Buoyancy control of four species of eleotrid fishes during aquatic surface respiration

Synopsis Four species of Australian Eleotridae from hypoxic habitats were examined in the laboratory to study buoyancy control in hypoxic water (<10 torr) when performing aquatic surface respiration (ASR; irrigating gills with upper millimeter of surface water). A conflict can arise here because O₂ can be reabsorbed from the swimbladder (reducing buoyancy) at a time when additional lift may be required to perform ASR. Three species were negatively buoyant and initially performed ASR while resting on the bottom in shallow water. After 24 h swimbladder lift increased to nearly neutral and ASR was performed while fish were pelagic. The fourth species remained pelagic at near neutral buoyancy in hypoxic water. With sudden exposure to hypoxia these physoclists reabsorbed between 5–27% (depending on species) of swimbladder volume (standard pressure) during the initial 30–90 min exposure to hypoxia. Additional experiments on one species (Hypseleotris galii) showed such loss to occur at O₂ tensions below 68 torr and when O₂ declined rapidly (2.17 torr min^-1). Secretion of gas compensated for losses under slower, natural rates of nocturnal O₂ decline. Eleotrids appear to reduce the conflict between respiration and buoyancy control in hypoxia by restricting gas reabsorbtion from the swimbladder and by rapidly secreting gases into the swimbladder.     

Gill chemoreceptors and cardio-respiratory reflexes in the neotropical teleost pacu, <Emphasis Type="Italic">Piaractus mesopotamicus</Emphasis>

This study examined the location and distribution of O₂ chemoreceptors involved in cardio-respiratory responses to hypoxia in the neotropical teleost, the pacu (Piaractus mesopotamicus). Intact fish and fish experiencing progressive gill denervation by selective transection of cranial nerves IX and X were exposed to gradual hypoxia and submitted to intrabuccal and intravenous injections of NaCN while their heart rate, ventilation rate and ventilation amplitude were measured. The chemoreceptors producing reflex bradycardia were confined to, but distributed along all gill arches, and were sensitive to O₂ levels in the water and the blood. Ventilatory responses to all stimuli, though modified, continued following gill denervation, however, indicating the presence of internally and externally oriented receptors along all gill arches and either in the pseudobranch or at extra-branchial sites. Chemoreceptors located on the first pair of gill arches and innervated by the glossopharyngeal nerve appeared to attenuate the cardiac and respiratory responses to hypoxia. The data indicate that the location and distribution of cardio-respiratory O₂ receptors are not identical to those in tambaqui (Colossoma macropomum) despite their similar habitats and close phylogenetic lineage, although the differences between the two species could reduce to nothing more than the presence or absence of the pseudobranch.     

Gill ventilation and O2 extraction during graded hypoxia in two ecologically distinct species of flatfish,the flounder (Platichthys flesus) and the plaice (Pleuronectes platessa)

Synopsis Gill ventilation, breathing frequency, breath volume, oxygen extraction from the ventilatory water current and oxygen uptake through the gills were measured in flounder, Platichthys flesus, and plaice, Pleuronectes platessa, at water O₂ tensions ranging from 35 to 155 mm Hg at 10° C. Ventilation volumes were similar in the two species at high water O₂ tension. Exposure to hypoxic water elicited a larger increase in ventilation in the flounder. The per cent extraction of O₂ from water decreased slightly in both species as water O₂ tension was lowered. At comparable levels of ventilation O₂ extraction was higher in flounder. At the higher levels of water O₂ tension, O₂ uptake across the gills of flounder was stable, the critical O₂ tension being between 60 and 100 mm Hg. The plaice behaved as an oxygen conformer over the entire range of O₂ tensions investigated. The superior ability of the flounder in maintaining OZ uptake across the gills during a reduction in water O₂ tension may in part explain why the species, unlike plaice, inhabits very shallow waters with large fluctuations in dissolved oxygen.     

