Respiratory Function of the Hemocyanins

Recent studies clearly demonstrate the respiratory importanceof the hemocyanins in each of the three animal phyla in whichthey occur. Despite their generally low oxygen affinity, hemocyaninscan be highly oxygenated at the site of gas exchange with themedium as well as deoxygenated at the tissues. The functionalrange of a hemocyanin oxygen transport system is severely limitedhowever by environmental change. These systems function underincipient hypoxia due largely to responses of blood pH whichare not fully understood a normal Bohr shift is accompaniedby a rise in blood pH and a reverse Bohr shift by a decreasein blood pH. In both instances blood oxygen affinity increasesand its oxygenation state at the gill remains high in spiteof its lower Po₂. Dilution of the blood at low salinity generallyalters its oxygenation properties both oxygen affinity and cooperativity.These properties may or may not be restored by concomitant changesin blood pH, which depend on the various mechanisms of osmoticadaptation. Within a homogeneous taxon the oxygenation properties of a hemocyaninappear to be highly conservative showing little interspecificadaptation except to extreme changes in the mode of gas exchange.Unlike that in vertebrates air-breathing in crustaceans is accompaniedby an increase in blood oxygen affinity. Similar oxygen affinitiesin latitudinally separated species result in optimal functioningof the system at the same temperature, corresponding to differentseasons. In eurythermal species a temperature acclimation ofoxygen affinity extends the operating range of the crustaceanhemocyanins but they cannot deoxygenate at very low temperatures. Unsolved problems of hemocyanin function include specific effectsof pH and CO₂ the basis of which is not entirely clear, andthe postulated occurrence in native blood of both dialyzableand non-dialyzable substances that modify oxygen affinity theidentity of which is unknown. With the exception of the crustacean oxygen carrier the hemocyaninsconfer a respiratory advantage over their predecessors. Butthe oxygen carrying capacity of crustacean blood never reachesthe levels found in the annelids and molluscs due to the colloidosmotic pressure of the relatively low molecular weight hemocyaninand to the drop in blood hydrostatic pressure accompanying theloss of a fluid skeleton. The selection of a blood oxygen carrierwith an apparently limiting combination of respiratory and osmoticproperties is obscured by the uncertain phylogenetic positionof the phylum.     

Blood Oxygen Transport and Delivery in Reptiles

Cenozoic reptiles are characterized by physiological morphologicaland ecological systems with low energy requirements comparedto those of mammals. Ectothermy and low resting rates of metabolismare the primary physiological adaptations of reptiles that producelow energy demand. Adjustments of the oxygen-transport systemto different thermoregulatory characteristics among reptilesmay be reflected in blood viscosity oxygen capacity oxygen affinityand the temperature sensitivity of oxygenation. Other adaptationsreduce the energy cost of oxygen transport. Reptiles have lowhematocrits and large, widely spaced capillaries that contributeto a low fluid resistance in the vascular system but also limitthe oxygen transport capacity. The low oxygen affinity characteristicof the blood of most reptiles appears to facilitate diffusionof oxygen to the tissues, overcoming the intrinsic limitationsimposed by the morphological specializations of the cardiovascularsystem. The low blood oxygen affinity permits virtually allof the oxygen carried by the blood to be delivered to the tissuesduring periods of stress. It may also help to maintain a relativelyhigh arterial Po₂ even when a right-to-left shunt occurs inthe heart. Reptilian erythrocytes are capable of reducing methemoglobinrapidly. The high concentrations of methemoglobin and polymerizedhemoglobin that occur in vivo may indicate that these compoundshave a functional role. In their blood physiology as in otheraspects of their biology reptiles are specialized animals thatreflect selective forces quite different from those that haveshaped the evolution of mammals.     

Acclimation of photosynthesis to low temperature in Spinacia oleracea L. II. Effects of nitrogen supply

