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Cardiac cholinergic and adrenergic tones were determined in minimally instrumented African lungfish Protopterus annectens. Mean ±S.E. routine heart rate ( fH) was 31·6 ± 1·4 beats min−1, cholinergic tone was 34·6 ± 5·2% and adrenergic tone was 9·4 ± 2·3%, while the intrinsic fH after blockade of both adrenergic and cholinergic control systems was 39·1 ± 1·3 beats min−1. It is demonstrated that routine cholinergic tone has probably been underestimated in previous studies on lungfishes, suggesting that withdrawal of vagal tone may provide an important mechanism to increase fH in this group of fishes during, for example, air breathing. [ABSTRACT FROM AUTHOR]

Abstract: The role of cathodic cover opening ratio on water management was investigated for micro air-breathing polymer electrolyte membrane fuel cells (PEMFCs). The results demonstrate the possibility to manage water content in micro-PEMFC using cover opening ratio variation. By measuring the internal resistance of a cell in various cover configurations (0.33Ωcm2 to 4.0Ωcm2), the influence of cover opening ratio on water management was shown. Indeed, for a cell situated in a 10% relative humidity atmosphere and operated at 0.5V, the addition of a 5% opening ratio cover allowed to reach similar current densities (270mAcm−2) to those recorded for the same potential at 70% relative humidity without cover. Although the starting current density for a cell operated at 60°C without gas humidification was extremely low (15mAcm−2), the total closure of the cover allowed to maintain the water produced and accumulated by the cell at the cathode, and current density of 800mAcm−2 were reached after height minutes of operation. The influence of the opening ratio on back-diffused water was also evaluated and the maximum of back-diffused water was observed for a cell operated with a 5% cover opening ratio and represented 33% of the total water product at 150mAcm−2. A new method of anodic water evacuation, which does not increase the cell volume and which does not require any control tool was carried out and experimentally evaluated. [Copyright &y& Elsevier]

Abstract: Two designs of flow fields/current collectors for a passive direct methanol fuel cell (DMFC) monopolar three-cell stack were investigated. The first one (A) consisted of two plastic plates covered by thin gold film current collectors in the area of electrodes with a distribution of holes through which methanol (from a reservoir) and air (from ambient) could diffuse into the electrodes. The second design (B) consisted of thin gold film deposited on the external borders of the fuel and oxidant apertures where the electrodes were placed in contact. A big central hole allowed a direct exposure of electrodes to ambient air (for the cathodes) and methanol solution (for the anodes). An investigation of the performance and discharge behaviour of the two designs was carried out. The advantages and disadvantages of each configuration were analysed. Similar performances in terms of maximum power were recorded; whereas, better mass transport characteristics were obtained with the design B. On the contrary, open circuit voltage (OCV) and stack voltage at low current were higher for the design A as a consequence of lower methanol cross-over. A longer discharge time (17h) with a unique MeOH charge was recorded with design B at 250mA compared to the design A (5h). This was attributed to an easier CO2 removal from the anode and better mass transport properties. [Copyright &y& Elsevier]

Abstract: A planar, air-breathing, proton-exchange membrane fuel cell (PEMFC) system using a humidified H2 gas released from the hydrolysis of a stabilized sodium borohydride (NaBH4) solution with a selected solid catalyst is extensively investigated as a promising energy storage system for mobile power sources. In economic terms, the passive air-breathing PEMFC is quite attractive because can its reliability, fuel utilization, and specific energy can be enhanced by removing auxiliary devices such as the air pump and the humidifier. The rate of hydrogen generation can be manually controlled by adjusting the amount of NaBH4 solution passed through the selected catalyst. In this study, the rate of hydrogen generation is varied from 0 to 18cm3 min−1 by controlling the mass transport of stabilized NaBH4 solution. The open-circuit voltage (OCV) and maximum power density are, respectively, 0.9V and 128mWcm−2 at ambient temperature and pressure. Assuming a fuel conversion efficiency of 100%, the Faradic efficiency and energy efficiency of this system estimated to be 75% and 46%, respectively. In a planar 8-cell series-connected format, the maximum power density and cathode temperature are 3.1W (103mWcm−2) at 103mWcm−2 and 46°C, respectively. [Copyright &y& Elsevier]

