Metabolic responses of the common mussel Mytilus edulis to hypoxia and anoxia

The effects of hypoxia and anoxia on the metabolic responses of mussels Mytilus edulis of different sizes (spat, 5 mm; juveniles, 13 mm; adults, 33 mm) were measured by simultaneous open-flow calorimetry and respirometry. A significant anaerobic contribution to the total metabolic rate of juveniles and adults was only apparent at PO, (oxygen partial pressure) values < 2.3 kPa. In 5 mm spat, however, the anaerobic component was relatively small and not statistically significant, even at 1.0 kPa. At the most severe level of hypoxia (1.0 kPa), the degree to which anaerobiosis contributed towards the total energy metabolism increased with body size from 11 % in spat to >50 % in adult mussels. These findings contrast with a previous study recording a large anaerobic contribution to total metabolism at moderate levels of hypoxia (ca 10 kPa). The present study showed that the metabolism of juvenile mussels held at 10 kPa for >50 h remained fully aerobic. Below 10 kPa the time required to induce a sigmficant anaerobic component declined from ca 42 h at 4.8 kPa to 5 h at 1.0 kPa. The 'heat increment' associated with the cost of feeding, digestion and growth was markedly reduced with declining PO, and was absent under anoxia.