The photosynthetic capacity of leaves of N-sufficent plantsof Spinacia oleracea L. increases following transfer a constanttemperature of 10C for 10 d compared to plants maintained at25C. The effects of nitrogen nutrition on this low temperatureacclimation have been investigated in respect of CO₂ assimilation,the activities and activation states of key enzymes and thepartitioning of recently fixed carbon. N-deficiency greatlyrestricted acclimation of photosynthetic CO₂ assimilation tolow temperature at both ambient and at saturating CO₂ concentrations,indicating a restriction on accilmatory changes in both ribulose1,5-bisphosphatecarboxylase-oxygenase (Rubisco) and the reactions of ribulose1,5-bisphosphateregeneration. Nitrogen limitation led to an increase in thepartitioning of recently-fixed carbon into starch. Total proteinincreased during acclimation in both N-sufficient and N-deficientleaves and was much less affected than were the activities ofenzymes. Increases in the activation state of Rubisco and thestromal fructose-1,6-bisphosphatase occurred in response tolow temperature, but increases in the activities of Rubisco,sucrose-phosphate synthase or the cytosolic fructose1,6-bisphosphatasecould not be sustained in N-deficient plants throughout theperiod of acclimation, although the activities of these enzymesdeclined less precipitately than in non-acclimated N-deficientplants. These data are all consistent with the view that increasesin the activities of key enzymes of carbon assimilation area pre-requisite for acclimation to low temperature and thatthese increases are restricted under N-limitation. Key words: Low temperature, nitrogen, photosynthesis, Rubisco, sucrose-phosphate synthase     

Oxygen-binding characteristics of Potamilla chlorocruorin

Accurate oxygen equilibrium curves of chlorocruorin of a marine polychaete annelid, Potamilla leptochaeta, were determined under a variety of experimental conditions. Like chlorocruorins from other species Potamilla chlorocruorin exhibited a low oxygen affinity, a large Bohr effect, and high cooperativity compared to those of human hemoglobin. However, in contrast to chlorocruorins from other species, the shape of the oxygen equilibrium curve for Potamilla chlorocruorin varied dramatically upon changes of pH or temperature. As observed in hemocyanins and annelid hemoglobins, cations, especially divalent ones such as Mg2+ and Ca2+, caused marked increase in oxygen affinity and cooperativity of Potamilla chlorocruorin. This finding together with the determination of cations in Potamilla blood has made clear the physiological role of chlorocruorin as an oxygen carrier. A graphical analysis based on the Monod-Wyman-Changeux allosteric model indicated that the number of sites for oxygen binding involved in heme-heme interactions is six, defining the functional unit of chlorocruorin molecule.     

Differential regulation of hexameric and dodecameric hemocyanin from A. leptodactylus

The oxygen binding properties of hemocyanins are regulated on a short time scale by effectors such as l-lactate, urate and protons, and on longer time scales by expression of the different types of subunits. For Astacus leptodactylus it was shown previously that acclimation to higher temperatures leads to increased levels of a 6-meric hemocyanin species, whereas at lower temperatures the 12-meric form prevails. Here we show that the temperature dependence of the two forms supports the idea, that the maintenance of high affinity towards oxygen is the driving force for the differential expression of these hemocyanins. Furthermore, the two different types of hemocyanin differ not only in the affinity to oxygen, but also with respect to their interaction with l-lactate: while the 12-meric form displays a normal shift in oxygen affinity upon the addition of l-lactate this allosteric regulation is absent in the 6-meric form. Exclusive binding of l-lactate to the 12-meric form was supported by isothermal titration calorimetry. These results indicate that l-lactate binds either at the interface between the two hexamers or at subunit α′ which is responsible for the formation of the 12-mers and is not present in the 6-meric form. Urate has a comparable effect on the oxygen affinity of 6-meric and 12-meric forms and also binds to a similar extent to the oxygenated state as determined by isothermal titration calorimetry. Thus, urate and l-lactate do not seem to share the same binding sites. Interestingly, urate binding sites with no allosteric effect seem to exist, which is unusual. This article is part of a Special Issue entitled: Oxygen Binding and Sensing Proteins.     

Effects of acclimation and acute temperature change on specific dynamic action and gastric processing in the green shore crab, Carcinus maenas

