A comparative study of the respiratory responses of the bivalves Arctica islandica (L.) and Mytilus edulis L. to declining oxygen tension

The bivalves Arctica islandica and Mytilus edulis generally maintain respiratory independence during hypoxia, but the mechanisms by which this is achieved are very different. In declining oxygen tension, Arctica increases the rate of ventilation down to a low PO2 of about 7 kPa, while oxygen utilization is gradually reduced. In contrast, Mytilus shows only a slight initial increase in ventilation rate, which then falls steadily as the PO2 of the medium declines, while the oxygen utilization increases sharply. Mytilus which have lost the ability to maintain respiratory independence show little change in oxygen utilization. The increase in ventilation rate of the subtidal Arctica is brought about principally by increasing the duration of the pumping periods, a mechanism not available to Mytilus which, like other intertidal species studied, normally pumps more or less continuously whenever it is immersed. The cardiac responses to hypoxia are very similar in both Arctica and Mytilus; heart rate and amplitude increase down to a PO2 of about 2-4 kPa, below which both decrease rapidly. In Arctica the ventilation/perfusion ratio remains approximately constant over the range of oxygen tension that respiratory independence is maintained. In Mytilus this ratio gradually declines as the ambient PO2 is reduced. It is suggested that the different respiratory responses to hypoxia of these two species may be due to the different patterns of pumping behaviour related to their different habitats, and to the relative energy costs of pumping blood and water.