Abstract: Small fuel cells have shown excellent potential as alternative energy sources for portable applications. One of the most promising fuel cell technologies for portable applications is air-breathing fuel cells. In this paper, a dynamic model of an air-breathing PEM fuel cell (AB-PEMFC) system is presented. The analytical modeling and simulation of the air-breathing PEM fuel cell system are verified using Matlab, Simulink and SimPowerSystems Blockset. To show the effectiveness of the proposed AB-PEMFC model, two case studies are carried out using the Matlab software package. In the first case study, the dynamic behavior of the proposed AB-PEMFC system is compared with that of a planar air-breathing PEM fuel cell model. In the second case study, the validation of the air-breathing PEM fuel cell-based power source is carried out for the portable application. Test results show that the proposed AB-PEMFC system can be considered as a viable alternative energy sources for portable applications. [Copyright &y& Elsevier]

Abstract: The air-breathing organ (ABO) of the Atlantic tarpon is formed by four parallel ridges of alveolar-like respiratory tissue that extend along the length of the physostomous gas bladder. The large and complex surface of each ridge is formed by a cartilage matrix that is completely infiltrated by a thin respiratory epithelium. Comparison of a size series of specimens demonstrates isometric growth of the ABO, and histological and SEM studies show comparable levels of tissue complexity. These findings suggest that air-breathing capacity, which is required for the survival of juvenile fish in their hypoxic nursery habitat, is retained in older tarpon inhabiting more oxygenated marine coastal habitats. The retention of air breathing in adult tarpon may be related to their occasional occurrence in hypoxic waters and their high rates of aerobic metabolism. [Copyright &y& Elsevier]

Abstract: An investigation of properties and operating parameters of a passive DMFC monopolar mini-stack, such as catalyst loading and methanol concentration, was carried out. From this analysis, it was derived that a proper Pt loading is necessary to achieve the best compromise between electrode thickness and number of catalytic sites for the anode and cathode reactions to occur at suitable rates. Methanol concentrations ranging from 1M up to 10M and an air-breathing operation mode were investigated. A maximum power of 225mW was obtained at ambient conditions for a three-cell stack, with an active single cell area of 4cm2, corresponding to a power density of about 20mWcm−2. [Copyright &y& Elsevier]

Abstract: An analysis of water management in air breathing small polymer electrolyte membrane fuel cells (PEMFCs) is presented. Comprehensive understanding of flooding and dry-out limiting phenomena is presented through a combination of analytical modeling and experimental investigations using a small PEMFC prototype. Configurations of the fuel cell with different heat and mass transfer properties are experimentally evaluated to assess the impact of thermal resistance and mass transport resistance on water balance. Manifestation of dry-out and flooding problems, as limiting phenomena, are explained through a ratio between these two resistances. Main conclusions are that decreasing the ratio between thermal and mass transport resistance under a certain point leads to flooding problems in air breathing PEMFC. Increasing this ratio leads to dry-out of the polymer electrolyte membrane. However, too high thermal resistance or too low mass transport resistance reduces the limiting current by pushing forward the dry-out problem. This work provides a framework to achieve the proper balance between thermal rejection and mass transport to optimize the maximum current density of free convection fuel cells. [Copyright &y& Elsevier]

Swimming in a flume at reduced water pO2 resulted in muscle and blood lactate levels in Pacific tarpon Megalops cyprinoides that were significantly higher when fish did not have access to air. Blood glucose and haematological variables were unchanged throughout the regimes of exercise at two swimming speeds and hypoxia. Strenuous exercise with bouts of burst swimming, however, resulted in both high blood lactate and glucose, and perturbed haematological status with elevated haemoglobin and reduced mean cell-haemoglobin concentration. Post-exercise recovery was achieved through aquatic breathing rather than by air breathing. The air-breathing organ in Pacific tarpon therefore prolonged aerobic activity, but gill breathing was used to repay oxygen debt. [ABSTRACT FROM AUTHOR]

