₅₀) of 1.56. Half-saturation oxygen tension (P₅₀) for pH = 7.56 (mean in vivo pH of venous blood) was 27 Torr. The blood showed a marked Bohr effect ($\text{Δ log}\text{P}{}_{50}\text{ΔpH}\text{= ?1.19}$) and also a Root effect which at the in vivo pH reduced oxygen carrying capacity by 20%. The Pv_CO2 was 3.2 Torr and the buffering power of the blood was low, the buffer value of true plasma averaging 5.43 mmol · 1^?1 · pH^?1. It is suggested that the large Bohr effect coupled with the low buffer value confers on the haemoglobin a flexibility, in terms of oxygen affinity, to withstand changes which occur in environmental oxygen tensions.     

A topical metabolic inhibitor to improve transcutaneous estimation of arterial oxygen tension in adults

The use of Eugenol as a contact medium in the measurement of transcutaneous oxygen (tcPO₂) partially inhibits the metabolism in the underlying tissue, thereby reducing oxygen consumption and increasing tcPO₂. Oxygen consumption in the tissue can be estimated from the rate at which tcPO₂ falls when blood flow is occluded, and blood flow in the tissue can be estimated from the rate at which tcPO₂ increases when the subject changes from breathing air to pure oxygen. Both these measurements have been made with Eugenol and distilled water as contact media. From these measurements it has proved possible to estimate the arterial oxygen tension (aPO₂) of healthy adults at a relatively low sensor temperature (43°C).     

Limitations in energy intake affect the ability of young turkeys to cope with low ambient temperatures

Young turkeys exposed to low ambient temperature (T_a) showed significantly reduced body weight, which coincided with a reduction in energy intake and with changes in the circulatory system to accommodate higher oxygen demand. These changes included a significant increase in hematocrit, hemoglobin concentration, plasma triiodothyronine (T₃) concentration, blood volume, and blood oxygen capacity. At the relatively high T_a, changes to accommodate heat dissipation included significant increases in plasma volume and panting rate. These compensations were sufficient to control body temperature (T_b). However, the higher energy expenditure for maintenance followed by significantly higher plasma triiodothyronine (T₃) concentration, but with lower energy intake at low T_a, suggest a physical limitation in the ability to further increase energy intake as T_a declines.     

The fatty acid composition of phospholipids and absorption spectra of lipid extracts from lamprey,frog, and rat erythrocytes

The work studies the content and fatty acid composition of phospholipids as well as the absorption spectra of lipid extracts from red blood cells of poikilothermal and homoiothermal animals at different evolutionary levels. The objects of study include two poikilothermal species, the river lamprey (Lampetra fluviatilis) that uses oxygen dissolved in water, and the common frog (Rana temporaria) that consumes oxygen both from water and from air. A homoithermal animal is the white rat (Rattus rattus) that inhabits the terrestrial-aerial environment. The animals are studied in winter and spring. The phospholipid content in lamprey blood plasma is found to be twice higher than that in its erythrocytes. In the frog and the rat, the ratio is reverse. Determination of the fatty acid lipid composition of red blood cell phospholipids suggests that membranes in the lamprey are denser than in the frog. As for the fatty acids in the erythrocyte fraction of rat blood, they appear to be less diverse, with a double prevalence of saturated acids over unsaturated ones and devoid of long chain (C22) ω3 fatty acids. All of this results in a lower degree of unsaturation and a denser packing of fatty acids in the membrane structures of rat erythrocytes. The mechanism of reversible binding of O₂ molecules to hemoglobin in erythrocytes is discussed. Presumably, the mechanism of interaction between molecules of O₂ and molecules of water prevents the exchange interaction of electrons of the hemoglobin iron atoms with an oxygen molecule. This is confirmed by our obtained absorption spectra, which show that in the lipid extract almost totally devoid of water the heme isolated from erythrocytes is converted to hemin.     

