Brian R. McMahon

Oxygen uptake and the potentiating effects of increased hemolymph lactate on oxygen transport during exercise in the blue crab,Callinectes sapidus

SummaryAt the onset of moderate swimming activity,Callinectes sapidus rapidly increased branchial ventilation, heart rate, and oxygen uptake, reaching steady state values in 2–3 min, with a half-time of 30 sec. Although O2 extraction efficiency decreased slightly (50% to 43%) upon reaching steady state, O2 uptake was increased 2.6 fold over resting (routine) levels. HemolymphPO2 did not change during sustained (30–60 min) exercise, but a marked decrease in pH (7.60 to 7.10), associated with a 14-fold increase in hemolymph lactate concentration, caused decreases in both pre-and postbranchial O2 content due to a large hemocyanin Bohr shift. The effect of the Bohr shift on O2 binding, however, was minimized by an increase in hemocyanin O2 affinity induced by lactate ions; the influence of lactate on hemocyaninP50 was shown to be the same in vivo and in vitro. As a result of the interaction between the Bohr and lactate effects, only slight increases were observed in the a-v O2 difference (13%) and the quantitative role of hemocyanin in oxygen transport (11%) during exercise. The increase in O2 delivery was therefore attributed primarily to a 2.3 fold increase in cardiac output (Fick estimate), resulting from increases in both heart rate (1.61 X) and stroke volume (1.42X). During exercise hemocyanin remained 21% oxygenated upon leaving the tissues, thus maintaining a substantial ‘venous O2 reserve’ which could be utilized to fuel more strenuous levels of exercise at least partly by aerobic pathways. The high hemolymph lactate levels, however, indicate that anaerobic metabolism makes a significant contribution to energy production even during moderate exercise. These results are similar to the respiratory and circulatory responses reported for other decapod crustaceans and fish during mild exercise.C. sapidus, however, appears to be highly resistant to fatigue, which correlates with its welldeveloped locomotor capabilities.

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.

Central Control of Cardiac and Scaphognathite Pacemakers in the Crab, Cancer magister

SummaryCommand fibers located in the circumesophageal connectives which modify scaphognathite and heart rhythms have been mapped and characterized in the crab,Cancer magister.Behavior: Crabs show a variety of responses to external stimuli often including simultaneous cessation of cardiac and scaphognathite “pumping”. Habituation and a return to prestimulus rhythms results from continued stimulation. The response to short stimulus durations, on the other hand, generally outlasts the stimulus indicating the playing-out of a motor program.Neurophysiology: Small bundles of fibers have been isolated from desheathed connectives. Activity in these fibers resulting from stimulation of various anterior sensory receptors was recordeden passant with suction electrodes. When sensory stimulation produced both electrical activity in the nerves under examination and a cardiac and/or scaphognathite response it was assumed such units were involved in inducing this response. This was tested by electrical stimulation delivered through the same electrode. Those units which produced similar responses to natural and artificial stimulation were deemed “command fibers”. It was invariably found that the minimum stimulating frequency needed to mimic naturally induced responses was much greater than the frequency at which the units discharged in response to those stimuli.During mapping experiments, command fibers were characterized with respect to their positions in the connectives and by the responses they produced at different frequencies of stimulation. 68% of the fibers identified affected both cardiac and scaphognathite systems, 29% the scaphognathites alone and 3% the heart alone. The frequency-response profiles of single bivalent command fibers were often different from the heart and scaphognathites. These findings help explain the responses of both systems to natural stimuli and also indicate that the circulatory and respiratory systems not only perform in concert, but are often under common control.