The jeju is a teleost fish with bimodal respiration that utilizes a modified swim bladder as an air-breathing organ (ABO). Like all air-breathing fish studied to date, jeju exhibit pronounced changes in heart rate (fH) during air-breathing events, and it is believed that these may facilitate oxygen uptake (MO2) from the ABO. The current study employed power spectral analysis (PSA) of fH patterns, coupled with instantaneous respirometry, to investigate the autonomic control of these phenomena and their functional significance for the efficacy of air breathing. The jeju obtained less than 5% of total MO2 (MtO2) from air breathing in normoxia at 26°C, and PSA of beat-to-beat variability in fH revealed a pattern similar to that of unimodal water-breathing fish. In deep aquatic hypoxia (water PO2=1 kPa) the jeju increased the frequency of air breathing (fAB) tenfold and maintained MtO2 unchanged from normoxia. This was associated with a significant increase in heart rate variability (HRV), each air breath (AB) being preceded by a brief bradycardia and then followed by a brief tachycardia. These fH changes are qualitatively similar to those associated with breathing in unimodal air-breathing vertebrates. Within 20 heartbeats after the AB, however, a beat-to-beat variability in fH typical of water-breathing fish was re-established. Pharmacological blockade revealed that both adrenergic and cholinergic tone increased simultaneously prior to each AB, and then decreased after it. However, modulation of inhibitory cholinergic tone was responsible for the major proportion of HRV, including the precise beat-to-beat modulation of fH around each AB. Pharmacological blockade of all variations in fH associated with air breathing in deep hypoxia did not, however, have a significant effect upon JAB or the regulation of MtO2. Thus, the functional significance of the profound HRV during air breathing remains a mystery. [ABSTRACT FROM AUTHOR]

Abstract: Air-breathing proton exchange membrane (PEM) fuel cells provide for fully or partially passive operation and have gained much interest in the past decade, as part of the efforts to reduce the system complexity. This paper presents a detailed physics-based numerical analysis of the transport and electrochemical phenomena involved in the operation of a stack consisting of an array of vertically oriented air-breathing fuel cells. A comprehensive two-dimensional, nonisothermal, multi-component numerical model with pressurized hydrogen supply at the anode and natural convection air supply at the cathode is developed and validated with experimental data. Systematic parametric studies are performed to investigate the effects of cell dimensions, inter-cell spacing and the gap between the array and the substrate on the performance of the stack. Temperature and species distributions and flow patterns are presented to elucidate the coupled multiphysics phenomena. The analysis is used to determine optimum stack designs based on constraints on desired performance and overall stack size. [Copyright &y& Elsevier]

Abstract: The objective of this article is to investigate the performance of an air-breathing proton exchange membrane (PEM) fuel cell operating with hydrogen fed at the anode and air supplied by natural convection at the cathode. Considering a dual-cell cartridge configuration with a common anode flow chamber, a comprehensive two-dimensional, non-isothermal, multi-component numerical model is developed to simulate the mass transport and electrochemical phenomena governing the cell operation. Systematic parametric studies are presented to investigate the effects of operating conditions, cell orientation and cell geometry on the performance. Temperature and species distributions are also studied to assist the understanding of the single cell performance for different conditions. It is shown that the cell orientation affects the local current density distribution along the cell and the average current density, particularly at lower cell voltages. The cell performance is shown to improve with increase of temperature, anode flow rate, anode pressure and anode relative humidity. [Copyright &y& Elsevier]

Abstract: In this paper water and air management systems were developed for a miniature, passive direct methanol fuel cell (DMFC). The membrane thickness, water management system, air management system and gas diffusion electrodes (GDE) were examined to find their effects on the water balance coefficient, fuel utilization efficiency, energy efficiency and power density. Two membranes were used, Nafion® 112 and Nafion® 117. Nafion® 117 cells had greater water balance coefficients, higher fuel utilization efficiency and greater energy efficiency. A passive water management system which utilizes additional cathode gas diffusion layers (GDL) and a passive air management system which makes use of air filters was developed and tested. Water management was improved with the addition of two additional cathode GDLs. The water balance coefficients were increased from −1.930 to 1.021 for a cell using a 3.0molkg−1 solution at a current density of 33mAcm−2. The addition of an air filter further increased the water balance coefficient to 1.131. Maximum power density was improved from 20mWcm−2 to 25mWcm−2 for 3.0molkg−1 solutions by upgrading from second to third generation GDEs, obtained from E-TEK. There was no significant difference in water management found between second and third generation GDEs. A fuel utilization efficiency of 63% and energy efficiency of 16% was achieved for a 3.0molkg−1 solution with a current density of 66mAcm−2 for third generation GDEs. [Copyright &y& Elsevier]