Tradeoffs between respiration and feeding in Sacramento blackfish,Orthodon microlepidotus

Synopsis Suspension-feeding fishes use gill structures for both respiration (lamellae) and food capture (rakers). During hypoxic exposure in eutrophic lakes or poorly circulated sloughs, many fishes, including Sacramento blackfish, Orthodon microlepidotus, increase their gill water flows, in part by increasing ventilatory stroke volumes. Stroke volume increases could compromise particle sieving efficiency by spreading interdigitated gill rakers from adjacent gill arches, although blackfish capture food particles by raker-guided water flows to a sticky buccal root. Using van Dam-type respirometers, blackfish respiratory variables and feeding efficiency (Artemia nauplii) were measured under normoxia (> 130 torr PO₂) and hypoxia (60 torr PO₂). Compared with non-feeding, normoxic conditions, gill ventilation volume, frequency, stroke volume, and gape all increased, while O₂ uptake efficiency decreased, during hypoxia and during feeding. O₂ consumption increased during feeding treatments, and % uptake of nauplii showed no difference between normoxic and hypoxic groups. Thus, blackfish display respiratory adaptations, including increased ventilatory stroke volumes, to survive in hypoxic environments such as Clear Lake, California. Importantly, they have also evolved a particle capture mechanism that allows efficient suspension-feeding under both normoxic and hypoxic conditions.     

Aquatic surface respiration,a widespread adaptation to hypoxia in tropical freshwater fishes

Synopsis Use of the surface water for aquatic respiration (aquatic surface respiration, ASR) is one of the few alternatives to aerial respiration which allow fish to survive extreme hypoxia, yet it has received very little attention. This report examines three generalizations concerning ASR on a phylogenetically and geographically diverse range of tropical freshwater fishes. It demonstrates that ASR greatly enhances survival in hypoxic water, even in fish not morphologically specialized to use the surface film, that ASR is initiated at a distinct threshold oxygen concentration, with time spent at the surface increasing rapidly as O₂ declines, and that with extreme deoxygenation fish perform ASR over 90% of the time. Ninety-four percent of the 31 species of non-air breathing fish tested showed ASR., with the threshold oxygen concentration ranging from 6 to 40 torr.Present address correspondence and reprint requests to D.L. Kramer.     

The relationship between O2 chemoreceptors, cardio-respiratory reflex and hypoxia tolerance in the neotropical fish Hoplias lacerdae

The localization, distribution and orientation of O₂ chemoreceptors associated with the control of cardio-respiratory responses were investigated in the neotropical, Hoplias lacerdae. Selective denervation of the cranial nerves (IX and X) was combined with chemical stimulation (NaCN) to characterize the gill O₂ chemoreceptors, and the fish were then exposed to gradual hypoxia to examine the extent of each cardio-respiratory response. Changes in heart rate (f_H) and ventilation amplitude (V_amp) were allied with chemoreceptors distributed on both internal and external surfaces of all gill arches, while ventilation rate (f_R) was allied to the O₂ chemoreceptors located only in the internal surface of the first gill arch. H. lacerdae exposed to gradual hypoxia produced a marked bradycardia (45%) and 50% increase in V_amp, but only a relatively small change in f_R (32%). Thus, the low f_R response yet high V_amp were in accord with the characterization of the O₂ chemoreceptors. Comparing these results from H. lacerdae with hypoxia-tolerant species revealed a relationship existent between general oxygenation of the individual species environment, its cardio-respiratory response to hypoxia and the characterization of O₂ chemoreceptors.     

PO2 and pH changes in the retina of the crab Ocypode ryderi: evidence for aerobic glycolysis

Summary The isolated retina of the terrestrial crab Ocypode ryderi exhibits a pronounced lactate production in spite of being supplied with sufficient O₂ (140 torr). To determine whether this lactate production is caused by hypoxic areas in the tissue or represents aerobic glycolysis, oxygen partial pressure and pH measurements with two-channel glass microelectrodes and additional biochemical analyses were carried out on this organ. Distinct profiles were obtained for O₂ partial pressure and pH inside the tissue. At a depth of 200 m different O₂ partial pressure levels could be observed depending on the O₂ partial pressure in the medium (85 torr at 280 torr and 36 torr at 130 torr, respectively). The extracellular pH displays a similar pattern; it reaches a stable value of 7.15 at 100 m inside the tissue. Lowering bath O₂ partial pressure from 280 torr to about 15 torr (hypoxia) induces a decrease of the O₂ partial pressure in the tissue with different time-courses for different tissue depths. However, hypoxia did not change the extracellular pH. Addition of antimycin A (100 mol · 1^-1) to the medium abolishes the O₂ partial pressure gradient and the delayed recovery of the tissue O₂ partial pressure after hypoxia. These results and the biochemical data suggest that in the crab retina a high glycolytic activity occurs simultaneously with oxydative carbohydrate degradation (aerobic glycolysis).Abbreviations AEC Atkinson energy charge - DC bioelectric potential - dw dry weight - HEPES N-[2-Hydroxyethyl]piperazine-N-[2-ethanesulphonic acid] - PCO₂ carbon dioxide partial pressure - PO₂ oxygen partial pressure - P _tO₂ oxygen partial pressure inside the tissue - P _mO₂ oxygen partial pressure in the medium - pH_t pH inside the tissue - pH_m pH in the superfusion medium     

