Normoxic Water Research Articles

The physiology of hibernation in common map turtles ( Graptemys geographica)

Map turtles from Wisconsin were submerged at 3°C in normoxic and anoxic water to simulate extremes of potential respiratory microenvironments while hibernating under ice. In predive turtles, and in turtles submerged for up to 150 days, plasma P o 2, P co 2, pH, [Cl −], [Na +], [K +], total Mg, total Ca, lactate, glucose, and osmolality were measured; hematocrit and body mass were determined, and plasma [HCO 3 −] was calculated. Turtles in anoxic water developed a severe metabolic acidosis, accumulating lactate from a predive value of 1.7 to 116 mmol/l at 50 days, associated with a fall in pH from 8.010 to 7.128. To buffer lactate increase, total calcium and magnesium rose from 3.5 and 2.0 to 25.7 and 7.6 mmol/l, respectively. Plasma [HCO 3 −] was titrated from 44.7 to 4.3 mmol/l in turtles in anoxic water. Turtles in normoxic water had only minor disturbances of their acid–base status and ionic statuses; there was a marked increase in hematocrit from 31.1 to 51.9%. This study and field studies suggest that map turtles have an obligatory requirement for a hibernaculum that provides well-oxygenated water (e.g. rivers and large lakes rather than small ponds and swamps) and that this requirement is a major factor in determining their microdistribution.

Blood viscosity and hematological changes during prolonged submergence in normoxic water of northern and southern musk turtles (Sternotherus odoratus

Musk turtles (Sternotherus odoratus) can survive at least 150 days of submergence in normoxic water at 3°C, during which time there are large increases in packed cell volume (PCV). We investigated the effects of submergence in normoxic water at 3°C on the blood viscosity of musk turtles from northern (Massachusetts) and southern (Alabama) locales. Blood was collected from air-breathing turtles and after 20, 50, 100, and 150 days of submergence in normoxic water at 3°C. Hematological responses to submergence were similar in the two groups, therefore the results were combined. Packed cell volume increased steadily above that of controls after 20, 50, 100, and 150 days of submergence. Hemoglobin concentration also progressively increased above that of controls after 20, 50, and 100 days of submergence but declined to near control values after 150 days. Blood viscosity increased with increasing PCV; however, blood viscosity of musk turtles appears less affected by PCV than is blood viscosity of mammalian species. As such, musk turtles appear to be able to maintain adequate blood flow to tissues while increasing the oxygen carrying capacity of the blood during prolonged submergence. However, after 150 days submergence, oxygen delivery should decrease due to a reduced oxygen carrying capacity of the blood and an increased resistance to blood flow, which may limit the length of time these turtles can remain viable during hibernation. J. Exp. Zool. 287:459–466, 2000. © 2000 Wiley-Liss, Inc.

The physiology of hibernation among painted turtles: the midland painted turtle ( Chrysemys picta marginata)

Midland painted turtles from Michigan were submerged at 3°C in normoxic and anoxic water. In predive, and in turtles submerged for up to 150 days, plasma PO 2, PCO 2, pH, [Cl −], [Na +], [K +], total Mg, total Ca, lactate, glucose, and osmolality were measured; hematocrit and mass were determined, and plasma [HCO 3 −] was calculated. Anoxic turtles developed a severe metabolic acidosis, accumulating lactate from a predive value of 4.4 mmol/L to a 150-day value of 185 mmol/L, associated with a fall in pH from 7.983 to 7.189. To buffer lactate increase, total calcium and magnesium rose from 3.7 and 2.6 to 58.9 and 11.8 mmol/L, respectively. Plasma [HCO 3 −] was titrated from 39.2 to 4.8 mmol/L in anoxic turtles. Turtles in normoxic water had only minor disturbances of their acid–base and ionic statuses, associated with a much smaller increase of lactate to 23 mmol/L; there was a marked increase in hematocrit from 29.1% to 42.1%. We suggest that it is ecologic, rather than phylogenetic, relationships that determine the responses of painted turtles to prolonged submergence associated with hibernation.