Oxygen and carbon isotope studies in recent foraminifera from the southwest Indian ocean

Eighteen species of planktonic foraminifera have been analyzed for their oxygen and carbon isotopic composition in five Recent samples of deep-sea sediment from the southwest Indian Ocean; one sample of glacial age and one mid-Holocene sample were also studied. On the basis of oxygen isotopic composition three groups are recognized. Species in the first group (Globigerinoides ruber, G. sacculifer and G. conglobatus; G. Globigerina rubescens and Globigerinita glutinata) calcity in the near-surface Tropical Water, so that the oxygen isotopic composition of their test carbonate may be used to indicate surface temperature. Species in the second group (Pulleniatina obliquiloculata, Neogloboquadrina dutertrei, Orbulina universa, Globigerinella siphonifera and Sphaeroidinella dehiscens) are associated with the sub-surface high-salinity Subtropical Water, so that their oxygen isotope composition indicates trends in the temperature of this water mass. The third group (the species of Globorotalia) calcity in the deeper Central Water. The average oxygen isotopic composition of each Globorotalia species is more or less constant over the range studied and does not reflect the surface temperature trend.The carbon isotopic composition of three species (Globigerina rubescens, Globigerinoides ruber and Globigerinita glutinata indicate departure from isotopic equilibrium by at least 3%₀. Among the remaining species the variation of carbon isotopic composition with depth (where depth is inferred from the temperature estimated from oxygen isotopic composition) implies that N. dutertrei, P. obliquiloculata and G. siphonifera occupy the shallow subsurface oxygen minimum, while the deeper-dwelling globorotaliids approach the deeper oxygen minimum. Hence it is possible, despite scatter among the data, to discern the pattern of oxygen content with depth in the overlying water masses from an examination of oxygen and carbon isotopic composition among foraminiferal species in the sediment. This promises to be an exciting new tool for palaeo-oceanographic investigations.     

Hypoxia impairs visual acuity in snapper (Pagrus auratus)

We investigated the effect of environmental hypoxia on vision in snapper (Pagrus auratus). Juvenile snapper inhabit estuarine environments where oxygen conditions fluctuate on a seasonal basis. Optomotor experiments demonstrated that visual acuity is impaired by environmental hypoxia, but not until levels approach the critical oxygen tension (P _crit) of this species (around 25 % air-saturated seawater). In 100, 80, and 60 % air-saturated seawater, a positive optomotor response was present at a minimum separable angle (M _SA) of 1°. In 40 % air-saturated seawater, vision was partially impaired with positive responses at M _SAs of 2° and above. However, in 25 % air-saturated seawater, visual acuity was seriously impaired, with positive responses only present at M _SAs of 6° and above. Snapper were found to possess a choroid rete, facilitating the maintenance of high ocular oxygen partial pressures (PO₂) during normoxia and moderate hypoxia (PO₂, between 269 and 290 mmHg). However, at 40 and 25 % water oxygen saturation, ocular PO₂ was reduced to below 175 mmHg, which is perhaps linked to impairment of visual acuity in these conditions. The ability to preserve visual function during moderate hypoxia is beneficial for the maintenance of a visual lifestyle in the fluctuating oxygen environments of estuaries.     

Factors affecting electrical impedance of internodal stem sections

A sample holder was designed and built to facilitate measuring the magnitude and phase angle of the electrical impedance of internodal stem sections from Cornus stolonifera Michx. A nonpolarizing, electrically conducting manganese dioxidecarbon paste used between the stem sample and the electrodes of the sample holder allowed measurement of impedance at frequencies from 50 hertz to 500 kilohertz without electrode polarization or electrical interference. The impedance magnitude was linearly dependent on the sample length, but this dependence was minimized by computing a normalized impedance magnitude. The normalized impedance magnitude (Z_nf) was calculated using the impedance magnitude (Z) at any specified frequency (f) and the impedance magnitude at 500 kilohertz (Z_{500 khz}) in the following formula: Z_nf = (Z - Z_{500 khz})/Z_{500 khz}. The normalized impedance magnitude was sensitive to injury produced by boiling and peeling the sample. Electrical impedance measurements on the bark and wood separately demonstrated that they have different electrical properties.     

