Aerial exposure and energy input in the bivalve Choromytilus meridionalis (Kr.)

The effects of aerial exposure on growth, filtration, and assimilation were examined in Choromytilus meridionalis (Kr.) from Bailey's Cottage, False Bay, South Africa. The maximum height at which these mussels occur on the shore corresponds to 50% aerial exposure. Growth rates declined with increasing shore height. The relationships between flesh and shell dry weight and shell length were not significantly different in littoral and sublittoral mussels. Littoral mussels did not show enhanced filtration rates or assimilation efficiencies relative to continually submerged individuals. Thus, littoral mussels experience a decline in ingestion ration which is directly correlated with increasing shore height. Under conditions of limited food availability littoral organisms frequently employ conservative mechanisms to limit energy expenditure. In C. meridionalis this is evidenced by limitation of the growth rate. Energy conservation may also be facilitated by quiescence and reduced respiratory cost during exposure. This requires investigation.