The effects of temperature acclimation and acute temperature change were investigated in postprandial green shore crabs, Carcinus maenas. Oxygen uptake, gut contractions and transit rates and digestive efficiencies were measured for crabs acclimated to either 10 °C or 20 °C and subsequently exposed to treatment temperatures of 5, 15, or 25 °C. Temperature acclimation resulted in a partial metabolic compensation in unfed crabs, with higher oxygen uptake rates measured for the 10 °C acclimated group exposed to acute test temperatures. The Q₁₀ values were higher than normal, probably because the acute temperature change prevented crabs from fully adjusting to the new temperature. Both the acclimation and treatment temperature altered the characteristics of the specific dynamic action (SDA). The duration of the response was longer for 20 °C acclimated crabs and was inversely related to the treatment temperature. The scope (peak oxygen consumption) was also higher for 20 °C acclimated crabs with a trend towards an inverse relationship with treatment temperature. Since the overall SDA (energy expenditure) is a function of both duration and scope, it was also higher for 20 °C acclimated crabs, with the highest value measured at the treatment temperature of 15 °C. The decline in total SDA after acute exposure to 5 and 25 °C suggests that both cold stress and limitations to oxygen supply at the temperature extremes could be affecting the SDA response. The contractions of the pyloric sac of the foregut region function to propel digesta through the gut, and contraction rates increased with increasing treatment temperature. This translated into faster transit rates with increasing treatment temperatures. Although pyloric sac contractions were higher for 20 °C acclimated crabs, temperature acclimation had no effect on transit rates. This suggests that a threshold level in pyloric sac contraction rates needs to be reached before it manifests itself on transit rates. Although there was a correlation between faster transit times and the shorter duration of the SDA response with increasing treatment temperature, transit rates do not make a good proxy for calculating the SDA characteristics. The digestive efficiency showed a trend towards a decreasing efficiency with increasing treatment temperature; the slower transit rates at the lower treatment temperatures allowing for more efficient nutrient absorption. Even though metabolic rates of 10 °C acclimated crabs were higher, there was no effect of acclimation temperature on digestive efficiency. This probably occurred because intracellular enzymes and digestive enzymes are modulated through different control pathways. These results give an insight into the metabolic and digestive physiology of Carcinus maenas as it makes feeding excursions between the subtidal and intertidal zones.     

Oxygen uptake,the circulatory system,and haemoglobin function in the intertidal polychaete Terebella haplochaeta (Ehlers)

The intertidal polychaete Terebella haplochaeta (Ehlers) shows a high degree of oxyregulation in declining pO₂ when confined to its burrow at low tide. This response is achieved by a number of adaptations to the respiratory system. The worm ventilates its burrow in a headward direction by rhythmical conractions of the body. The rate of these pulsations increases at low pO₂ and assists the circulation of the coelomic and vascular fluids. Haemoglobin in the vessels has a high affinity for oxygen and a sigmoidal equilibrium curve. Both the shape and position of the oxygen-binding curve are sensitive to changes in pH, pCO₂, and temperature in a way that suggests augmentation of oxygen delivery at low tide. The concentration of haemoglobin in the vessels is high and is further raised following warm acclimation, presumably to meet an increase in oxygen demand. The ultrastructure of the gills and blood vessels indicates a design for function at low oxygen tensions where diffusion distances must be short and surface areas large in order to enhance the rate of diffusion of oxygen from the near environment.     

Unusual oxygen binding behavior of a 24-meric crustacean hemocyanin

Hemocyanins from Crustacea usually are found as 1 × 6 or 2 × 6-meric assemblies. An exception is the hemocyanin isolated from thalassinidean shrimps where the main component is a 24-meric structure. Our analysis of oxygen binding data of the thalassinidean shrimp Upogebia pusilla based on a three-state MWC-model revealed that despite the 24-meric structure the functional properties can be described very well based on the hexamer as allosteric unit. In contrast to the hemocyanins from other thalassinidean shrimps the oxygen affinity of hemocyanin from U. pusilla is increased upon addition of l-lactate. A particular feature of this hemocyanin seems to be that l-lactate already enhances oxygen affinity under resting conditions which possibly compensates the rather low intrinsic affinity observed in absence of l-lactate. The fast rate of oxygen dissociation might indicate that in this hemocyanin a higher cooperativity is less important than a fast response of saturation level to changes in oxygen concentration.     

The influence of environmental salinity on hemocyanin function in the blue crab, Callinectes sapidus.

The effects of inorganic ions and of the hydrogen ion on oxygen-binding properties of most respiratory pigments are opposite. The addition of salt to the medium increases oxygen affinities, and the addition of H+ decreases oxygen affinities of crustacean hemocyanins. These oxygenation properties, as observed in vitro, suggest that the oxygen-transport system must adapt to ionic changes in the blood. In fact, decreases in the salt concentration of the blood of estaurine blue crabs are accompanied by increases in pH, probably resulting from the input of ammonia produced in deamination of the intracellular pool of free amino acids as the cells conform to osmotic changes in body fluids. The result is a stability of hemocyanin function until the blood becomes very dilute. As the acclimation salinity is reduced from 35 to 15 o/ooo, the ionic effects on respiratory transport are balanced and there is no change in total oxygen uptake. At 5 o/ooo salinity, however, the higher blood pH is manifested in an elevation of the total oxygen concentration of prebranchial blood, probably because the Bohr shift is no longer opposed by a critical level of salt in the blood. Under these conditions the role of hemocyanin in aerobic respiration is reduced at high environmental oxygen levels, but it may be enhanced in hypoxic uaters.     