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

Hemolymph acid-base variables (pH, PCO2 and CCO2), hemolymph Ca2+ and Na+ concentrations, and osmolality were measured in unrestrained crabs, Cancer productus, before, during and following 4 hr emersion and 43 hr hyperoxia (460-510 Torr), both at 10 degrees C. Emersion and hyperoxia provoked an acidosis associated with elevation of hemolymph CCO2 and PCO2, yet attempts to calculate PCO2 from measured pH and CCO2 always resulted in values greater than those measured directly. This discrepancy between measured and calculated PCO2, was associated with base excess, and was eliminated upon in vitro equilibration of the hemolymph and more slowly in vivo, suggesting that metabolic compensation for the acidosis occurred more rapidly than could acid-base equilibration. During emersion, increases of CCO2 and [Ca2+] provide evidence that the internal CaCO3 stores, possibly from the exoskeleton, were mobilized during acid-base compensation. Hyperoxia provoked no such increase in Ca2+, and branchial uptake of HCO3- may make a major contribution to the elevation of CCO2 during hyperoxia. It is suggested that shell buffering by aquatic crustaceans provides a means of compensation for acidosis under conditions during which branchial function is impaired.

Gas Exchange, Hemolymph Acid-Base Status, and the Role of Branchial Water Stores during Air Exposure in Three Littoral Crab Species

O₂ uptake and hemolymph acid-base status, together with branchial water volume, CO₂ content, and titratable alkalinity, were measured in three species of intertidal crabs. In Pachygrapsus crassipes, a grapsid crab that actively moves between air and water, O₂ uptake increased on emersion. In Eurytium albidigitum, a mud-burrowing xanthid crab that is air exposed by tidal action, O₂ uptake declines dramatically on emersion. There is no significant lactate production by either crab following emersion. An emersion-induced respiratory acidosis was fully compensated in P. crassipes and another grapsid, Hemigrapsus nudus, but uncompensated in E. albidigitum. Branchial water volume 10 min after emersion was 0.013 ml/g crab weight in P. crassipes and 0.072 ml/g in E. albidigitum. The CO₂ content of branchial water in P. crassipes increased rapidly during air exposure and was accompanied by an increase in titratable alkalinity (TA). The CO₂ content of branchial water in E. albidigitum remained constant for at least 4 h and increased slightly after 8 h. TA remained unchanged for up to 8 h. We suggest that the ability of the crabs to compensate for the respiratory acidosis and to increase branchial water TA is correlated with osmoregulation in P. crassipes and H. nudus. On the other hand, E. albidigitum is an osmoconformer and neither compensates for the respiratory acidosis nor changes its TA during air exposure. Possible adaptive advantages of the two different strategies may be related to the relatively short duration of emersion and active habits of P. crassipes and the longer periods of air exposure and inactivity of E. albidigitum.

The effects of progressive hypoxia on the crustacean cardiovascular system: a comparison of the freshwater crayfish, (Procambarus clarkii ), and the lobster (Homarus americanus)

Abstract The heart rate of crayfish (Procambarus clarkii) and lobsters (Homarus americanus) decreased (bradycardia) as partial pressure of O2 (PO2) decreased, yet cardiac output (Vb) was maintained via an increased stroke volume (Sv) to PO2s of 40 mmHg and 75 mmHg for crayfish and lobsters, respectively. Vb was redistributed in both animals. Flow through the anterior aorta increased while flow dropped through the posterior aorta and sternal artery to a PO2 of 30 mmHg; below this flow was no longer maintained in crayfish. In the lobster, flow increased to the lateral arteries and the ventral thoracic artery while flow through the anterior and posterior aortas, sternal artery and ventral abdominal artery decreased to a PO2 of 75 mmHg. Anterior hemolymph flow was maintained or increased in both animals presumably to supply nervous tissue and cephalic sense organs better. Crayfish showed an increase in intracardiac and mean arterial hemolymph pressures as PO2 declined. The increased pressures combined with the net increase in cardiac filling pressure and diastolic filling time could have accounted for the increased SV. The cardiovascular response exhibited by both the crayfish and lobster was PO2 dependent; below a critical water PO2 active compensation was no longer observed.

Ventilation and control of acid-base status during temperature acclimation in the crab,Cancer magister

SummaryFrequencies of scaphognathite (ventilatory,fsc) and heart (fh) pumping, oxygen consumption (

Patterns of haemolymph-flow variation in decapod crustaceans

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