Non-equilibrium acid-base status in C. productus: role of exoskeletal carbonate buffers

Meta-analysis suggests negative, but pCO2-specific, effects of ocean acidification on the structural and functional properties of crustacean biomaterials

Crustaceans comprise an ecologically and morphologically diverse taxonomic group. They are typically considered resilient to many environmental perturbations found in marine and coastal environments, due to effective physiological regulation of ions and hemolymph pH, and a robust exoskeleton. Ocean acidification can affect the ability of marine calcifying organisms to build and maintain mineralized tissue and poses a threat for all marine calcifying taxa. Currently, there is no consensus on how ocean acidification will alter the ecologically relevant exoskeletal properties of crustaceans. Here, we present a systematic review and meta-analysis on the effects of ocean acidification on the crustacean exoskeleton, assessing both exoskeletal ion content (calcium and magnesium) and functional properties (biomechanical resistance and cuticle thickness). Our results suggest that the effect of ocean acidification on crustacean exoskeletal properties varies based upon seawater pCO2 and species identity, with significant levels of heterogeneity for all analyses. Calcium and magnesium content was significantly lower in animals held at pCO2 levels of 1500-1999 µatm as compared with those under ambient pCO2. At lower pCO2 levels, however, statistically significant relationships between changes in calcium and magnesium content within the same experiment were observed as follows: a negative relationship between calcium and magnesium content at pCO2 of 500-999 µatm and a positive relationship at 1000-1499 µatm. Exoskeleton biomechanics, such as resistance to deformation (microhardness) and shell strength, also significantly decreased under pCO2 regimes of 500-999 µatm and 1500-1999 µatm, indicating functional exoskeletal change coincident with decreases in calcification. Overall, these results suggest that the crustacean exoskeleton can be susceptible to ocean acidification at the biomechanical level, potentially predicated by changes in ion content, when exposed to high influxes of CO2. Future studies need to accommodate the high variability of crustacean responses to ocean acidification, and ecologically relevant ranges of pCO2 conditions, when designing experiments with conservation-level endpoints.

Exercise and emersion in air, and recovery in seawater in the green crab (Carcinus maenas): metabolic, acid-base, cardio-ventilatory, and ionoregulatory responses

In nature, the green crab exhibits emersion and terrestrial activity at low tide. Treadmill exercise in air (20-23°C) of crabs acclimated to 32ppt seawater (13°C) revealed an inverse relationship between velocity and duration: 2.0 BL sec−1sustainable for several minutes, and 0.25 BL sec−1 for long periods. Fatigue was not due to dehydration. Physiological responses over 18-h recovery in seawater after near-exhaustive exercise (0.25 BL sec−1, 1h) in air were compared with responses after quiet emersion (1h) in air. Exercising crabs exhibited transient scaphognathite slowing and progressive increases in heart rate, whereas emersed crabs exhibited persistent inhibition of ventilation and transient heart slowing. Upon return to seawater, all these rates increased above both control and treatment levels. Post-exercise disturbances were more marked and/or longer lasting (e.g. EPOC, hyperventilation, tachycardia, metabolic acidosis, lactate elevation, ionic disturbances) than those after simple air exposure. However, an increase in net acidic equivalent excretion to the environment occurred after emersion but not after exercise. Instead, post-exercise crabs relied on carapace buffering, signaled by elevated haemolymph Ca2+ and Mg2+. Prolonged lowering of haemolymph PCO2 associated with hyperventilation also played a key role in acid-base recovery. EPOC after exercise was 3-fold greater than after emersion, sufficient to support control M˙O2for>14h. This reflected clearance of a large lactate load, likely by glycogen re-synthesis rather than oxidation. We conclude that the amphibious green crab uses a combination of aquatic and terrestrial strategies to support exercise in air, emersion in air, and recovery in seawater.

