Abstract 1. 1. Oxygen uptake and ammonia loss were monitored during responses to reductions of both salinity and oxygen tension ( PO 2) in the marine mussels Perna viridis and Perna indica from southern India. 2. 2. The proportional contribution of protein to total catabolic substrates under natural environmental conditions was as much as 96% in P. viridis, relative to only 19% in P. indica. 3. 3. Normoxic oxygen consumption remained statistically unchanged in P. viridis conditioned to salinities between 32 and 15‰, with no obvious signs of distress. Although equally unaffected at salinities between 32 and 20‰, P. indica showed significantly reduced oxygen uptake following transfer from 32 to 15‰, and had died within the next 7 days. 4. 4. At salinities greater than 20‰, P. viridis was better able than P. indica to regulate oxygen consumption independent of PO 2. 5. 5. P. indica showed a compensatory increase in oxyregulatory capacity at 15‰. This exceeded unstressed abilities, helping to maintain albeit reduced oxygen uptake throughout wider ranges of PO 2. 6. 6. Different responses recorded in each of these tropical and often intertidal species were in accordance with their natural distributions. Nevertheless, the oxyregulatory capacity in both species was higher than in bivalves from temperate and/or subtidally restricted habitats.