Respiratory and Cardiovascular Responses of Two Species of Deep-Sea Crabs, Chaceon Fenneri and C. Quinquedens, in Normoxia and Hypoxia

Abstract

The respiratory and cardiovascular variables in Chaceon fenneri and C. quinquedens were examined in animals during normoxia, animals exposed to hypoxia, and animals allowed to recover in normoxia. Chaceonfenneri was characterized by a relatively low metabolic rate (low oxygen uptake), and low ventilatory frequency. This species displayed a pattern of oxy-independent 02 uptake when exposed to hypoxia: MO2 was maintained primarily through hyperventilation. At an environmental PO2 of about 25 torr, respiratory and cardiovascular functions shut down, and the animal appeared to make the transition to exclusively anaerobic energy production. Lactate accumulated in the hemolymph during hypoxia and during recovery. Recovery was also characterized by elevated 02 uptake, and prolonged hyperventilation, indicating the repayment of an oxygen debt. Oxygen uptake in resting C. quinquedens was roughly double that in C. fenneri, and ventilatory frequency was 3-fold higher. MO2 declined in hypoxia, and hyperventilation was absent, a more typical oxy-dependent pattern of respiration. The ambient PO2 at which ventilatory and cardiovascular shutdown occurred was about 17 torr, but oxygen uptake continued at very low levels even in severe hypoxia. Lactate buildup in the hemolymph was not observed. During recovery, normal levels of 02 uptake were quickly restored, and hyperventilation was absent. Chaceon quinquedens appeared not to utilize anaerobic metabolism during hypoxia or to incur an oxygen debt. The differences in respiratory and cardiovascular responses to hypoxia in these two species appear to be due to a combination of differences in metabolic rate and body morphology. The basic mechanisms of crustacean respiration, ventilation, and cardiovascular function have been the subjects of extensive investigation for more than a decade, especially among the decapods (for review, see Taylor, 1982; Cameron and Mangum, 1983; McMahon and Wilkens, 1983; McMahon, 1988). The picture that has emerged is one of a common mechanism that displays a graded pattern of responses to varying environmental conditions. One of the most commonly studied environmental variables has been low oxygen tensions. In response to declining oxygen tensions, decapods show one of two general patterns: (1) animals exhibiting oxy-independent respiration, maintaining normal rates of oxygen uptake (MO2), or (2) animals exhibiting oxy-dependent respiration, in which MO2 declines with ambient oxygen tension (Mangum and Van Winkle, 1973). These two patterns have also been termed oxyregulation and oxyconformity, respectively (Taylor, 1982). Most decapods display some degree of oxy-independent respiration with MO2 being maintained by a combination of hyperventilation and increased stroke volume of the heart. This preserves both the PO2 gradient across the gills and hemolymph PO2 until ambient oxygen tensions reach a critical low value (Mangum and Van Winkle, 1973; Taylor, 1976, 1982; McMahon and Wilkens, 1983; McMahon, 1988). Below that critical value (termed Pc) ventilatory and cardiovascular compensation no longer provide the necessary oxygen for aerobic respiration; hemolymph PO2 declines as the animal uses up its venous 02 reserve, and ultimately ventilation and cardiac function are shut down as the animal makes the transition to anaerobic energy production. Patterns of oxy-independent and oxy-dependent respiration, ventilatory and cardiovascular responses, and values for Pc have been described for species of decapod crustaceans from a wide variety of habitats and which display different life-styles. Studies