Effects of single and dual-stressor elevation of environmental temperature and PCO2 on metabolism and acid-base regulation in the Louisiana red swamp crayfish, Procambarus clarkii

Elevation of temperature and CO₂ levels within the world's aquatic environments is expected to cause numerous physiological challenges to their inhabitants. While effects on marine ecosystems have been well studied, freshwater ecosystems have rarely been examined using a dual-stressor approach leaving our understanding of its inhabitants upon these challenges unclear. We aimed to identify the affects of elevated temperature and hypercapnia in isolation and in combination on the metabolic and acid-base regulatory processes of a freshwater crayfish, Procambarus clarkii. Crayfish were exposed to freshwater conditions that may be prevalent by the year 2100 and metabolic responses were determined after 14-days of exposure. In addition, changes in branchial mRNA expression of acid-base linked transporters were investigated. Interactions between exposure conditions influenced extracellular pH as well as the nitrogen physiology and routine metabolic rate of the crayfish. Crayfish exposed to individual and combined elevations in temperature and/or hypercapnia maintained an extracellular pH similar to that of control crayfish. Dual-stressor exposed crayfish seem to elevate the importance of ammonium as an excretable acid-equivalent based on an overall increase in the branchial mRNA expression of transporters related to ammonia excretion including the Na⁺/K⁺-ATPase, Rhesus-protein, and the V-type H⁺-ATPase. Overall, hypercapnia and dual-stressor conditions caused a metabolic depression that may have long-lasting consequences such as limited locomotion, growth, and reproduction. Future generations of crayfish given the chance to adapt over several generations may ameliorate these consequences.

Ocean acidification alters properties of the exoskeleton in adult Tanner crabs, Chionoecetes bairdi

Ocean acidification can affect the ability of calcifying organisms to build and maintain mineralized tissue. In decapod crustaceans, the exoskeleton is a multilayered structure composed of chitin, protein and mineral, predominately magnesian calcite or amorphous calcium carbonate (ACC). We investigated the effects of acidification on the exoskeleton of mature (post-terminal-molt) female southern Tanner crabs, Chionoecetes bairdi. Crabs were exposed to one of three pH levels – 8.1, 7.8 or 7.5 – for 2 years. Reduced pH led to a suite of body region-specific effects on the exoskeleton. Microhardness of the claw was 38% lower in crabs at pH 7.5 compared with those at pH 8.1, but carapace microhardness was unaffected by pH. In contrast, reduced pH altered elemental content in the carapace (reduced calcium, increased magnesium), but not the claw. Diminished structural integrity and thinning of the exoskeleton were observed at reduced pH in both body regions; internal erosion of the carapace was present in most crabs at pH 7.5, and the claws of these crabs showed substantial external erosion, with tooth-like denticles nearly or completely worn away. Using infrared spectroscopy, we observed a shift in the phase of calcium carbonate present in the carapace of pH 7.5 crabs: a mix of ACC and calcite was found in the carapace of crabs at pH 8.1, whereas the bulk of calcium carbonate had transformed to calcite in pH 7.5 crabs. With limited capacity for repair, the exoskeleton of long-lived crabs that undergo a terminal molt, such as C. bairdi, may be especially susceptible to ocean acidification.

Specialized adaptations allow vent-endemic crabs (Xenograpsus testudinatus) to thrive under extreme environmental hypercapnia