Overwintering behavior and physiology of eastern painted turtles (Chrysemys picta picta) in Rhode Island

We equipped 20 eastern painted turtles (Chrysemys picta picta) with radio transmitters and recovered them from a pond in Rhode Island during the winter of 1998-1999. Each month, three turtles were collected without permitting them to breathe. Blood was sampled by cardiac puncture and analyzed for pH, PCO2, PO2, and hematocrit; plasma was analyzed for [Na+], [K+], [Cl–], total [Ca], total [Mg], [lactate], and osmolality. In December 1998, mean values for pH, PO2, PCO2, [HCO3–], [lactate], total [Ca] and [Mg], hematocrit, and osmolality were 7.84 ± 0.02, 4.7 ± 1.9 mmHg (1 mmHg = 133.3 Pa), 10.2 ± 1.2 mmHg, 25.4 ± 2.6 mmol·L–1, 2.78 ± 1.16 mmol·L–1, 3.2 ± 0.4 mmol·L–1, 2.5 ± 0.1 mmol·L–1, 23% red blood cells, and 271 mosmol·kg–1H2O, respectively, values similar to those for turtles submerged in normoxic water for 10 days at the prevailing water temperature (2-3°C). Throughout the winter, ice intermittently covered approximately 80% of the pond, water PO2was greater than 75% of air saturation, and blood PO2was maintained between 0.8 ± 0.3 and 10.1 ± 1.1 mmHg. Between December and February, there were no changes in most of the measured blood variables but, in March 1999, although the pond was largely free of ice, plasma [lactate], total [Ca], and total [Mg] averaged nearly 30, 8.1 ± 1.7, and 4.5 ± 0.8 mmol·L–1, respectively, although with a large variation among individuals. The turtles did not bury in the substrate during the winter and, despite the increase in plasma lactate, there was no significant acidemia. However, the winter of 1998–1999 was relatively mild, and it is possible that more significant acid-base and ionic perturbations could occur during more severe winters, particularly in small ponds that cool sooner in late autumn and may have more prolonged ice cover than larger bodies of water.

Effects of hypoxia, anoxia, and endogenous ethanol on thermoregulation in goldfish, Carassius auratus.

Effects of hypoxia, anoxia, and endogenous ethanol (EtOH) on selected temperature (T(sel)) and activity in goldfish were evaluated. Blood and brain EtOH concentrations ([EtOH]) and brain oxygen partial pressure (PO(2)) were quantified at crucial ambient oxygen pressures. Below a threshold value near 31 Torr, T(sel) decreased as a function of environmental PO(2). T(sel) of 15 degrees C-acclimated fish was approximately 10 degrees C at the onset of anoxia and changed little over 2 h. Activity showed a similar response pattern. Brain [EtOH] was significantly elevated above control levels after 1 h anoxia. In normoxic water, T(sel) remained different in previously anoxic and normoxic control fish for approximately 20 min. Blood [EtOH] of previously anoxic fish remained significantly elevated ([EtOH] >4.0 micromol/g blood), and activity was significantly depressed at 20 min. Brain PO(2) reached normal levels in <3 min. We conclude that [EtOH] (brain or blood) and brain PO(2) are not proximal causes of either behavioral anapyrexia (hypothermia) or inactivity in goldfish exposed to oxygen-depleted environments.

The influence of haemoglobin on behavioural thermoregulation and oxygen consumption in Daphnia carinata.

When placed in a temperature gradient, most ectotherms have a strict thermal preference that is lowered on exposure to hypoxia. Branchiopods, small aquatic crustaceans, are known to synthesise haemoglobin (Hb) when exposed to hypoxia; hypoxia can occur diurnally and seasonally in ponds. The effect of Hb on behavioural thermoregulation in the branchiopod Daphnia carinata following exposure to both normoxia and hypoxia was examined. Control animals raised in normoxia (Po2=150 mmHg, [Hb]=0.026+/-0.007 mg g dry wt-1) and Hb-rich animals raised in hypoxia (Po2=70 mmHg, [Hb]=0.080+/-0.017 mg g dry wt-1) were placed (N=30) in a tube (length=500 mm, diameter=8 mm) filled with pond water. In the absence of a thermal gradient, control and Hb-rich animals in normoxic water were uniformly distributed along the tube. The presence of a thermal gradient (13 degrees -28 degrees C) elicited clustering at a preferred temperature, T approximately 23 degrees C for both groups. Exposure to hypoxic water in a thermal gradient resulted in a behavioural shift: T approximately 16 degrees C for controls and T approximately 19 degrees C for Hb-rich animals. Measurements of oxygen consumption (V&d2;o2) at fixed temperatures revealed that Hb is associated with a metabolic acclimation to hypoxia.

