Topics dealing with life in the intertidal zone include gas exchange considered here, and TEMPERATURE EFFECTS, HEAT-SHOCK PROTEINS, WAVE EFFECTS, SALINITY EFFECTS, TRANSLOCATION STUDIES, and WATER-CHEMISTRY EFFECTS, considered in other sections.

Studies at Friday Harbor Marine Laboratories, Washington on sea mussels Mytilus californianus indicate that several things happen with respect to gas exchange when a mussel is emersed by the falling tide and remains tightly closed.  First, oxygen concentration in the trapped mantle-cavity water drops to about one-third of its normal level within 1hr. At the same time, heart rate decreases from about 20 beats ^. min^-1 to about 8 beats ^. min^-1 (at 13^oC).   On re-immersion after 6hr in the air, both return within about 10min to normal levels (see graphs on Left).

Gaping, thought to be a possible strategy for evaporative cooling, may alternatively be a way of enabling at least some aerobic respiration during air exposure. Within 5hr after air exposure, oxygen consumption in air is 2.5 times the previous rate in water.   On re-immersion, oxygen consumption returns to normal levels within 2h (see graph on Left). 

As a consequence of a mussel being closed when emersed, excretory products in the form of ammonia have no way of escaping, so concentrations rise. On re-immersion after 6hr, the ammonia is quickly flushed out and levels quickly return to normal.  Bayne et al. 1976 Oecologia 22: 229.

NOTE  part of this increase may owe to air temperature being 19^oC as compared with water temperature of 13^oC