Shallow hydrothermal vent environments are typically very warm and acidic due to the mixing of ambient seawater with volcanic gasses (> 92% CO₂) released through the seafloor making them potential ‘natural laboratories’ to study long-term adaptations to extreme hypercapnic conditions. Xenograpsus testudinatus, the shallow hydrothermal vent crab, is the sole metazoan inhabitant endemic to vents surrounding Kueishantao Island, Taiwan, where it inhabits waters that are generally pH 6.50 with maximum acidities reported as pH 5.50. This study assessed the acid–base regulatory capacity and the compensatory response of X. testudinatus to investigate its remarkable physiological adaptations. Hemolymph parameters (pH, [HCO₃⁻], \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{P}}_{{{\text{CO}}_{2} }}$$\end{document}PCO2, [NH₄⁺], and major ion compositions) and the whole animal’s rates of oxygen consumption and ammonia excretion were measured throughout a 14-day acclimation to pH 6.5 and 5.5. Data revealed that vent crabs are exceptionally strong acid–base regulators capable of maintaining homeostatic pH against extreme hypercapnia (pH 5.50, 24.6 kPa \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{P}}_{{{\text{CO}}_{2} }}$$\end{document}PCO2) via HCO₃⁻/Cl⁻ exchange, retention and utilization of extracellular ammonia. Intact crabs as well as their isolated perfused gills maintained \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{P}}_{{{\text{CO}}_{2} }}$$\end{document}PCO2tensions below environmental levels suggesting the gills can excrete CO₂ against a hemolymph-directed \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{P}}_{{{\text{CO}}_{2} }}$$\end{document}PCO2 gradient. These specialized physiological mechanisms may be amongst the adaptations required by vent-endemic animals surviving in extreme conditions.

No compromise between metabolism and behavior of decorator crabs in reduced pH conditions

Many marine calcifiers experience metabolic costs when exposed to experimental ocean acidification conditions, potentially limiting the energy available to support regulatory processes and behaviors. Decorator crabs expend energy on decoration camouflage and may face acute trade-offs under environmental stress. We hypothesized that under reduced pH conditions, decorator crabs will be energy limited and allocate energy towards growth and calcification at the expense of decoration behavior. Decorator crabs, Pelia tumida, were exposed to ambient (8.01) and reduced (7.74) pH conditions for five weeks. Half of the animals in each treatment were given sponge to decorate with. Animals were analyzed for changes in body mass, exoskeleton mineral content (Ca and Mg), organic content (a proxy for metabolism), and decoration behavior (sponge mass and percent cover). Overall, decorator crabs showed no signs of energy limitation under reduced pH conditions. Exoskeleton mineral content, body mass, and organic content of crabs remained the same across pH and decoration treatments, with no effect of reduced pH on decoration behavior. Despite being a relatively inactive, osmoconforming species, Pelia tumida is able to maintain multiple regulatory processes and behavior when exposed to environmental pH stress, which underscores the complexity of responses within Crustacea to ocean acidification conditions.

Physiological responses of postprandial red rock crabs (Cancer productus) during emersion

The physiological responses of unfed and postprandial red rock crabs ( Cancer productus J.W. Randal, 1840) were investigated during periods of emersion. During aerial exposure, oxygen uptake quickly fell to very low levels and was no longer detectable in the haemolymph after 12 h. The resulting anaerobic respiration led to a build up in lactic acid and the resulting acidosis was more pronounced in the postprandial crabs. There was also a concomitant rise in PCO2and CCO2, and in both cases these were higher in postprandial animals. Higher ammonia levels in postprandial crabs showed that cellular activities were still proceeding anaerobically, suggesting that although crabs can delay mechanical digestion during emersion, once intracellular digestion occurs they may be committed to these processes. Increased mortality rates of postprandial animals were probably due to a combination of the high lactate and CO2levels coupled with an increased ammonia concentration. For C. productus stranded in the intertidal zone there may be little effect of feeding, as they are only exposed for short periods and recovery occurs during re-immersion. The crabs are more likely to become moribund and death ensue during longer term exposure such as commercial live shipment.

Physiological responses of Daphnia pulex to acid stress

BACKGROUND

Acidity exerts a determining influence on the composition and diversity of freshwater faunas. While the physiological implications of freshwater acidification have been intensively studied in teleost fish and crayfish, much less is known about the acid-stress physiology of ecologically important groups such as cladoceran zooplankton. This study analyzed the extracellular acid-base state and CO2 partial pressure (P(CO2)), circulation and ventilation, as well as the respiration rate of Daphnia pulex acclimated to acidic (pH 5.5 and 6.0) and circumneutral (pH 7.8) conditions.