The hemocyanin of the shamefaced crab Calappa granulata: structural-functional characterization

Arthropod hemocyanins (Hcs) transport and store oxygen and are composed of six subunits, or multiples thereof depending on the species. Calappa granulata Hc is found as a mixture of dodecamers (95%) and hexamers (5%). Removal of calcium ions and alkaline pH induce an incomplete partially reversible dissociation of dodecameric Hc. Two-dimensional electrophoretic pattern of dissociated Hc indicated a large heterogeneity in Hc subunit: most differences are likely to be explained by post-translational modifications. Dodecameric Hc showed a large Bohr effect (Deltalog P50/DeltapH = -0.95) and a normal cooperativity (h50 values = 2.7 +/- 0.2) in the presence of 10 mM CaCl2. The hexameric molecule displayed lower Bohr effect and cooperativity than the dodecamer. Lactate effect on the oxygen affinity (Deltalog P50 = 0.55) and the increase of lactate concentrations in animals kept in emersion were related to the increased oxygen requirements that occur during hypoxia in vivo. Calcium affects oxygen affinity only at high concentrations: this Hc appeared to lack the calcium high-affinity binding sites found in other species. The effect of temperature on both oxygen affinity and cooperativity was measured in the absence and presence of 10 mM lactate, allowing calculation of the exothermic contribution of lactate binding (DeltaH = -25 kJ mol(-1)).     

Adaptation of the Oxygen Transport System to Hypoxia in the Blue Crab, Callinectes sapidus

Among the oxygen carrying proteins, two groups are known toadapt to environmental challenges in the adult stage. Both vertebratehemoglobins and arthropod hemocyanins adapt to chronic hypoxiaby responding to the actions of allosteric co-factors. The designstrategy, however, differs fundamentally in the two groups.Even within the arthropods, chelicerate and crustacean hemocyaninsrespond to co-factors very differently. Only in the crustaceansdoes the oxygen carrier adapt by shifts in intrinsic molecularproperties. In hypoxic blue crabs, increases in the ratio ofthe primitive 1 x 6-meric oligomer bring about a higher oxygenaffinity relative to that in normoxic animals, in which greaterproportions of the derived 2 x 6-meric oligomer are responsiblefor a lower oxygen affinity     

Built-in co-operativity of oxygen binding by arthropod hemocyanins

Oxygen binding by arthropod hemocyanin from the scorpion Leirus quinquestriatus and the crabs Telphusa fluviatilis and Ocypoda cursor was studied in Ca²⁺, Mg²⁺-free solutions. The binding was found to be co-operative in all three cases. Our results and a re-examination of the literature lead us to conclude that co-operative oxygen binding is a built-in feature common to arthropod hemocyanins, distinguishing them from mollusc hemocyanins where co-operativity is conditional upon the presence of Ca²⁺ or Mg²⁺.     

Respiration and Circulation in Land Crabs: Novel Variations on the Marine Design

Both the "true" crabs (Brachyura) and hermit crabs (Anomura)include species that show numerous behavioral, morphological,and physiological specializations permitting terrestrial life.This paper examines respiratory and circulatory adaptationsfor air breathing in these land crabs. Respiratory specializationsinclude modification of gas exchange structures for air breathing(gills and elaborated branchial chamber linings), ventilatorymechanisms permitting effective air pumping, an elevated hemolymphoxygen capacity, and a primarily CO₂- rather than O₂- sensitiveventilatory control system. The qualitative aspects of hemolymphoxygen transport and metabolic rate are apparently unchangedfrom that of marine crabs. While the basic cardiovascular morphologyof land crabs appears similar to that of marine forms, thereis considerable elaboration of the vasculature of the branchialchamber lining, which in some species includes a unique doubleportal system. Cardiac output is lower in land crabs (probablyrelated to their higher hemolymph O₂ capacity), but insufficientdata on hemolymph pressures prevent comparisons with marineforms. In general, land crabs have modified (sometimes extensively)existing structures and processes found in their marine relativesrather than evolving structures for terrestrial life de novo.Accordingly, land crabs present a useful model for the evolutionof terrestriality, showing that even subtle anatomical changescan result in the large changes in physiological function necessaryfor the terrestrial invasion.     