Differences in Hematological Traits between High- and Low-Altitude Lizards (Genus Phrynocephalus)

Phrynocephalus erythrurus (Lacertilia: Agamidae) is considered to be the highest living reptile in the world (about 4500-5000 m above sea level), whereas Phrynocephalus przewalskii inhabits low altitudes (about 1000-1500 m above sea level). Here, we report the differences in hematological traits between these two different Phrynocephalus species. Compared with P. przewalskii, the results indicated that P. erythrurus own higher oxygen carrying capacity by increasing red blood cell count (RBC), hemoglobin concentration ([Hb]) and hematocrit (Hct) and these elevations could promote oxygen carrying capacity without disadvantage of high viscosity. The lower partial pressure of oxygen in arterial blood (PaO₂) of P. erythrurus did not cause the secondary alkalosis, which may be attributed to an efficient pulmonary system for oxygen (O₂) loading. The elevated blood-O₂ affinity in P. erythrurus may be achieved by increasing intrinsic O₂ affinity of isoHbs and balancing the independent effects of potential heterotropic ligands. We detected one α-globin gene and three β-globin genes with 1 and 33 amino acid substitutions between these two species, respectively. Molecular dynamics simulation results showed that amino acids substitutions in β-globin chains could lead to the elimination of hydrogen bonds in T-state Hb models of P. erythrurus. Based on the present data, we suggest that P. erythrurus have evolved an efficient oxygen transport system under the unremitting hypobaric hypoxia.     

The Dependence of the Oxygen-Concentrating Mechanism of the Teleost Eye (Salmo gairdneri) on the Enzyme Carbonic Anhydrase

Microoxygen polarographic electrodes were constructed and used to measure oxygen tension (P_OO2) in the eyes of rainbow trout (Salmo gairdneri). The values obtained are compared with arterial blood and environmental water P_OO2 and indicate that there is an oxygen-concentrating mechanism in the eye supplying oxygen to the avascular retina. Anatomically similar retes suggest that the mechanism is similar to the one which exists in the swim bladder. Elimination of the arterial blood supply to the choroidal gland rete mirabile of the eye (through pseudobranchectomy) and the consequent lowering of ocular oxygen tensions implicate the choroidal gland as one of the major components of the oxygen-concentrating mechanism. After pseudobranchectomy the presence of ocular P_OO2 above that of arterial blood is indicative of a secondary structure in the eye capable of concentrating oxygen. Inhibition of carbonic anhydrase, using acetazolamide, is shown to result in complete suppression of the oxygen-concentrating mechanism. A hypothesis is advanced for the participation of retinal-choroidal and erythrocyte carbonic anhydrase in the oxygen-concentrating mechanism.     

Whole Blood Thallium Determination by GFAAS with High-Frequency Modulation Polarization Zeeman Effect Background Correction

A new technique of blood thallium direct determination based on graphite furnace atomic absorption spectrometry with Zeeman background absorption correction system was designed. The developed technique does not require sample digestion. Sample treatment includes only a fivefold per volume dilution of blood sample with 0.1% (m/v) Triton X-100. L’vov integrated platform was modified with 400 μg of Rh. Matrix modifier (200 μg NH₄NO₃ and 160 μg Pd(NO₃)₂) was suggested for coping chloride and blood organic matter interferences. Standard reference material (Clincheck® Plasma Control for trace elements) analysis was used for validation. Additional validation was performed by analyzing spiked blood samples in the whole dynamic range. The dynamic range was 2–50 μg/L. Precision (RSD) was found <12%. Blood thallium limit of detection was 0.2 μg/L.     

Oxygen transport and cardiovascular responses in skipjack tuna (Katsuwonus pelamis) and yellowfin tuna (Thunnus albacares) exposed to acute hypoxia