The physiology of hibernation among painted turtles: the Eastern painted turtle Chrysemys picta picta.

Eastern painted turtles (Chrysemys picta picta) from Connecticut were submerged at 3 degrees C in normoxic and anoxic water to simulate potential respiratory environments within their hibernacula. Those in normoxic water could survive submergence for at least 150 d, while those in anoxic water could survive for a maximum of about 125 d. Turtles in normoxic water developed a slight metabolic acidosis as plasma lactate accumulated to about 50 mM in 150 d, while anoxic turtles developed a severe lactic acidosis as plasma lactate reached about 200 mM in 125 d; there was no respiratory acidosis in either group. Plasma [Na+] changed little in either group, [Cl-] fell by about one-third in both, and [K+] increased by about fourfold in anoxic turtles but only slightly in those in normoxic water. Total plasma magnesium and calcium increased profoundly in anoxic turtles but moderately in those in normoxic water. Consideration of charge balance indicates that all major ions were measured in both groups. Plasma glucose remained unchanged in anoxic turtles until after about 75 d of submergence, when it increased and continued to increase with the duration of anoxia, with much variation among individuals; glucose remained unchanged throughout in turtles in normoxic water. Hematocrit doubled in 150 d in turtles in normoxic water; in anoxic turtles, an initial increase was no longer significant by day 100. Plasma osmolality increased markedly in anoxic turtles, largely because of accumulation of lactate, but anoxic turtles only gained about half the mass of turtles in normoxic water, who showed no increase in osmolality. The higher weight gain in the latter group is attributed to selective perfusion and ventilation of extrapulmonary gas exchange surfaces, resulting in a greater osmotic influx of water. The physiologic responses to simulated hibernation of C. picta picta are intermediate between those of Chrysemys picta bellii and Chrysemys picta dorsalis, which correlates with the severity of the winter each subspecies would be expected to encounter.

Arterial blood gas levels and cardiovascular function during varying environmental conditions in a mudskipper, periophthalmodon schlosseri

Changes in blood gas levels, blood pressure and heart rate were studied in chronically cannulated mudskippers, Periophthalmodon schlosseri, subjected to air exposure (6 h), aquatic hypoxia with access to air (water PO2 <0.9 kPa, 6 h) and forced submersion in normoxic water (12 h) at 30 degrees C. Air exposure did not affect either blood O2 and had little effect on blood CO2 levels, but blood pH increased slightly, but significantly. Blood ammonia concentration was elevated sixfold during air exposure. Aquatic hypoxia caused no significant changes in blood gas levels. When the fish was forcibly submerged, blood O2 saturation decreased rapidly to approximately 30 %. Blood PCO2 and total CO2 also decreased, but blood pH was unaffected by forcible submersion. Air exposure did not affect blood pressure or heart rate. Aquatic hypoxia did not affect blood pressure but transiently increased heart rate. In contrast, forced submersion significantly depressed heart rate throughout the period of submersion, while blood pressure decreased only transiently. Upon emersion, the heart rate immediately increased to above the control level when the fish took its first air breath.

The emersion response of the Australian Yabby Cherax destructor to environmental hypoxia and the respiratory and metabolic responses to consequent air-breathing

The Australian Yabby Cherax destructor voluntarily emerges from water to breathe air with increased frequency as water PO2 decreases. When the water PO2 declined below 2.7 kPa the crayfish spent >50% of time breathing air. The respiratory gas transport, acid-base, ionic and energetic status were quantified in simulations of this emersion behaviour to determine the benefits that the crayfish may gain from switching to air-breathing. C. destructor initially showed an elevated O2 uptake rate on emerging from hypoxic water, but after 1 h the O2 uptake rate was not different from that of crayfish in normoxic water. During 3 h of air breathing, subsequent to 2.7 kPa aquatic hypoxia, the haemolymph PO2 increased while oxygen content was essentially unchanged, although cardiac output increased 5-fold. The haemolymph PCO2 increased from 0.44 to 1.21 kPa after 3 h while the CO2 content increased from 3.47 to 8.66 mmol · l−1 and the pH decreased from 7.73 to 7.57 after 1 h in air. In air C. destructor eventually achieved an O2 uptake rate similar to that achieved in water. A general hyperglycaemia occurred without anaerobiosis. In air-breathing C. destructor, small changes in lactate appear to offset the decrease in haemocyanin-O2 affinity caused by acid Bohr shift. During air-breathing, decreased haemocyanin-O2 affinity assisted in maintaining O2 diffusion into the tissues, but the ATP content of the tail muscle decreased so that after 3 h in air the energy charge was only 0.59. The data are consistent with a specific depression of the Emden-Meyerhof pathway, preventing either lactate formation or oxidative phosphorylation in the tail muscle, despite a concomitant glycogenolysis.