RESULTS

D. pulex had a remarkably high extracellular pH of 8.33 and extracellular P(CO2) of 0.56 kPa under normal ambient conditions (pH 7.8 and normocapnia). The hemolymph had a high bicarbonate concentration of 20.9 mM and a total buffer value of 51.5 meq L(-1) pH(-1). Bicarbonate covered 93% of the total buffer value. Acidic conditions induced a slight acidosis (DeltapH = 0.16-0.23), a 30-65% bicarbonate loss, and elevated systemic activities (tachycardia, hyperventilation, hypermetabolism). pH 6.0 animals partly compensated the bicarbonate loss by increasing the non-bicarbonate buffer value from 2.0 to 5.1 meq L(-1) pH(-1). The extracellular P(CO2) of pH 5.5 animals was significantly reduced to 0.33 kPa, and these animals showed the highest tolerance to a short-term exposure to severe acid stress.

CONCLUSION

Chronic exposure to acidic conditions had a pervasive impact on Daphnia's physiology including acid-base balance, extracellular PCO2, circulation and ventilation, and energy metabolism. Compensatory changes in extracellular non-bicarbonate buffering capacity and the improved tolerance to severe acid stress indicated the activation of defense mechanisms which may result from gene-expression mediated adjustments in hemolymph buffer proteins and in epithelial properties. Mechanistic analyses of the interdependence between extracellular acid-base balance and CO2 transport raised the question of whether a carbonic anhydrase (CA) is involved in the catalysis of the CO2-HCO3(-)-H(+) reaction, which led to the discovery of 31 CA-genes in the genome of D. pulex.

Stress effect of different temperatures and air exposure during transport on physiological profiles in the American lobster Homarus americanus

Homarus americanus is an important commercial species that can survive 2-3 days out of water if kept cool and humid. Once caught for commercial purpose and shipped around the world, a lobster is likely to be subjected to a number of stressors, including emersion and air exposure, hypoxia, temperature changes and handling. This study focused on the effect of transport stress and specifically at different animal body temperature (6 and 15 degrees C) and air exposure during commercial transport and recovery process in water. Animals were monitored, by hemolymph bleeding, at different times: 0 h (arrival time at plant) 3 h, 12 h, 24 h and 96 h after immersion in the stocking tank with a water temperature of 6.5+/-1.5 degrees C. We analysed the effects by testing some physiological variables of the hemolymph: glucose, cHH, lactate, total protein, cholesterol, triglycerides, chloride and calcium concentration, pH and density. All these variables appeared to be influenced negatively by high temperature both in average of alteration from the physiological value and in recovering time. Blood glucose, lactate, total protein, cholesterol were significantly higher in the group with high body temperature compared to those with low temperature until 96 h after immersion in the recovery tank.

Acid-base balance during hypoxic hypometabolism: selected vertebrate strategies

An important functional advantage of hypoxic hypometabolism is that it blunts the acid-base consequences of hypoxia. Hypoxia can lead to anaerobiosis and metabolic acidosis and, in animals that are apneic, to respiratory acidosis. A fall in blood and tissue pH is a major limiting factor in hypoxic tolerance and a variety of strategies occur in vertebrates, in concert with hypometabolism, to respond to this acid-base challenge. These include sequestering of lactic acid away from the circulating blood during the hypoxic exposure, either in underperfused tissues or in mineralized tissues, supplementing extracellular buffering by releasing bone mineral into the circulation, and utilizing alternative metabolic pathways for anaerobiosis to produce ethanol rather than lactate as the principal end-product. For submerged air-breathing ectotherms, effective cutaneous O2 and CO2 exchange can also allow an animal to avoid or minimize both anaerobiosis and respiratory acidosis. These responses serve to maintain a viable acid-base state in the body and to extend the time that the hypoxic stress can be endured.