Summary Responses to acute hypoxia were measured in skipjack tuna (Katsuwonus pelamis) and yellowfin tuna (Thunnus albacares) (1–3 kg body weight). Fish were prevented from making swimming movements by a spinal injection of lidocaine and were placed in front of a seawater delivery pipe to provide ram ventilation of the gills. Fish could set their own ventilation volumes by adjusting mouth gape. Heart rate, dorsal and ventral aortic blood pressures, and cardiac output were continuously monitored during normoxia (inhalant water (PO ₂>150 mmHg) and three levels of hypoxia (inhalant water PO ₂130, 90, and 50 mmHg). Water and blood samples were taken for oxygen measurements in fluids afferent and efferent to the gills. From these data, various measures of the effectiveness of oxygen transfer, and branchial and systemic vascular resistance were calculated. Despite high ventilation volumes (4–71·min^-1·kg^-1), tunas extract approximately 50% of the oxygen from the inhalant water, in part because high cardiac outputs (115–132 ml·min^-1·kg^-1) result in ventilation/perfusion conductance ratios (0.75–1.1) close to the theoretically ideal value of 1.0. Therefore, tunas have oxygen transfer factors (ml O₂·min^-1·mmHg^-1·kg^-1) that are 10–50 times greater than those of other fishes. The efficiency of oxygen transfer from water in tunas (65%) matches that measured in teleosts with ventilation volumes and order of magnitude lower. The high oxygen transfer factors of tunas are made possible, in part, by a large gill surface area; however, this appears to carry a considerable osmoregulatory cost as the metabolic rate of gills may account for up 70% of the total metabolism in spinally blocked (i.e., non-swimming) fish. During hypoxia, skipjack and yellowfin tunas show a decrease in heart rate and increase in ventilation volume, as do other teleosts. However, in tunas hypoxic bradycardia is not accompanied by equivalent increases, in stroke volume, and cardiac output falls as HR decreases. In both tuna species, oxygen consumption eventually must be maintained by drawing on substantial venous oxygen reserves. This occurs at a higher inhalant water PO₂ (between 130 and 90 mmHg) in skipjack tuna than in yellowfin tuna (between 90 and 50 mmHg). The need to draw on venous oxygen reserves would make it difficult to meet the oxygen demand of increasing swimming speed, which is a common response to hypoxia in both species. Because yellowfin tuna can maintain oxygen consumption at a seawater oxygen tension of 90 mmHg without drawing on venous oxygen reserves, they could probably survive for extended periods at this level of hypoxia.Abbreviations BP_da, BP_va dorsal, ventral aortic blood pressure - C _aO₂, C _vO₂ oxygen content of arterial, venous blood - DO₂ diffusion capacity - E_b, E_w effectiveness of O₂ uptake by blood, and from water, respectively - Hct hematocrit - HR heart rate - PCO₂ carbon dioxide tension - P _aCO₂, P _vCO₂ carbon dioxide tension of arterial and venous blood, respectively - PO₂ oxygen tension - P _aO₂, P _vO₂, P _iO₂, P _cO₂ oxygen tension of arterial blood, venous blood, and inspired and expired water, respectively - pHa, pHv pH of arterial and venous blood, respectively - P_w—b effective water to blood oxygen partial pressure difference - Pg partial pressure (tension) gradient - cardiac output - R vascular resistance - SV stroke volume - SEM standard error of mean - TO₂ transfer factor - U utilization - _g ventilation volume - O₂ oxygen consumption     

Effects of silicon on Zea mays plants exposed to water and oxygen deficiency

Effects of shoot and root supplementation with silicon on the response of Zea mays L. plants to matric water potential (Ψ_m) and oxygen deficiency (waterlogging) stresses were studied. The soil water limitation (Ψ_m) and oxygen deprivation significantly reduced shoot dry weight, chlorophyll (Chl) content, ascorbic acid content, as well as leaf relative water content. Both soil drying and waterlogging caused a significant increase in the leaf membrane injury by heat (51°C) and dehydration (40% PEG) stresses. The levels of lipid peroxidation (POL) and hydrogen peroxide (H₂O₂) content were increased by excess soil drying and oxygen deficiency. Supplementary silicon at 1.0 mM significantly increased Chl content and improved water status. Concentrations of H₂O₂, MDA, and proline and leaf membrane injury were significantly reduced by Si application. The reverse helds true for ascorbic acid. The results of this study indicate that application of silicon might improve growth attributes, effectively mitigate the adverse effect of drought and waterlogging, and increase tolerance of maize plants. The silicon-induced improvement of drought and anoxia tolerance was associated with the increase in oxidative defense abilities.     

Oxygen consumption by the bivalve Donax vittatus (da Costa)

The acute oxygen consumption of Donax vittatus (da Costa) freshly collected at different times from a beach at Barrassie, Ayrshire, Scotland, has been measured at different temperatures. The logarithmic relationship between oxygen consumption and body weight showed a significant difference on only one occasion, and a common regression coefficient (b) of 0.865 could be used for regressions of oxygen consumption on weight. Over the temperature range 2.9–20 °C oxygen consumption rose with temperature. There was a linear decline of Q₁₀ with temperature in the range 2.9 –20 °C. Differences in values of the constant (a) in the regression equation suggest that there is some acclimation to temperature, resulting in rotation of the rate/temperature curve counterclockwise for warm-acclimated animals, and a reduction of Q₁₀ in cold-acclimated animals. The differences in oxygen consumption which result are small and appear to have little practical significance. High levels of metabolically-inactive materials such as stored glycogen reserves lead to a reduction in the weight-specific oxygen consumption. Spawning animals show an increased oxygen consumption.     