Oxygen uptake in bullfrog tadpoles (Rana catesbeiana)

Weight-specific rates of aquatic oxygen consumption (VO2, microliter O2 g-1 h-1) at 23 degrees C were determined for water-breathing (e.g., forcibly submerged) bullfrog tadpoles as functions of stage of development and O2 tension (PO2). The VO2 at an O2 tension near that of air-saturated water (PO2 approximately 154 mmHg) was independent of stage of development throughout the premetamorphic stages (I-XIX). Aquatic VO2 increased by approximately 24%, relative to the average of the preceding stages, during the first metamorphic stage (XX) and thereafter decreased steadily with developmental stage. The decline in aquatic VO2 resulted in a shift from facultative air-breathing to obligate air-breathing at about stage XXII. Changes with developmental stage of the critical O2 tension (Pc) corresponded to changes in aquatic VO2. The Pc was low and relatively constant at 29-36 mmHg through stage XVI, started to increase (to 51 mmHg) during the final premetamorphic stages (XVII-XIX), reached a value near air saturation (159 mmHg) at stage XXII, and was in excess of air saturation for stages XXIII-XXV. The ability to survive continuous submergence paralleled the changes in Pc, as tadpoles could survive in air-saturated water without access to air through stages XXII-XXIII, when they drowned. Whole-body lactate concentrations of tadpoles in normoxic water with access to air averaged 0.56 mg/g through stage XX, comparable to that of froglets (stage XXV, 0.67 mg/g) in shallow water. Animals in anoxic water with access to air exhibited an approximate doubling (1.05 mg/g) of lactate concentration for all stages, as did metamorphic stages XXI-XXV in normoxic water with air access. Recently, metamorphosed frogs (stage XXV) could survive continuous submergence for up to 200 h without accumulating lactate if the water was hyperoxic (600-700 mmHg), suggesting that cutaneous O2 exchange in normoxic water is diffusion-limited. Stages I-XXI in normoxic water breathed air regularly but infrequently (0.4-6.2 surfacings/h), with earlier stages breathing more frequently than later stages. While not required for oxygen uptake during these stages, air-breathing may serve to promote lung development, to prevent lung collapse, or to prevent accumulation of fluid in the lungs. Surfacing rates increased as the PO2 of the water decreased, but we could discern no clear relationship between the Pc for surfacing and developmental stage. Stages XXII-XXV spent most of their time floating. The changes in aquatic VO2, Pc, surfacing behavior, and survivorship when forcibly submerged, all suggest that stage XXII is a critical developmental stage during which bullfrog tadpoles switch from a primarily aquatic to a primarily terrestrial mode of existence. When provided with a choice of being in or out of water, early stages abruptly change from a preference for water (e.g., 92.5% of stage XXI) to a preference for land by stage XXII (74%), further indicating the transitional nature of the latter stage.

The occurrence of aerial respiration in Rhinelepis strigosa during progressive hypoxia

Rhinelepis strigosa did not surface for air breathing in normoxic or moderate hypoxic water. This species initiated air breathing when the Pio2 in the water reached 22 ± 1 mmHg. Once begun, the air‐breathing frequency increased with decreasing Pio2. Aquatic oxygen consumption was 21·0 ± 1·9ml O2 kg−1h−1 in normoxic water, and was almost constant during progressive hypoxia until the Pio2 reached 23·9 mmHg, considered the critical oxygen tension (Pco2). Gill ventilation increased until close to the Pco2 (7·9‐fold) as a consequence of a greater increase in ventilatory volume than in breathing frequency. Gill oxygen extraction was 42 ± 5% and decreased with hypoxia, but under severe hypoxia returned to values characteristic of normoxic. The critical threshold for air breathing was coincident with the Pco2 during aquatic respiration. This suggests that the air‐breathing response is evoked by the aquatic oxygen tension at which the respiratory mechanisms fail to compensate for environmental hypoxia, and the gill O2 uptake becomes insufficient to meet O2 requirements.