Behavioural responses of a south-east Australian floodplain fish community to gradual hypoxia

1. Hypoxic conditions occur frequently during hot, dry summers in the small lentic waterbodies (billabongs) that occur on the floodplains of the Murray‐Darling River system of Australia. Behavioural responses to progressive hypoxia were examined for the native and introduced floodplain fish of the Ovens River, an unregulated tributary of the Murray River in south‐east Australia. 2. Given the high frequency of hypoxic episodes in billabongs on the Ovens River floodplain, it was hypothesised that all species would exhibit behaviours that would confer a degree of hypoxia‐tolerance. Specifically, it was hypothesised that as hypoxia progressed, gill ventilation rates (GVRs) would increase and aquatic surface respiration (ASR) would become increasingly frequent. Fish were subjected to rapid, progressive hypoxia from normoxia to anoxia in open tanks. 3. All tested species exhibited behaviours consistent with their use of potentially hypoxic habitats. As hypoxia progressed, GVRs increased and all species, with the exception of oriental weatherloach, began to switch increasingly to ASR with 90% of individuals using ASR at various oxygen concentrations below 1.0 mg O₂ L⁻¹. Australian smelt, redfin perch and flat‐headed galaxias were the first three species to rise to ASR, with 10% of individuals using ASR by 2.55, 2.29 and 2.21 mg O₂ L⁻¹ respectively. Goldfish and common carp were the last two species to rise to ASR, with 10% of individuals using ASR by 0.84 and 0.75 mg O₂ L⁻¹ respectively. In contrast to other species, oriental weatherloach largely ceased gill ventilation and used air‐gulping as their primary means of respiration during severe hypoxia and anoxia. 4. Australian smelt, redfin perch and flat‐headed galaxias were unable to maintain ASR under severe hypoxia, and began exhibiting erratic movements, termed terminal avoidance behaviour, and loss of equilibrium. All other species continued to use ASR through severe hypoxia and into anoxia. Following a rise to ASR, GVRs either remained steady or decreased slightly indicating partial or significant relief from hypoxic stress for these hypoxia‐tolerant species. 5. Behavioural responses to progressive hypoxia amongst the fish species of the Ovens River floodplain indicate a generally high level of tolerance to periodic hypoxia. However, species‐specific variation in hypoxia‐tolerance may have implications for community structure of billabong fish communities following hypoxic events.     

The effects of endogenous or exogenous catecholamines on blood respiratory status during acute hypoxia in rainbow trout (Oncorhynchus mykiss)