The Optimal Oxygen Equilibrium Curve: A Comparison Between Environmental Hypoxia and Anemia

SYNOPSIS. Internal hypoxia in vertebrates occurs during anemia,when blood oxygen (0₂) carrying capacity is reduced, or duringexposure to environmental hypoxia. In non-altitude adapted vertebrates,exposure to environmental hypoxia results in a change in bloodO₂ affinity which, in some cases is beneficial to tissue O₂delivery. In contrast, the elevation in blood O₂ carrying capacityobserved in almost all vertebrates is always beneficial to tissueO₂ delivery (assuming no large changes in blood viscosity) andmay be more important than changes in blood O₂ affinity in maintainingtissue O₂ delivery. Experimental anemia in vertebrates results in a decrease inblood O₂ affinity which is always beneficial to tissue O₂ delivery.The reduction in affinity is brought about by an increase inthe organic phosphate to hemoglobin ratio (NTP:Hb) within thered cell. In fish NTP:Hb decreases during environmental hypoxiabut increases during anemia indicating that NTP regulation isquite different between these treatments despite the similarityof these treatments at the tissue level.     

Thermoacclimatory changes in blood oxygen binding properties and gill secondary lamellar structure ofSalmo gairdneri

Summary The blood oxygen binding properties and gill secondary lamellar structure of rainbow trout acclimated to several temperatures were studied. The blood oxygen carrying capacity decreased as acclimation temperature increased from 2 to 15 °C; the decrease was probably caused by an increase in plasma volume. Also the blood oxygen affinity decreased as the acclimation temperature increased from 2 to 15 °C. This change had no effect on the oxygen loading in gills, since the efferent arterial oxygen tension was adequate for approximately 100% erythrocytic O₂ saturation at all acclimation temperatures, but facilitated the oxygen unloading in tissues. At the highest acclimation temperature (18 °C) the oxygen loading in gills was facilitated by the changes in the secondary lamellar structure; the proportion of erythrocytes in the secondary lamellar capillaries was higher than at the other acclimation temperatures (2 and 10 °C).     

The oxygen-binding modulation of hemocyanin from the Southern spiny lobster <Emphasis Type="Italic">Palinurus gilchristi</Emphasis>

Arthropod hemocyanins transport and store oxygen and are composed of six subunits, or multiples thereof depending on the species. Palinurus gilchristi hemocyanin is found only as 1 × 6-mers, as normally occurs in spiny lobsters. An alkaline pH and removal of calcium ions induce a wholly reversible dissociation into monomers. The oxygen-binding properties of 1 × 6-meric hemocyanin from P. gilchristi were investigated with respect to pH and modulating effect exerted by calcium, lactate and urate. The oxygen affinity was highly affected by pH in the presence of calcium ions, while in its absence the Bohr coefficient became 60% lower. The protein is insensitive to lactate, but affected by urate which markedly increased hemocyanin–oxygen affinity, acting as the physiological major positive effector. Calcium ions decrease oxygen affinity at low concentration range (0–1 mM), while as concentration becomes higher than 100 mM, the oxygen affinity increases, indicating the presence of two independent types of calcium-binding sites with high and low affinity, respectively. The previous hypothesis, that the presence of high-affinity binding sites in addition to low affinity ones could be a characteristic feature of Palinuran hemocyanins, has been tested by analyzing, with respect to calcium–hemocyanin interaction, three other species belonging to Palinura.     

Acclimation of photosynthesis to low temperature in Spinacia oleracea L. I. Effects of acclimation on CO2 assimilation and carbon partitioning

Acclimation of spinach plants grown at 25C to a temperatureof 10C for 10 d resulted in an increased capacity for leafphotosynthesis in saturating light and CO₂ but not at ambientCO₂ concentrations. Gas exchange and chlorophyll fluorescencemeasurements indicated that acclimation was accompanied by anincreased capacity for the regeneration of ribulose-1,5-bisphosphate.Changes in starch, soluble carbohydrates and activities of sucrose-Psynthase and ADP-glucose pyrophosphorylase were measured duringthe acclimation process. There was an initial increase in starchand sucrose during the first 2 d, but these then declined. Therewas an increase in the capacity for sucrose synthesis duringlow temperature acclimation, evidenced by an increase in themaximum activity of sucrose-P synthase activity and an increasein partitioning of ¹⁴CO₂ into sucrose, but there was no increasein the activity of ADP-glucose pyrophosphorylase or carbon partitioninginto starch. Key words: Acclimation, carbon metabolism, gas exchange, low temperature, spinach, Spinacia oleracea     