Maintenance of the Adh polymorphism in Ambystoma tigrinum nebulosum (tiger salamanders). I. Genotypic differences in time to metamorphosis in extreme oxygen environments

Populations of gilled tiger salamanders (Ambystoma tigrinum nebulosum) living in ephemeral ponds were studied in order to determine what evolutionary processes might be affecting genetic variation at the alcohol dehydrogenase (Adh) locus. Ponds frequently dried in late summer; salamanders that did not metamorphose and leave drying ponds died of desiccation. Low levels of water oxygen significantly slowed metamorphosis of tiger salamanders in both natural and laboratory populations. Adh- SS frequency was significantly positively correlated with daily oxygen maxima in ponds, Adh-FF frequency demonstrated a nonsignificant trend to be negatively correlated with this measure, and Adh-SF frequency was unrelated to pond oxygen. Genotype frequency changes across the season in hypoxic and supersaturated ponds suggested that differential mortality of genotypes might explain the relationships with pond oxygen, although other mechanisms cannot be ruled out. Individuals of the Adh-SS genotype metamorphosed more slowly in hypoxic water than in normoxic water in the laboratory, and Adh-FF individuals metamorphosed more slowly in supersaturated water in the field, and in hypoxic water in the laboratory, than they did in normoxic water. Adh-SF individuals metamorphosed at the same rate in all oxygen environments. These data suggest that the Adh polymorphism in tiger salamanders is being maintained by selection for Adh or for a linked locus through differential metamorphosis in extreme oxygen environments.

Maintenance of Enzyme Activity Levels during Long-Term Aerobic Diving in the Red-Eared Turtle Trachemys scripta elegans

Energy metabolism in red-eared turtles (Trachemys [=Pseudemys] scripta elegans) was monitored in air-breathing animals at 11°-14° C, in animals forced to dive over a 16-wk period in normoxic water at 3° C, and in animals recovering for up to 8 wk from forced dives. Diving had no effect on blood glucose levels; hematocrit and blood lactate were marginally higher during diving Heart and brain glycogen levels did not decrease during diving These findings suggest that anaerobic metabolism was not activated and that survival was more likely dependent on metabolic depression and/or a low level of aquatic respiration. The absence of an activation of anaerobic metabolism in this species is in contrast to the usual response in diving turtles. Heart and brain total RNA levels remained constant during the diving period, but there was an increase in total RNA during recovery, which suggests a burst in protein synthesis at this time. Activity levels of hexokinase, pyruvate kinase, lactate dehydrogenase, citrate synthase, 3-hydroxyacyl-CoA dehydrogenase, and cytochrome oxidase were measured. In heart, pyruvate kinase levels showed a tendency to decline during the diving period; pyruvate kinase and cytochrome oxidase increased during recovery. In brain, cytochrome oxidase declined during diving; pyruvate kinase, lactate dehydrogenase, citrate synthase, and cytochrome oxidase increased during recovery. The increase in activity of various enzymes during recovery was consistent with the elevation in RNA content and may be associated with increased cardiovascular activity, neural activity, and nutrient supply of amino acids.

Air Breathing by the Purple Shore Crab, Hemigrapsus nudus (Dana). IV. Aquatic Hypoxia as an Impetus for Emersion? Oxygen Uptake, Respiratory Gas Transport, and Acid-Base State