Lactate sequestration by osteoderms of the broad-nose caiman, Caiman latirostris, following capture and forced submergence

Lactate accumulation in osteoderms of the broad-nose caiman, Caiman latirostris, was determined following capture and surgery and after a period of forced submergence and related to concurrent values in blood. Control samples of bone and blood were taken after recovery from surgery and before submergence. In addition, samples of osteoderm were incubated in a lactate solution to determine equilibrium concentration, and additional samples were analyzed for elemental and CO(2) concentrations. The composition of the osteoderms closely resembles that of typical vertebrate bone, with a high concentration of calcium and phosphate. Plasma and osteoderm lactate concentrations were both elevated following surgery and decreased significantly after 1 day of recovery. Submergence produced a typical lactate pattern in the plasma, with only a modest increase during the dive and then a sharp increase during recovery to a peak of 31.2+/-1.9 micromol ml(-1) after 1 h. When caimans were anesthetized 2 h after submergence, osteoderm lactate in the same animals was significantly increased to 14.8 micromol g(-1) wet mass. The ratio of the osteoderm:plasma lactate concentration after submergence was similar to the ratio observed in the incubated samples, suggesting that osteoderm lactate concentrations in vivo were equilibrated with circulating plasma levels. We conclude that caiman osteoderms sequester lactate during lactic acidosis and that the time course is fast enough to have benefit to these animals following normal anaerobic burst activity.

Effects of controlled pH on organic and inorganic composition in haemolymph, epidermal tissue and cuticle of mud crab Scylla serrata

Analysis of organic and inorganic compounds in plasma, epidermal tissue and cuticle were accomplished in the intermolt (C3 stage) of crab Scylla serrata incubated in different pH media. Significant changes with similar trends for protein, carbohydrates, glycosaminoglycans (GAG), sulphur, calcium, magnesium, potassium, phosphorus and copper in the plasma suggested higher dissolution in an acidic medium while the deposition increased in alkaline medium. Similar decreases in protein, carbohydrate and GAG in the epidermal compartment were observed from pH 4 to pH 12. However, significantly higher contents of sodium, chloride, potassium, phosphorus, magnesium, sulphur and copper were measured at pH 7.5 with a symmetrical decrease profile in both acidic and alkaline media, resulting from synergistic effects in the osmotic regulation. Clear changes in calcium concentrations were observed with a sharp increase from lower contents at pH 7.5 to higher at pH 12. In the cuticle, the acidic condition induced a significant dissolution of HCl-protein, GAG, calcium and magnesium contents. On the other hand, the alkaline condition induced a significant decrease in carbohydrate, calcium, chloride, sulphur and potassium. A reduction trend is seen for NaOH and H(2)O-protein contents in the cuticle. These observations suggest that GAG and HCl-protein might constitute the most soluble fraction with high affinity for calcium binding and easily removed in acidic conditions. Additionally, it is possible to speculate that the carbohydrates associated with the NaOH and H(2)O-proteins may form an interface between the soluble matrix fraction and the chitin framework. Sulphur groups seem to present a strong linkage role in this interface fraction, maybe only broken by a specific enzyme in extreme alkaline conditions with subsequent release of significant calcium from the shell.

Respiratory and circulatory compensation to hypoxia in crustaceans

Crustaceans are often tolerant of hypoxic exposure and many regulate O(2) consumption at low ambient O(2). In acute hypoxia, most increase branchial water flow, and many also increase branchial haemolymph flow, both by an increase in cardiac output and by shunting flow away from the viscera. The O(2)-binding affinity of crustacean O(2) carriers increases in hypoxic conditions, as a result of hyperventilation induced alkalosis. In chronic hypoxic exposure some crustaceans do not sustain high ventilatory pumping levels but increased effectiveness of O(2)-uptake across the gills is maintained as a result of the build up of metabolites such as lactate and urate which also function to increase the haemocyanin O(2)-binding affinity. Chronic exposure to hypoxia also may increase O(2)-binding capacity and promote the synthesis of new high O(2)-affinity carrier molecules. Exposure to untenable rates or levels of O(2) depletion causes many decapodan crustaceans to surface and ventilate the gills with air. Burrowing crayfish provide an example of animals, which excel in all these mechanisms. Control mechanisms involved in compensatory responses to hypoxia are discussed.