Decreased affinity for oxygen of cytochrome-c oxidase in Leigh syndrome caused by SURF1 mutations

Mutations in the gene SURF1 prevent synthesis of cytochrome-c oxidase (COX)-specific assembly protein and result in a fatal neurological disorder, Leigh syndrome. Because this severe COX deficiency presents with barely detectable changes of cellular respiratory rates under normoxic conditions, we analyzed the respiratory response to low oxygen in cultured fibroblasts harboring SURF1 mutations with high-resolution respirometry. The oxygen kinetics was quantified by the partial pressure of oxygen (PO₂) at half-maximal respiration rate (P₅₀) in intact coupled cells and in digitonin-permeabilized uncoupled cells. In both cases, the P₅₀ in patients was elevated 2.1- and 3.3-fold, respectively, indicating decreased affinity of COX for oxygen. These results suggest that at physiologically low intracellular PO₂, the depressed oxygen affinity may lead in vivo to limitations of respiration, resulting in impaired energy provision in Leigh syndrome patients. oxygen kinetics; mitochondrial disease     

Egg-Mass Size and Cell Size: Effects of Temperature on Oxygen Distribution

Two processes strongly influence the distribution of oxygenwithin egg masses and cells: the supply of oxygen by diffusionand the consumption of oxygen by embryos and mitochondria. Theseprocesses are differentially sensitive to temperature. The diffusioncoefficient of oxygen depends only weakly on temperature, havinga Q₁₀ of approximately 1.4. In contrast, the consumption ofoxygen depends strongly on temperature, having a Q₁₀ between1.5 and 4.0. Thus, at higher temperatures, the ratio of oxygensupply to demand decreases. I show, by extending a model ofoxygen distribution within metabolizing spheres, that maximalegg-mass sizes and cell sizes are predicted to be smaller athigher temperatures. For egg masses, definitive data are notyet available. For ectothermic cells, this prediction appearsto be supported; cells from a variety of ectothermic organisms,unicellular and multicellular, are smaller when the cells areproduced at warmer temperatures. Establishing a specific connectionbetween this pattern and oxygen distributions requires demonstrationof (1) oxygen concentration gradients within metabolizing spheresand (2) central oxygen concentrations low enough to affect function.Egg masses from a variety of taxa show steep oxygen concentrationgradients and often are severely hypoxic or anoxic in centrallocations. Severe hypoxia appears capable of retarding developmentor killing embryos. Similar kinds of data for ectothermic cellshave not yet been collected, but the literature on oxygen gradientswithin mammalian cells suggests that intracellular gradientsmay be important.     

Effects of Temperature and Water Loss on Terrestrial Locomotor Performance in Land Crabs: Integrating Laboratory and Field Studies

SYNOPSIS. Terrestrial and semi-terrestrial crustaceans are exposedto fluctuations in ambient temperature and conditions that favorevaporative water loss. These environmental stresses alter performancelimits in the laboratory and behavior in the field. The maximalrate of oxygen consumption, maximum aerobic speed, and endurancecapacity are greater at a body temperature (T_b) of 24°Cthan at 15°C or 30°C in the ghost crab, Ocypode quadrata.The total metabolic cost to move at the same relative speedis greater at a T_b of 24°C than at 15°C. Slower aerobickinetics at 15°C result in a smaller relative contributionof oxidative metabolism to total metabolic cost. However, therelative contributions from accelerated glycolysis are similarat both temperatures. When locomotion is intermittent, the totaldistance traveled before fatigue can be similar at Tbs of 15and 24°C but result from different movement and pause durationsat these temperatures. Performance limits of the ghost crabare negatively affected by dehydration and are sensitive torates of water loss. In the laboratory, endurance capacity ofthe fiddler crab, Uca pugilator, is greater at a T_b of 30°Cthan at 25°C. In the field, freely moving fiddler crabswith a T_b of 30°C travel at faster mean preferred speeds,as determined by motion analysis, than crabs at 25°C. Datafor land crabs support and advance general ectothermic modelsfor the effects of temperature and dehydration on locomotorperformance.