Summary An extracorporeal circulation of rainbow trout (Oncorhynchus mykiss) was utilized to continuously monitor the rapid and progressive effects of endogenous or exogenous catecholamines on blood respiratory/acid-base status, and to provide in vivo evidence for adrenergic retention of carbon dioxide (CO₂) in fish blood (cf. Wood and Perry 1985). Exposure of fish to severe aquatic hypoxia (final P _wO₂=40–60 torr; reached within 10–20 min) elicited an initial respiratory alkalosis resulting from hypoxia-induced hyperventilation. However, at a critical arterial oxygen tension (P _aO₂) between 15 and 25 torr, fish became agitated for approximately 5 s and a marked (0.2–0.4 pH unit) but transient arterial blood acidosis ensued. This response is characteristic of abrupt catecholamine mobilization into the circulation and subsequent adrenergic activation of red blood cell (RBC) Na⁺/H⁺ exchange (Fievet et al. 1987). Within approximately 1–2 min after the activation of RBC Na⁺/H⁺ exchange by endogenous catecholamines, there was a significant rise in arterial PCO₂ (P _aCO₂) whereas arterial PO₂ was unaltered; the elevation of P _aCO₂ could not be explained by changes in gill ventilation. Pre-treatment of fish with the -adrenoceptor antagonist phentolamine did not prevent the apparent catecholamine-mediated increase of P _aCO₂. Conversely, pre-treatment with the -adrenoceptor antagonist sotalol abolished both the activation of the RBC Na⁺/H⁺ antiporter and the associated rise in P _aCO₂, suggesting a causal relationship between the stimulation of RBC Na⁺/H⁺ exchange and the elevation of P _aCO₂. To more clearly establish that elevation of plasma catecholamine levels during severe hypoxia was indeed responsible for causing the elevation of P _aCO₂, fish were exposed to moderate hypoxia (final P _wO₂=60–80 torr) and then injected intraarterially with a bolus of adrenaline to elicit an estimated circulating level of 400 nmol·l^-1 immediately after the injection. This protocol activated RBC Na⁺/H⁺ exchange as indicated by abrupt changes in arterial pH (pHa). In all fish examined, P _aCO₂ increased after injection of exogenous adrenaline. The effects on P _aO₂ were inconsistent, although a reduction in this variable was the most frequent response. Gill ventilation frequency and amplitude were unaffected by exogenous adrenaline. Therefore, it is unlikely that ventilatory changes contributed to the consistently observed rise in P _aCO₂. Pretreatment of fish with sotalol did not alter the ventilatory response to adrenaline injection but did prevent the stimulation of RBC Na⁺/H⁺ exchange and the accompanying increases and decreases in P _aCO₂ and P _aO₂, respectively. These results suggest that adrenergic elevation of P _aCO₂, in addition to the frequently observed reduction of P _aO₂ are linked to activation of RBC Na⁺/H⁺ exchange. The physiological significance and the potential mechanisms underlying the changes in blood respiratory status after addition of endogenous or exogenous catecholamines to the circulation of hypoxic rainbow trout are discussed.Abbreviations P _aCO₂ arterial carbon dioxide tension - P _aO₂ arterial oxygen tension - P _da dorsal aortic pressure - pHa arterial pH - P _wO₂ water oxygen tension - RBC red blood cell - V _f breathing frequency     

Influence of hypoxia on cardiac functions in the grass shrimp (Palaemonetes pugio Holthuis)

Crustaceans frequently encounter hypoxic water and have evolved a variety of compensatory mechanisms to deal with low O₂ conditions. Typically, large decapod crustaceans attempt to maintain cardiac output by increasing stroke volume to compensate for the hypoxia-induced bradycardia. Grass shrimp (Palaemonetes pugio), small hypoxic tolerant decapod crustaceans, were used to investigate cardiac responses to hypoxia in a smaller crustacean using videomicroscopy and dimensional analysis techniques. In addition, these techniques were compared to the more established dye dilution technique for calculation of cardiac output. No significant difference was found between the two methods for determining cardiac output in grass shrimp. Cardiac parameters (heart rate f_H, stroke volume V_S, and cardiac output V_b) were monitored in grass shrimp exposed to progressive hypoxia (P_O2s=20, 13.3, 10, 5.3, and 2 KPa O₂). Shrimp exhibit a cardiac response to hypoxia that is atypical when compared to larger crustaceans. Cardiac output was maintained until water P_O2 fell below 10 KPa O₂. This maintenance of V_b is consistent in both large and small decapods, however the mechanism differs. In grass shrimp, V_S was P_O2 dependent and declined significantly while f_H increased significantly when P_O2 was reduced to 13.3 KPa O₂.     