Lactate sequestration in the carapace of the crayfish Austropotamobius pallipes during exposure in air

When held in air for up to 24 h, crayfish accumulated Ca(2+) and Mg(2+) in their haemolymph in direct proportion to raised levels of lactate. K(+) levels were highly variable, with elevated levels associated with morbidity. Lactate accumulation in the haemolymph was reflected in proportional increases in lactate levels in the carapace and muscle. Pieces of carapace incubated in saline containing elevated levels of lactate accumulated lactate to up to half the dissolved concentration. Measured levels in the carapace, relative to its water content, implied that lactate accumulated in the carapace in a combined form, possibly complexed to calcium. The exoskeleton seems to provide a reserve of buffering capacity and a sink for lactate during anaerobic metabolism. A similar mechanism has been identified in pond turtles.

Bone and shell contribution to lactic acid buffering of submerged turtles Chrysemys picta bellii at 3 degrees C

To evaluate shell and bone buffering of lactic acid during acidosis at 3 degrees C, turtles were submerged in anoxic or aerated water and tested at intervals for blood acid-base status and plasma ions and for bone and shell percent water, percent ash, and concentrations of lactate, Ca(2+), Mg(2+), P(i), Na(+), and K(+). After 125 days, plasma lactate concentration rose from 1.6 +/- 0.2 mM (mean +/- SE) to 155.2 +/- 10.8 mM in the anoxic group but only to 25.2 +/- 6.4 mM in the aerated group. The acid-base state of the normoxic animals was stable after 25 days of submergence. Plasma calcium concentration (¿Ca(2+)) rose during anoxia from 3.2 +/- 0.2 to 46.0 +/- 0.6 mM and ¿Mg(2+) from 2.7 +/- 0.2 to 12.2 +/- 0.6 mM. Both shell and bone accumulated lactate to concentrations of 135.6 +/- 35.2 and 163.6 +/- 5.1 mmol/kg wet wt, respectively, after 125 days anoxia. Shell and bone ¿Na(+) both fell during anoxia but the fate of this Na(+) is uncertain because plasma ¿Na(+) also fell. No other shell ions changed significantly in concentration, although the concentrations of both bone calcium and bone potassium changed significantly. Control shell water (27.8 +/- 0.6%) was less than bone water (33.6 +/- 1.1%), but neither changed during submergence. Shell ash (44.7 +/- 0.8%) remained unchanged, but bone ash (41.0 +/- 1.0%) fell significantly. We conclude that bone, as well as shell, accumulate lactate when plasma lactate is elevated, and that both export sodium carbonate, as well as calcium and magnesium carbonates, to supplement ECF buffering.

Living without oxygen: lessons from the freshwater turtle

Freshwater turtles, and specifically, painted turtles, Chrysemys picta, are the most anoxia-tolerant air-breathing vertebrates. These animals can survive experimental anoxic submergences lasting up to 5 months at 3 degrees C. Two general integrative adaptations underlie this remarkable capacity. First is a profound reduction in energy metabolism to approximately 10% of the normoxic rate at the same temperature. This is a coordinated reduction of both ATP generating mechanisms and ATP consuming pathways of the cells. Second is a defense of acid-base state in response to the extreme lactic acidosis that results from anaerobic glycolysis. Central to this defense is an exploitation of buffer reserves within the skeleton and, in particular, the turtle's shell, its most characteristic structure. Carbonates are released from bone and shell to enhance body fluid buffering of lactic acid and lactic acid moves into shell and bone where it is buffered and stored. The combination of slow metabolic rate and a large and responsive mineral reserve are key to this animal's extraordinary anaerobic capacity.