The sensing and processing of hypoxic signals, the responses to these signals and the modulation of these responses by other physical and physiological factors are an immense topic filled with numerous novel and exciting discoveries. Nestled among these discoveries, and in contrast to mammals, is the unusual cardiac response of many fish to environmental hypoxia — a reflex slowing of heart rate. The afferent and efferent arms of this reflex have been characterised, but the benefits of the hypoxic bradycardia remain enigmatic since equivocal results have emerged from experiments examining the benefit to oxygen transfer across the gills. The main thesis developed here is that hypoxic bradycardia could afford a number of direct benefits to the fish heart, largely because the oxygen supply to the spongy myocardium is precarious (i.e. it is determined primarily by the partial pressure of oxygen in venous blood, PvO2) and, secondarily, because the fish heart has an unusual ability to produce large increases in cardiac stroke volume (VSH) that allow cardiac output to be maintained during hypoxic bradycardia. Among the putative benefits of hypoxic bradycardia is an increase in the diastolic residence time of blood in the lumen of the heart, which offers an advantage of increased time for diffusion, and improved cardiac contractility through the negative force-frequency effect. The increase in VSH will stretch the cardiac chambers, potentially reducing the diffusion distance for oxygen. Hypoxic bradycardia could also reduce cardiac oxygen demand by reducing cardiac dP/dt and cardiac power output, something that could be masked at cold temperature because of a reduced myocardial work load. While the presence of a coronary circulation in certain fishes decreases the reliance of the heart on PvO2, hypoxic bradycardia could still benefit oxygen delivery via an extended diastolic period during which peak coronary blood flow occurs. The notable absence of hypoxic bradycardia among fishes that breathe air during aquatic hypoxia and thereby raise their PvO2, opens the possibility that that the evolutionary loss of hypoxic bradycardia may have coincided with some forms of air breathing in fishes. Experiments are needed to test some of these possibilities. Ultimately, any potential benefit of hypoxic bradycardia must be placed in the proper context of myocardial oxygen supply and demand, and must consider the ability of the fish heart to support its routine cardiac power output through glycolysis. [ABSTRACT FROM AUTHOR]

Abstract: A passive (air breathing) planar three-cell direct methanol fuel cell was designed, fabricated and tested. The design concepts for plates, current collectors, seals and membrane electrode assemblies are discussed in this paper. Testing of single cells and stacks with parallel and serial connections were carried out. The results show that a reliable areal power density of 8.6mWcm−2 can be achieved at ambient temperature with passive operation. Stacks with a serial connection of the single cells gave significantly higher performance than a parallel connection. It was also identified that high electrical resistance was the dominant factor in reducing performance. The major causes of the high resistance were the stainless steel hardware used and poor contact between the electrode and current collector. Future work will involve material and design studies of current collectors and seals to minimize the electrical resistance, and architectural design studies to effectively utilize the heat generated to raise operation temperature. [Copyright &y& Elsevier]

Abstract: This paper presents a two-dimensional, steady state, single phase, non-isothermal and complete model for a vertical, fully planar, Air Breathing Proton Exchange Membrane Fuel Cell (AB-PEMFC) with hydrogen fuel supplied by forced convection at the anode. It is applied to a cell with an active area of 6cm2 operating at ambient atmospheric conditions of 23°C and 20% relative humidity. The transport characteristics in terms of the velocity, and heat and mass transfer coefficients in the various components of the fuel cell are reported for several values of the operating current density. The effect of cathode height (1–5cm) and operating atmospheric conditions (10–40°C and 20–80% relative humidity), on the cell performance is also reported. Further, the applicability of the model to a non-planar AB-PEMFC is examined by comparison with available experimental data. The average mass transfer coefficients for oxygen and water vapor at the cathode GDL surface are found to be of the order of 10−3 ms−1. The operating current density is seen to substantially affect the variation of the local current density distribution and the cathode surface temperature along the height of the fuel cell as also the temperature variation across the MEA thickness. The maximum power density and the corresponding current density, herein defined as the optimum current density, are found to increase with decreasing height of the fuel cell, decreasing ambient temperature and increasing ambient relative humidity. However, the local cell temperature at high current densities is found to increase beyond the safe operating limits for short fuel cells. Comparison of the model predictions with available experimental data points to its applicability in the ohmic polarization zone of a non-planar cell. The cell performance at high current densities deteriorates due to mass transport limitation and electrolyte dehydration. [Copyright &y& Elsevier]

Abstract: Directed methanol fuel cells (DMFCs) are fabricated on printed circuit board (PCB) substrates by means of a photolithography process. The effects of channel pattern, channel width and methanol flow rate on the performance of the fabricated DMFC are evaluated over a range of flow-channel widths from 200 to 400μm and flow rates of methanol from 2 to 80mlmin−1. A micro-DMFC with a cross-stripe channel pattern gives superior performance compared with zig-zag and serpentine type of pattern. A single cell with a 200-μm wide channel delivers a maximum power density of 33mWcm−2 when using 2M methanol feed at 80°C. An air-breathing multi-DMFC composed of eight single unit cells gave 180mWcm−2 by using a methanol reservoir. It is considered that this may be the first reported attempt to develop a multi-DMFC with micro-channels fabricated on a PCB substrate. [Copyright &y& Elsevier]