Developmental changes in the responses of O2 uptake and ventilation to acutely declining O2 tensions in larval krill Meganyctiphanes norvegica

The effect of exposure to acutely declining oxygen tensions on O₂ uptake (M_O2) and ventilation has been investigated in different larval stages of Northern krill Meganyctiphanes norvegica (calytopis III/early furcilia I, late furcilia I, furcilia III and V). An ability to regulate M_O2 during acutely declining P_O2 began to appear about furcilia III (critical O₂ tension or P_c=15.4±0.73 kPa) and had improved by furcilia V (P_c=12.6±0.39 kPa). Hypoxia-related hyperventilation was achieved by an increase in pleopod (but not thoracic limb) activity (P_c∼11 kPa), a sensitivity which also appeared at, or just before, furcilia V even though an earlier stage (furcilia III) had a full compliment of functional setose pleopods. While this regulatory ability appeared as the gills were beginning to form, furcilia V is still early in gill ontogeny compared with adults. Preexposure to very moderate hypoxia (60% and 70% O₂ saturation) of furcilia III and V resulted in substantial mortality, but where it did not (furcilia V, 80% O₂ saturation), there was no effect of keeping krill at this P_O2 on either M_O2 or ventilation, suggesting that the development of respiratory regulation in M. norvegica is not open to environmental influence in the same way as for other crustaceans. We suggest that ontogeny of pleopod control provides furcilia V+ with both a stronger means of propulsion, allowing the ontogeny of DVM but also with an ability to regulate M_O2 during exposure to acutely declining P_O2s. The onset of respiratory regulation (furcilia V) preceded the onset of DVM (furcilia VI+). As pleopod ontogeny is associated intimately with the ontogeny of DVM and respiratory regulation, in the Gullmarsfjord this co-occurrence is fortuitous as krill can be required during DVM to migrate into hypoxic water which they are not equipped to deal with, in physiological terms, before furcilia V.     

Comparative study of gill neuroepithelial cells and their innervation in teleosts and Xenopus tadpoles

Peripheral O₂ chemoreceptors initiate adaptive cardiorespiratory responses to hypoxia in vertebrates. Morphological and physiological evidence suggests that, in fish, neuroepithelial cells (NECs) of the gill perform this role. We conducted a comparative examination in three species of teleosts (zebrafish, goldfish and trout) and larvae of the amphibian Xenopus laevis, using whole-mount gill preparations and confocal immunofluorescence, to elucidate the distribution, morphology and innervation of gill NECs. Nerve fibres were immunolabelled with the neuronal marker zn-12 and were associated with serotonin-immunoreactive NECs in the gills of all species tested. With the exception of trout, innervated NECs were present on all gill arches in the filaments and respiratory lamellae in fish and on homologous structures in Xenopus (i.e. gill “tufts”, including respiratory terminal branches). Thus, the distribution and innervation of NECs of the internal gills of amphibians and teleosts are relatively well conserved, suggesting an important role for gill NECs as O₂ chemoreceptors in aquatic vertebrates. Furthermore, the size and density of gill NECs is variable among teleosts and developmental stages of Xenopus larvae and may be dependent on general gill dimensions or environmental conditions. This report constitutes the first comparative study of gill NECs in fish and amphibians and highlights the significance of gill NECs as an evolutionary model for studying O₂ sensing in vertebrates. We acknowledge the Natural Sciences and Engineering Research Council (NSERC) of Canada for funding through an operating grant to C.A.N., and the NSERC and the Ontario Graduate Scholarship (OGS) program for postgraduate scholarships to M.G.J.     

Dependence of oxygen uptake on ambient PO 2 in isolated perfused frog skin

Summary Rates of O₂ uptake across isolated perfused skin of bullfrogs (Rana catesbeiana) were measured in relation to blood flow at three levels of ambient O₂ tension: normoxia (O₂ tension=152 torr), hypoxia (12% O₂, 87 torr) and hyperoxia (42% O₂, 306 torr). At bulk perfusion rates ranging from 3.4 to 10.1 l·cm^-2·min^-1, O₂ uptake was positively correlated with hemoglobin delivery rate in both normoxia and hyperoxia, but was independent of delivery rate in hypoxia. Mean O₂ uptake in normoxia was 3.8 nmol O₂·cm^-2·min^-1 at a delivery rate of 9.8 nmol·cm^-2·min^-1 and 6.5 nmol O₂·cm^-2·min^-1 at a delivery rate of 28.3 nmol·cm^-2·min^-1. At any given bulk perfusion rate, oxygen uptake averaged about 49% lower in hypoxia than in normoxia, decreasing in proportion to the reduction of O₂ tension difference between medium and blood. In hyperoxia, O₂ uptake did not increase proportionally with the difference in O₂ tension between blood and medium, averaging only 50% higher at a 2.4-fold greater O₂ tension difference. Cutaneous diffusing capacity for O₂ averaged 0.041 nmol O₂·cm^-2·torr^-1·min^-1 during the first hour of perfusion in normoxia, and was not affected by reduction of ambient O₂ tension. The results indicate that cutaneous O₂ uptake in hypoxia is highly diffusion limited, and consequently, increases in cutaneous perfusion can not effectively compensate for reduction of ambient O₂ tension. In hyperoxia, O₂ uptake may be substantially perfusion limited because of reduced blood O₂ capacitance at high O₂ saturations.Abbreviations O₂ capacitance - C _Hb hemoglobin concentration - D diffusing capacity - PO₂ medium-blood PO₂ difference - Hb flow, hemoglobin delivery rate - Hepes N-[2-Hydroxyethyl]piperacine-N-[2 ethanesulfonic acid] - L _diff extent of diffusion limitation - MO₂ oxygen uptake rate - PO₂ oxygen tension - S O₂ saturation     