Patterns of energy metabolism in the stone crab, Menippe mercenaria, during severe hypoxia and subsequent recovery

Specimens of the stone crab, Menippe mercenaria, survived severe hypoxia (PO2 less than 8mm Hg) for at least 12 hr at 28-30 degrees C. During the time course of 12 hr of hypoxia, hemolymph L-lactate levels rose to 30-50 mumoles/g wet wt. There was a slight elevation of L-alanine levels, whereas succinate was found in only trace quantities in the hemolymph. Pronounced metabolic changes took place in the heart, cheliped closer, and leg socket muscles during severe hypoxia. L-lactate accumulated to levels ranging from 16-20 mumoles/g wet wt. There were pronounced changes in high-energy phosphate levels in the cheliped closer and leg socket muscles. Taking into account expected intra- and extracellular water content, the calculated intracellular lactate content in the three muscles investigated is substantially less than the hemolymph lactate concentrations. Part of this reverse concentration gradient may be accounted for by the reduction in lactate activity due to cation-lactate complex formation. Hemolymph calcium and magnesium concentrations rose considerably during severe hypoxia. During recovery from severe hypoxia, approximately 50% of the accumulated lactate in the hemolymph was cleared in 6 hr. Hemolymph lactate and alanine levels returned to near control levels after 24 hr of recovery. This study shows that the stone crab, M. mercenaria, survives severe hypoxia by a reliance on glycogen fermentation to lactate. This species is capable of tolerating high levels of accumulated lactate.

Acid-base regulation during exercise and recovery in the blue crab, Callinectes sapidus

During swimming activity Callinectes sapidus incurred a severe hemolymph acidosis due to elevated levels of PCO2 and lactate. Although hemolymph lactate concentration rose steadily during 1 h of exercise, hemolymph pH was maximally depressed within the first 15 min. A discrepancy between the quantities of lactate and H+ released from the tissues into the hemolymph is explained by a large apparent efflux of H+ into the ambient seawater, presumably via a branchial ion exchange process. Ammonia excretion increased 6 fold during exercise, but it is not clear if this contributed to the excretion of H+. The total quantity of H+ excreted into the environment during exercise and recovery far exceeded that which could be attributed to hemolymph lactic acid, indicating that some of the excreted H+ must have originated from other sources, such lactic acid which dissociated intracellularly, with the lactate anions remaining in the cells. Because lactate anions increase hemocyanin O2 affinity when unopposed by the Bohr shift, the excretion of a large portion of the metabolic H+ load, leaving lactate behind in the hemolymph, has important consequences for the regulation of O2 transport during swimming.

Plasma ion balance of submerged anoxic turtles at 3 degrees C: the role of calcium lactate formation

Freshwater turtles, Chrysemys picta bellii, were submerged in groups of 7 at 3 degrees C in O2-free water for 1, 2, 4, 8 and 12 weeks. Blood samples from these turtles and from 10 normoxic turtles at 3 degrees C were analyzed for plasma concentrations of lactic acid, total CO2, Na+, K+, Cl-, Ca2+, total calcium, total magnesium and osmolality. Total lactate rose during anoxia to a mean peak value of 145 mM, but the decrease in HCO-3 and Cl- and increase in K+ balanced less than 40% of the lactate. Total calcium and total magnesium rose respectively by 9.5 and 6.0 times the normoxic values after 12 weeks, at which time free [Ca2+] was 25.0 mEq (37% of the total calcium). To evaluate the possible role of bound calcium in ion balance, test solutions with calcium, but with and without 145 mM lactate, were tested for free Ca2+. In the presence of lactate, over two-thirds of the total calcium combined with lactate- to form a calcium lactate complex (possibly CaLactate+). Based on these data, it is concluded that most of the bound plasma calcium in the anoxic turtles was combined with lactate. By assuming that magnesium reacts similarly with lactate, a complete account of plasma ion balance is accomplished and the turtle's plasma ionic response to extreme lactic acidosis is described. Plasma osmolality increased during anoxia by 100 mOsm and matched the mM rise in total measured and calculated ions.