Abstract: In this paper, design criteria and development techniques for planar air breathing direct methanol fuel cell stacks are described in detail. The fuel cell design in this study incorporates a window-frame structure that provides a large open area for more efficient mass transfer and is modular, making it possible to fabricate components separately. The membrane electrode assembly and gas diffusion layers are laminated together to reduce contact resistance, which eliminates the need for heavy hardware. The composite current collector is low cost, has high electrical conductivity and corrosion resistance. In the interest of quick and cost-efficient prototyping, the fabrication techniques were first developed on a single cell with an active area of 1.0cm2. Larger single cells with active areas of 4.5 and 9.0cm2 were fabricated using techniques based on those developed for the smaller single cell. Two four-cell stacks, one with a total active area of 18.0cm2 and the other with 36.0cm2, were fabricated by inter-connecting four identical cells in series. These four-cell stacks are suitable for portable passive power source applications. The performance analysis of single cells as well as stacks is presented. Peak power outputs of 519.0 and 870.0mW were achieved in the stacks with active areas of 18.0 and 36.0cm2, respectively. The effects of methanol concentration and fuel cell self-heating on the fuel cell performance are emphasized. [Copyright &y& Elsevier]

Abstract: For portable applications, the, characteristics of passive air-breathing PEMFCs were investigated by examining effects of cathode open area and relative humidity on the cell performance. Among the single cells with cathode open area from 52 to 94%, the single cell with the open area of 77% exhibited the highest performance. The cell performance was improved with increasing RH of atmosphere from 20 to 100% in the low current region while lowered in high current region. Those results were related with the mass transport of oxygen from the atmosphere to the catalyst layer and the degree of membrane hydration determining the ionic conductivity of the membrane. [Copyright &y& Elsevier]

Abstract: This paper presents a two-dimensional computational model of micro-structured cathodic electrodes featuring nano-porous gas diffusion media that increase the surface to volume ratio of the deposited catalyst layer. A key objective of the modeling effort is to elucidate the Knudsen diffusion within the nano-porous electrode. In addition, the model is non-isothermal and resolves the natural convection in the ambient air directly above the cathode. A major component of the analysis is the identification of mass transport limitations introduced by the micro-structured electrode and the nano-porous gas diffusion layer. Furthermore, the potential for passive regulation of temperature and oxygen supply is investigated. This analysis of the natural convection is unique because it examines buoyancy effects above a horizontal plate with variation in thermal conductivity, segregated regions of porous media, and discrete heating locations. The results of this study indicate that this electrode configuration is feasible and does not suffer from insurmountable mass transfer limitations. [Copyright &y& Elsevier]

In callichthyid catfishes, the posterior intestine is modified to function as an air breathing organ by being air-filled, thin-walled and highly vascularized. These modifications make it unsuitable for digestive functions and digesta has to be transported quickly through this region to minimize disruption of vital respiratory functions. However, the weak muscles of the wall of the respiratory intestine make this problematic. It is hypothesized that the unidirectional ventilatory air current within the respiratory intestine is responsible for the quick transport of digesta through the respiratory intestine. To verify this, movement of digesta through the alimentary tract was examined in Hoplosternum littorale and Corydoras aeneus that were either allowed to breathe air or prevented from air breathing. When air breathing was prevented, digesta was not transported to the rectum in H. littorale and there was a 94% reduction in the amount of digesta in the rectum of C. aeneus. This study suggests that the anterior digestive intestine facilitates the passage of air although it is filled with digesta. The anterior digestive intestine packages digesta into a string of slightly compressed boluses, creating an air channel in the digestive intestine thus allowing air to pass unimpeded. [ABSTRACT FROM AUTHOR]

Abstract: The use of various Nafion membranes, including Nafion 117, 115 and 112 with respective thicknesses of 175μm, 125μm and 50μm, in a passive direct methanol fuel cell (DMFC) was investigated experimentally. The results show that when the passive DMFC operated with a lower methanol concentration (2.0M), a thicker membrane led to better performance at lower current densities, but exhibited lower performance at higher current densities. When the methanol concentration was increased to 4.0M, however, the three membranes exhibited similar cell voltages over a wide range of current densities. In contrast, this work also shows the polarization behaviors in an active DMFC when the three membranes were substantially different. Finally, the test of fuel utilization indicates that the passive DMFC with a thicker membrane exhibited higher efficiency. [Copyright &y& Elsevier]