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     

Short‐term effect of hypoxia on ammonia excretion and respiration rates in the crab carcinus maenas

The metabolic response of the crab Carcinus maenas to short‐term hypoxia (60% and 35% saturated seawater) was studied at 17.5°C in fed, 3 day‐unfed and 6 day‐unfed crabs.

Ammonia excretion rate decreased under hypoxia: a 40% and 45% decrease in the normoxic rate was observed in fed crabs at 35% saturation and in 3 day‐unfed crabs at both hypoxic levels respectively. In the 6 day‐unfed crabs, the effect of hypoxia was concealed by the effect of starvation.

Oxygen consumption rate was directly related to the external O₂ tension irrespective of the crab's nutritional state. Stressed crabs behaved as a whole, as oxygen‐conformers.

A strong relationship was observed between ammonia excretion and oxygen consumption rates in fed crabs under hypoxia but not in starved crabs.     

Brood protection at a cost: mouth brooding under hypoxia in an African cichlid

This study quantifies the behavioral response of the widespread mouth brooding African cichlid Pseudocrenilabrus multicolor victoriae to progressive hypoxia. We exposed four gender/stage classes of P. multicolor (males, brooding females, females that had just released young, and non-brooding females) to progressive hypoxia and recorded the percent time spent using aquatic surface respiration (surface skimming, ASR) and gill ventilation rates. This was done for fish collected from three sites in Uganda (lake, swamp, and river) after long-term acclimation to normoxia. There was no effect of site of origin on response to hypoxia, but ASR thresholds did differ between gender/stage classes. The oxygen level (threshold) at which spent 10, 50, and 90% of their time at the surface using ASR was much higher for brooding females than for males, whereas ASR thresholds did not differ between non-brooding females and males. Similarly, the level at which ASR was initiated was much higher in brooding females than males, but did not differ between males and non-brooders, or between males and females than had just released young. The rate of gill ventilation dropped significantly in males and all stages of females after initiation of ASR, suggesting that surface skimming increases efficiency of oxygen acquisition. These results suggest that mouth brooding in female P. multicolor ASR improves oxygen uptake but imposes a cost in terms of time spent at the water surface, and this may affect maternal predation risk in low-oxygen habitats.     

