The Anaerobic Metabolism of the Common Shore Crab, Carcinus maenas (L.)

Hill, Andrew Douglas (1989) The Anaerobic Metabolism of the Common Shore Crab, Carcinus maenas (L.). PhD thesis, University of Glasgow.

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The object throughout this study has been to investigate the importance of anaerobic metabolism in the decapod crustacean, Carcinus maenas (L.), as a result of exposure to environmental anoxia. An integrated approach was therefore adopted, spanning the subjects of biochemistry, physiology and field ecology. The study was initiated by an investigation into diel and seasonal environmental extremes that occur within intertidal rock pools. It was found that the diel ranges shown by the various physico-chemical factors become larger during the summer months. The fluctuations in the partial pressure of dissolved gases within the rock pools were the result of algal photosynthesis elevating the oxygen levels during the day and respiration of the pools' fauna and flora, in the absence of photosynthesis, causing a depletion at night. This depletion was most pronounced during the summer, when the PO2 of the rock pool, used in the field experiments, decreased to approximately 5 Torr. Studies investigating the distribution of Carcinus maenas throughout the year, revealed that far greater numbers of crabs (mainly male), were present in exposed intertidal rock pools during the summer than during the winter. This peak in the number of crabs present on the littoral zone coincided with the time of year that the pools became most hypoxic. Severe hypoxia (PO2 10 Torr) lasted up to 4 h and required the crabs to either employ a behavioural response to avoid these extreme conditions or to resort to anaerobic metabolism. Field observations showed that, in common with several other intertidal species of decapod crustaceans, Carcinus maenas exhibited partial and full emersion responses, when exposed to hypoxic or anoxic conditions in rock pools. Laboratory experiments demonstrated that under conditions of severe hypoxia, the concentration of L-lactate increased in fully immersed crabs, but no accumulation was observed in those crabs that had become partially or fully emersed. The possible advantages of these three different behavioural responses to environmental anoxia were briefly considered. There are situations, however, in which either the crab is not able to become emersed (e. g. deep rock pool with steep walls) or in which it would be dangerous to do so (e. g. high predation risks). The physiological responses which enable Carcinus maenas to survive exposure to hypoxia were investigated. An increase in the frequency of beating of the scaphognathites (hyperventilation) during mild hypoxia, resulting in an increase of the ventilatory flow, was implied from measurements of the pH and PCO2 of postbranchial haemolymph. Once the environmental PO2 decreased to below the critical oxygen tension (Pc point), the heart rate declined and the crabs were unable to maintain a rate of oxygen uptake that was sufficient to sustain the normal rate of oxygen consumption. As the degree of hypoxia became more severe, the biochemical responses associated with hypoxia and anoxia assumed a greater importance. The concentration of L-lactate started to increase, indicating the utilisation of anaerobic metabolism. During the early stages of severe hypoxia the transphosphorylation of ADP by phospho-1-arginine was observed to supplement the glycolytically derived energy. Anaerobic metabolism is energetically very wasteful, owing to the incomplete oxidation of the carbohydrate precursor, resulting in a very much lower efficiency of ATP production than under aerobic conditions. If an animals' anoxic energy demand remains at the pre-anoxic rate, then the rate of glycolysis must be enhanced (i.e. 'Pasteur effect'), which would rapidly deplete the carbohydrate pool. The need for this Pasteur effect could be overcome by reducing the energy demand of the animal. In the present study, after 2 hours of anoxia, the metabolic rate in Carcinus maenas had decreased to 16 % of the rate under aerobic conditions. It appears that this reduction in metabolic rate is probably the most important response of Carcinus maenas to explain the crabs' high anoxia-tolerance. Although the mechanism by which this depression occurs was not investigated in the present study, a brief review in Chapter 7 summarises some of the more recent work on this subject. Although L-lactate undoubtedly accumulates during anaerobic metabolism, there had been no previous comprehensive study to investigate the occurrence of other anaerobic pathways in decapod crustaceans. A major aim of the present study was therefore to establish their relative importance.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Zoology, Physiology, Biological oceanography
Date of Award: 1989
Depositing User: Enlighten Team
Unique ID: glathesis:1989-78014
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 15:43
Last Modified: 30 Jan 2020 15:43

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