Abstract: This paper quantifies the relationship between respiratory allocation (air vs. water) and the standard rate of metabolism (SMR) in the primitive air-breathing lungfish, Protopterus aethiopicus. Simultaneous measurements of oxygen consumed from both air and water were made to determine the SMR at ecologically relevant aquatic oxygen levels for juveniles 2 to 221 g. Total metabolic rate was positively correlated with body mass with a scaling exponent of 0.78. Aerial oxygen consumption averaged 98% (range=94% to 100%) of total respiratory allocation under low aquatic oxygen levels. Measurements of oxygen consumption made across a gradient of dissolved oxygen from normoxia to anoxia showed that P. aethiopicus maintains its SMR despite a change in respiratory allocation between water and air. [Copyright &y& Elsevier]

Abstract: A passive, air-breathing liquid feed direct methanol fuel cell (DMFC), with no external pumps or other auxiliary devices, was designed, fabricated and tested with different methanol concentrations. It was found that the cell performance was improved substantially with an increase in methanol concentration; a maximum of power density of 20mW/cm2 was achieved with 5.0M methanol solution. The measurements indicated that the better performance with higher methanol concentrations was mainly attributed to the increase in the cell operating temperature caused by the exothermic reaction between permeated methanol and oxygen on the cathode. This finding was subsequently confirmed by the fact that the cell performance was degraded, when the cell running with higher methanol concentrations was cooled down to room temperature. [Copyright &y& Elsevier]

The present study investigated the respiratory rates, frequency of air breathing and ability to extract oxygen from air compared to body mass of three neotropical air-breathing fishes (Lepidosiren paradoxa, Hoplerythrinus unitaeniatus and Hoplosternum littorale) with different accessory organs, under the same conditions of hypoxia (PO2<5 mm Hg) and temperature (25 °C), using the same apparatus. The results indicated a superior capacity of H. littorale (16.01 ml O2 breath−1 kg−1) compared to H. unitaeniatus (7.03 ml O2 breath−1 kg−1) and L. paradoxa (4.95 ml O2 breath−1 kg−1). [Copyright &y& Elsevier]

The increasing use of electronic technologies by the dismounted soldier is placing a growing burden on the power sources required to operate them. Battery technology, which is close to the limit of development cannot provide for the soldiers power needs at an acceptable weight. Air breathing proton exchange membrane (PEM) fuel cells in combination with advanced hydrogen generation technologies, are suggested as a viable alternative, providing greatly increased energy densities. An application as a portable field generator is suggested, with the fuel cell system acting as a charger coupled to rechargeable batteries. [Copyright &y& Elsevier]

To succeed on land rather than in water, crabs require a suite of physiological and morphological changes, and ultimately the ability to reproduce without access open water. Some species have modified gills to assist in gas exchange but accessory gas exchange organs, usually lungs, occur in many species. In accomplished air-breathers the lung becomes larger and more vascularised with pulmonary vessels directing oxygenated haemolymph to the heart. The relative abundance of O2 in air promotes relative hypoventilation and thus an internal hypercapnia to drive CO2 excretion. Land crabs have a dual circulation via either lungs or gills and shunting between the two may depend on respiratory media or exercise state. During their breeding migration on Christmas Island Gecarcoidea natalis maintained arterial Po2 by branchial O2 uptake, while pulmonary O2 pressure was reduced; partly because exercise doubled relative haemolymph flow through the gills. Related species rely on elevated haemocyanin concentration and affinity for O2 to assist uptake but this compromises unloading at the tissues and thus the aerobic scope of tissues. Aquatic crabs exchange salt and ammonia with water via the gills but in land crabs this is not possible. Birgus latro has adopted uricotelism but other species excrete ammonia in either the urine or as gas. Land crabs minimise urinary salt loss using a filtration-reabsorption system analogous to the kidney. Urine is redirected across the gills where salt reabsorption occurs in systems under hormonal control, although in G. natalis this is stimulatory and in B. latro inhibitory. While crabs occupy a range of habitats from aquatic to terrestrial, these species do not comprise a physiological continuum but across the crab taxa individual species possess appropriate and specific physiological features to survive in their individual habitat. [Copyright &y& Elsevier]