Hypoxia tolerance in two juvenile estuary-dependent fishes

Hypoxia events, or low dissolved oxygen (DO) conditions, occur frequently in North Carolina estuaries during the summer. These events may have harmful effects on important fish stocks, including spot (Leiostomus xanthurus) and Atlantic menhaden (Brevoortia tyrannus), but their consequences are not well understood. We investigated direct mortality due to hypoxia in juvenile spot and Atlantic menhaden to determine how the extent of mortality varies with the severity of hypoxia and the duration of exposure, and to explore how vulnerability to hypoxia changes across species, fish size, and temperature.Atlantic menhaden and spot were tested at two temperatures, 25 and 30 °C, and three dissolved oxygen concentrations, 0.6, 0.9, and 1.2 ppm. Survival analyses were performed on the data relating survival rate of each species to dissolved oxygen concentration, duration of exposure, fish size, and temperature. The data were analyzed using an LC₅₀ approach for comparative purposes, and 12-h LC₅₀ estimates ranged from 0.9 to 1.1 ppm O₂. Spot and menhaden exposed to 1.2 ppm O₂ showed no mortality in 24 h at 25 °C, and only 30-40% mortality at 30 °C. In contrast, both species experienced 100% mortality in 2-6 h at 0.6 ppm O₂. There was an effect of size on hypoxia tolerance, with small spot being less tolerant than large spot, while the converse size effect was observed for menhaden. Spot were consistently less tolerant to hypoxia than menhaden and both species were less tolerant to hypoxia at 30 °C than at 25 °C. Preliminary experiments showed a 24-h acclimation to sublethal levels of hypoxia significantly reduced mortality upon subsequent exposure to lethal hypoxia concentrations.Our results indicate that direct mortality due to hypoxia will vary with species, size, and temperature, but will likely only be substantial when these species are exposed to oxygen concentrations less than about 1 ppm O₂. Given the severity of hypoxia necessary to cause mortality and the ability of fish to behaviorally avoid hypoxia, direct mortality due to hypoxia may have limited impacts on fish population dynamics. Therefore, the greatest effects due to hypoxia may be caused by the stress imposed by sublethal hypoxic conditions alone or in concert with other stressors, or by indirect effects incurred by avoiding hypoxic areas.     

Thyroid function during intermittent exposure to hypobaric hypoxia

Circulatory levels of triiodothyronine (T₃) and thyroxine (T₄) and their kinetics were studied in rabbits exposed to intermittent hypobaric hypoxia (5200 m, 395 mm Hg,PO₂ 83 mm Hg) 6 h daily for 5 weeks in a decompression chamber maintained at room temperature of 22°–24° C. Kinetics of T₃ and T₄ were studied on days 21 and 28 of hypoxic exposure. The T₃ and T₄ values were found to be significantly lower on day 8 of exposure to hypoxia compared to the pre-exposure values. The decreased levels were maintained throughout the entire period of hypoxic stress. The metabolic clearance rate, production rate, distribution space and extrathyroidal T₃ and T₄ pools were significantly decreased in animals under hypoxic stress compared to the control animals. The decline in thyroid hormone levels and their production in rabbits under hypoxic stress indicate an adaptive phenomenon under conditions of low oxygen availability.     

Oxygen and carbon dioxide exchanges in crassulacean-acid-metabolism-plants

The 24 h O₂ uptake and release together with the CO₂ balance have been measured in two CAM plants, one a non-succulent Sempervivum grandifolium, the other a succulent Prenia sladeniana. The O₂ uptake was estimated by the use of ¹⁸O₂. It was found that the mean hourly O₂ uptake in the light was 7 times that in the dark for Sempervivum and 5 times that for Prenia, after correction for the lightdark temperature difference. It was estimated that oxygen uptake in the light was 2.4 times greater than oxygen release (=net photosynthesis) in Sempervivum and 1.4 times greater in Prenia. In both plants there was a positive carbon balance over the 24 h period under the experimental conditions. It was estimated that malate formed during the night could, if completely oxidized to CO₂ and water, account for 74% of the light phase O₂ uptake in Sempervivum. In Prenia the O₂ uptake was more than sufficient to account for a full oxidation of malate.Abbreviations CAM Crassulacean acid metabolism - PAR photosynthetically active radiation - PEP phosphoenolpyruvate - RrBP ribulose-1,5-bisphosphate - TCA tricarboxylic acid cycle     

Respiratory characters of three species of haplochromine cichlids: Implications for use of wetland refugia

Respiratory characters of three east African haplochromine cichlid species that differ in their use of hypoxic wetlands were examined to consider the potential of dissolved oxygen as one factor affecting habitat use. All three species had a large gill surface area, ranging from the 67th ( Pseudocrenilabrus multicolor victoriae ) to 98th ( Astatotilapia velifer ) percentile of the known gill size range for freshwater fishes. Pseudocrenilabrus multicolor victoriae was the most tolerant to hypoxia exhibiting the lowest aquatic surface respiration ( Rs )thresholds and lowest critical oxygen tension of the three species. Astatotilapia velifer had the highest ASR thresholds, gill ventilation rates, and level of surface activity of the three species, indicating a relatively low tolerance to hypoxia. Prognathochromis venator was intermediate in its response to hypoxia. These findings are discussed in light of survivorship and distribution patterns of these species following Nile perch introduction into Lake Nabugabo.