A biomechanically derived minimum work model of the fish gill lamellar system exhibits its exquisite morphological arrangement and perfusate regulation for oxygen uptake from water

To evaluate the efficiency of oxygen (O₂) uptake from water through the fish gill lamellar system, a cost function (CF) representing mechanical power expenditure for water ventilation and blood circulation through the gill was formulated, by applying steady-state fluid mechanics to a homogeneous lamellar-channel model. This approach allowed us to express CF as the function of inter-lamellar water channel width (w) and to derive an analytical solution of the width (w_min) at the minimum CF. Morphometric and physiological data for rainbow trout in the literature were referred to calculate CF(w) curves and their w_min values at five intensity stages of swimming exercise. Obtained w_min values were evenly distributed around the standard measure of the width (w_s = 24 μm) in this fish. Individual levels of CF(w_min) were also fairly close to the corresponding CF(w_s) values within a 10% deviation, suggesting the reliability of approximating [CF(w_min) = CF(w_s)]. The cost-performance of O₂ uptake through the gill (η_g) was then assessed from reported data of total O₂ uptake/CF(w_s) at each intensity stage. The η_g levels at any swimming stage exceeded 95% of the theoretical maximum value, implying that O₂ uptake is nearly optimally performed in the lamellar-channel system at all swimming speeds. Further analyses of O₂ transport in this fresh water fish revealed that the water ventilation by the buccal/opercular pumping evokes a critical limit of swimming velocity, due to confined O₂ supply to the peripheral skeletal muscles, which is avoided in ram ventilators such as tuna.     

Energy coupling for membrane hyperpolarization in Lemna: respiration rate,ATP level and membrane potential at low oxygen concentrations

Respiration rate, ATP content and membrane potential of Lemna have been measured as a function of the concentration of dissolved oxygen. Kinetic analysis showed that within the range from 1 μM to 20 μM O₂, the respiration rate of isolated mitochondria and intact plants was a hyperbolic function of the oxygen concentration. The apparent Michaelis constant (K _m ) for the oxygen of respiration of intact plants (1.15±0.08 μM) is close to that for isolated mitochondria (1.07±0.06 μM), so that diffusion of oxygen within the tissue was obviously not rate-limiting under the applied experimental conditions. The ATP level decreased in parallel with the respiration rate when the oxygen concentration was reduced. In contrast, the hyperpolarization of the membrane potential above the diffusion potential had already decreased at oxygen concentrations where the respiration rate and ATP level remained practically unchanged and was completely abolished at oxygen concentrations above the K _m of respiration. This result is discussed according to the current models for electrogenic pumps. It is concluded that ATP cannot be the fuel for the electrogenic process under investigation.     

Endophytic Bacterium-Triggered Reactive Oxygen Species Directly Increase Oxygenous Sesquiterpenoid Content and Diversity in Atractylodes lancea

Oxygenous terpenoids are active components of many medicinal plants. However, current studies that have focused on enzymatic oxidation reactions cannot comprehensively clarify the mechanisms of oxygenous terpenoid synthesis and diversity. This study shows that an endophytic bacterium can trigger the generation of reactive oxygen species (ROS) that directly increase oxygenous sesquiterpenoid content and diversity in Atractylodes lancea. A. lancea is a famous but endangered Chinese medicinal plant that contains abundant oxygenous sesquiterpenoids. Geo-authentic A. lancea produces a wider range and a greater abundance of oxygenous sesquiterpenoids than the cultivated herb. Our previous studies have shown the mechanisms behind endophytic promotion of the production of sesquiterpenoid hydrocarbon scaffolds; however, how endophytes promote the formation of oxygenous sesquiterpenoids and their diversity is unclear. After colonization by Pseudomonas fluorescens ALEB7B, oxidative burst and oxygenous sesquiterpenoid accumulation in A. lancea occur synchronously. Treatment with exogenous hydrogen peroxide (H₂O₂) or singlet oxygen induces oxidative burst and promotes oxygenous sesquiterpenoid accumulation in planta. Conversely, pretreatment of plantlets with the ROS scavenger ascorbic acid significantly inhibits the oxidative burst and oxygenous sesquiterpenoid accumulation induced by P. fluorescens ALEB7B. Further in vitro oxidation experiments show that several oxygenous sesquiterpenoids can be obtained from direct oxidation caused by H₂O₂ or singlet oxygen. In summary, this study demonstrates that endophytic bacterium-triggered ROS can directly oxidize oxygen-free sesquiterpenoids and increase the oxygenous sesquiterpenoid content and diversity in A. lancea, providing a novel explanation of the mechanisms of oxygenous terpenoid synthesis in planta and an essential complementarity to enzymatic oxidation reactions.     