Blood gas and acid-base status in the purple shore crab, Hemigrapsus nudus (Dana), was determined before, during, and after 3 h of exposure to water with a PO₂ of 20 mmHg. The ventilatory, circulatory, and O₂ uptake responses were assessed during progressive hypoxia to a PO₂ of 5 mmHg. The respiratory rate was independent of PO₂ to 10 mmHg and was achieved by increasing cardiac output rather than hyperventilation or tachycardia. In contrast to the respiratory alkalosis seen in most other hypoxic crustaceans, Hemigrapsus exhibited a large and rapid metabolic alkalosis within 30 min of exposure to water with a PO₂ of 20 mmHg. Subsequently, there was a smaller respiratory alkalosis so that by 3 h of elapsed time haemolymph pH had increased to a pH of 8.19. Return to normoxia prompted an almost immediate removal of both the respiratory and metabolic alkaloses. There was no evidence of any anaerobiosis. A small increase in circulating urate toward the end of the exposure period (∼0. 05 mmol L⁻¹ maximum) suggested that O₂ use by urate oxidase might be limiting. Urate accumulation in Hemigrapsus was shown to be temperature-dependent. Exposure to 20 mmHg PO₂ induced decreases in both arterial and venous O₂ pressure and content, while O₂ delivery was maintained, as was an arterial-venous difference. Fitting the in vivo data to O₂ equilibrium curves indicated increased haemocyanin O₂ affinity due to the alkalosis and increased urate, which served to maintain arterial oxygenation near 100%. Oxygen delivery switched from a reliance on dissolved O₂, in normoxic water to a dependency on haemocyanin O₂ transport. The apparent perfusion-based compensation for reduced O₂ availability, the rapid metabolic alkalosis, and the ability of the haemocyanin in Hemigrapsus to remain unusually well oxygenated are features quite different from those reported for other species of similar habit and habitat. This strongly implies that Hemigrapsus is well adapted to hypoxic exposure and that avoidance of hypoxic water is not a stimulus for this crab to assume air breathing. The role of aquatic hypoxia as a selective pressure in crustaceans evolving to life on land must be questioned.

Gill morphometry of the facultative air-breathing loricariid fish,Hypostomus plecostomus (Walbaum) with, special emphasis on aquatic respiration

Gill respiratory surface area and oxygen consumption during aquatic respiration were measured in the facultative air-breathing loricariid fish,Hypostomus plecostomus. The fish did not surface to breathe atmospheric air in normoxic water; air-breathing was evoked by environmental hypoxia (water oxygen tension=35±2, mmHg) and did not show size-related threshold differences for air breathing.During gradual hypoxia, without access to atmospheric, air,H. plecostomus was found to be an oxyregulator and showed a reduced range of water oxygen tension in which the oxygen consumption remained constant in smaller fish. The critical oxygen tensions were 55 and 33 mmHg at 25°C for fish of 14-30 g and 31-80g body weight, respectively.The gill respiratory surface area (total lamellae area) is reduced, however, the lamellar frequency per mm of gill filament is high which facilitates the gas exchange. Moreover, the increase of gill respiratory surface area (b=0.666) is higher than the increase in routine VO2 (b=0.338) showing a positive relationship between the gill respiratory surface area /VO2 ratio and body mass (b=0.328); this indicates that the fish have greater gill respiratory surface area per unit of routine VO2 as they grow.

A field versus laboratory study of blood oxygen status in normoxic crabs at different temperatures

The blood oxygen status of two species of active crabs (Carcinus maenas and Necora puber) was studied in the field and compared with the results of previous laboratory experiments performed on a wide spectrum of physiologically different water-breathers. The aim was to determine whether, as in the laboratory, the functioning of the O2supply system in the field could be based on maintaining the arterial [Formula: see text] in the low range, 1–3 kPa. The O2partial pressures and concentrations in the arterial and venous blood, arterial blood pH, and blood respiratory pigment concentration were measured in normoxic water at various temperatures ranging from 10 to 20 °C and in various seasons. In the field, [Formula: see text] values in normoxic C. maenas and N. puber were in the low range, 1–3 kPa, independently of temperature, season, and blood haemocyanin concentration. It is concluded that in the field as in the laboratory, [Formula: see text] values mainly in the low range provide a head pressure sufficient to meet O2needs. The changes that appear to occur in other respiratory variables are discussed in relation to field versus laboratory conditions and temperature differences. The consequences for analysing problems of hypoxaemia in hypoxic waters or situations are discussed.

