Cardio-respiratory function of swimming blue crab Callinectes danae Smith, during normoxia and graded hypoxia

Swimming blue crabs Callinectes danae Smith were studied as to the respiratory (oxygen uptake, MO 2 and scaphognathite beat frequency, f sc ) and cardiac (ECG and heart beat frequency, f H ) responses to normoxia (154 mmHg) and graded hypoxia (130, 100, 70, 50, 30 and 20 mmHg). C. Danae kept constant MO 2 from normoxia until the critical oxygen tension (PcO 2) of 97 mmHg was reached, below which it declined markedly. Scaphognathite beat frequency ( f sc ) remained constant at all PwO 2 values studied. Cardiac frequency decreased gradually below 70 mmHg down to the more severe hypoxic levels. The same tendency was recorded for the ECG amplitude, suggesting a causal relationship between cardiac dysfunction and hypoxic bradycardia. Simple scaphognathite reversals remained constant independent of the PwO 2 reductions but significant increases in the frequencies of reversals associated with cardiac arrest were recorded below the PcO 2. When compared with earlier studied exclusively water breathing crabs, C. danae demonstrated high PcO 2 and limited cardio-respiratory adjustments in response to hypoxia.