Testing optimal foraging models for air-breathing divers

Models of diving optimality qualitatively predict diving behaviours of aquatic birds and mammals. However, none of them has been empirically tested. We examined the quantitative predictions of optimal diving models by combining cumulative oxygen uptake curves with estimates of power costs during the dives of six tufted ducks, Aythya fuligula. The effects of differing foraging costs on dive duration and rate of oxygen uptake (V_O2up) at the surface were measured during bouts of voluntary dives to a food tray. The birds were trained to surface into a respirometer after each dive, so that changes in V_O2up over time could be measured. The tray held either just food or closely packed stones on top of the food to make foraging energetically more costly. In contrast to predictions from the Houston & Carbone model, foraging time (t_f) increased after dives incorporating higher foraging energy costs but surface time (t_s) remained the same. While optimal diving models have assumed that the cumulative oxygen uptake curve is fixed, V_O2up increased when the energy cost of the dive increased. The optimal breathing model quantitatively predicted ts in both conditions and oxygen consumption during foraging (m₂t_f) in the control condition, for the mean of all ducks. This offers evidence that the ducks were diving optimally and supports the fundamentals of optimal diving theory. However, the model did not consistently predictt_s or m₂t_f for individual birds. We discuss the limits of optimal foraging models for air-breathing divers caused by individual variation. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Respiratory and hematological responses of the bonnethead shark, Sphyrna tiburo, to acute changes in dissolved oxygen

Behavioral and physiological responses to acute changes in dissolved oxygen were examined in the bonnethead shark, Sphyrna tiburo. In two sets of respirometry experiments, sharks were randomly exposed to seawater in oxygen contents of 6.0, 5.0, 4.0 and 3.0 mg l⁻¹. During exposure, bonnetheads increased mouth gape from 0.8 cm at 6.0 mg l⁻¹ to 2.2 cm at 3.0 mg l⁻¹, while ventilation volume increased from 0.61 to 5.28 l min⁻¹ kg⁻¹. Standard oxygen consumption remained unchanged (163-181 mg O₂ kg⁻¹ h⁻¹) throughout all treatments and was not significantly different. Utilization (%) declined from 52.3% at 6.0 mg l⁻¹ to 21.3% when oxygen levels reached 3.0 mg l⁻¹. Changes in oxygen content of ambient water also caused no significant change in either blood oxygen content or hematocrit. Using cellulose acetate electrophoresis, a single hemoglobin profile was identified at seawater of 6.0 mg l⁻¹ and hypoxic conditions. Results suggest bonnetheads are physiologically able to tolerate moderate levels of hypoxia.     

Responses to respiratory stress in relation to blood pigment affinity in Goniopsis cruentata (Latreille) and (to a lesser extent) in Cardisoma guanhumi Latreille

The crabs Goniopsis cruentata (Latreille) and Cardisoma guanhumi Latreille are shown to be metabolic regulators at environmental temperatures.Haemocyanin P₅₀ in Goniopsis shows strong correlation with maximum standard respiratory rates (R_max) and critical oxygen tension (P_c) and also with lethal oxygen tensions (P_L), at different temperatures. The hypothesis that the P_c represents that P_o2 at which the blood pigment fails to become saturated in the gills, though qualitatively in accord with these correlations, is irreconcilable with metabolic regulation in these animals. An alternative hypothesis is tentatively suggested from which, like the previous hypothesis, the experimental relationship P_c ∝ (R_max) (P₅₀) can be derived. The new hypothesis leads to the conclusion that the P_c reflects the initiation of anaerobiosis. Altogether, the evidence is thought to suggest that the graphical correlations are causal and that pigment affinity plays a determining role in the responses to respiratory stresses, both thermal and anoxic.