Effects of experimental ventilation and ambient Po 2 on O 2 uptake of the isolated cutaneous tissue in the carp, Cyprinus carpio

Effects of experimental ventilation and ambient Po 2 on cutaneous O 2 uptake in vitro were studied in the carp, Cyprinus carpio. Oxygen uptake rate of the isolated cutaneous tissue was determined by ventilating the epidermis side of the skin with normoxic water in flow-through respirometers. Oxygen uptake rate of the skin increased with ventilation rate across the skin between 2.5 and 40 ml/min and became 3.2 nmol/cm 2/min at a flow rate of 40 ml/min, which corresponds to an apparent water velocity of 1.1 cm/sec. At a ventilation rate of 10 ml/min, oxygen uptake rate of the skin increased with the ambient Po 2 between 115 and 230 Torr and became constant (3.8 nmol/cm 2/min) between 230 and 295 Torr. When both sides of the skin were ventilated with normoxic water, oxygen uptake rate of the skin increased and became 3.7 nmol/cm 2/min at a flow rate of 20–40 ml/min. These results suggest that the oxygen requirement of the skin is 3.7–3.8 nmol/cm 2/min at 21.3°C and that cutaneous O 2 uptake in vitro depends on experimental ventilation and ambient Po 2, consistent with values measured in vitro in the carp (ref).

The control of ventilatory and cardiac responses to changes in ambient oxygen tension and oxygen demand in octopus

Octopus vulgaris can regulate its oxygen uptake down to a PO2 of around 6.7 kPa. As the tension falls from 18.6 to 6.7 kPa (140 to 50 mmHg), Pv (the pressure pulse driving the ventilatory flow, measured inside the mantle cavity) can more than double while fv (the ventilation frequency) increases by a few per cent at most. Both changes are reversed when the ambient oxygen tension is returned to normal. Cutting the visceral nerves linking the hearts and gills to the brain prevents these adaptive changes in Pv and fv, as does section of the branchial nerves linking the cardiac ganglia to the gills. Responses to changes in ambient oxygen tension are very fast, beginning within two or three ventilation cycles. It is concluded that changes to Pv and fv depend upon receptors in the gills and on the integrity of a nervous pathway to the brain. Changes in oxygen tension also affect the hearts, where aortic pulse amplitude (Pa) and, to a lesser extent, heartbeat frequency (fh) fall and rise with the ambient PO2. In this case, section of the visceral or branchial nerves has no effect. Responses are again very rapid. It is concluded that the observed fall and return to normoxic values of Pa and fh are local responses to a fall and rise in the oxygen tension of blood coming from the gills into the systemic heart. Changes to ventilation and heartbeat can also occur in normoxic water when oxygen demand rises after feeding. These responses are not prevented by section of the visceral or branchial nerves. Possible control of ventilation and heartbeat through the neurosecretory system in the anterior vena cava is discussed.

Energetics of fish larvae, the smallest vertebrates.

In this review recent findings on the energetics of fish larvae are presented, highlighting some of the physiological problems linked to small body size. The existence of a mass-independent phase of specific metabolic rate is confirmed but it is pointed out that in young fish ontogenetic transitions of metabolic scaling have so far been documented only for the routine level of activity. Maximum metabolic rate is limited by mitochondrial density in the swimming muscles which scales with a mass exponent of approximately 0.9. Mitochondrial density in the swimming muscles of a species of fish, from larva to adult, covers about the same range as mitochondrial density in the skeletal muscles of mammals. However, the aerobic capacity (power density) of mitochondria is one order of magnitude lower in fish than in mammals. Energy metabolism in embryos and early larvae of fish is almost entirely aerobic. Anaerobic power in the fast muscle fibres is low after hatching but increases during the transition from larva to juvenile with a mass exponent greater than one. In hypoxic water fish larvae swim more economically (i.e. their cost of transport is lower) than in normoxic water. If the rate of growth exceeds a critical threshold (about 10% d-1) fish larvae are capable of increasing the apparent efficiency of growth, probably by reducing the costs of other energy-consuming functions of maintenance.

SPONTANEOUS SWIMMING ACTIVITY OF ATLANTIC COD GADUS MORHUA EXPOSED TO GRADED HYPOXIA AT THREE TEMPERATURES

The spontaneous swimming activity of Atlantic cod Gadus morhua was investigated at graded levels of hypoxia at three temperatures (5, 10 and 15 &deg;C) by using a computerized system monitoring animal activity. The fish were tested individually, and swimming distance was used as a measure of activity. No significant effect of temperature on swimming distance in normoxic water was found. At all temperatures, activity level decreased with decreasing oxygen saturation. Swimming behaviour at normoxia and 50 % and 25 % oxygen saturation is described. No apparent avoidance of hypoxic water was found based on the distribution of swimming speeds and turning angles. The possible benefits of a decreased activity level in a hypoxic environment are discussed.