Respiratory Response to Periodic Emergence in Intertidal Molluscs

SYNOPSIS: The ratio of aerial: aquatic was computed for species of intertidal molluscs.This ratio was <1 for sub- and lower littoral species suggestingpartial anaerobiosis in air and >1 for high littoral archeogastropodssuggesting high metabolic demands in air. ratios were near unity for meso- and neogastropodspecies regardless of zonation. Littoral fringe mesogastropodshad ratios <1 reflecting reduced activity on emergence. Amajor gastropod adaptation to increasing emergence is reductionof ctenidium surface area and formation of a mantle cavity lung.Mid- and high littoral pulmonates with both a mantle cavitylung and secondary gills have ratios near unity. In contrast, littoral fringe pulmonates withoutsecondary gills are partially anaerobic in water. Emerged lowand mid-littoral bivalves close the valves, are almost entirelyanaerobic, have ratios 0.14:1and conserve energy by greatly reducing metabolic demand inair. In contrast, emerged high littoral bivalves remain aerobicby periodic gaping and mantle cavity ventilation. Such behaviorssupport an aerobic metabolism while minimizing evaporative waterloss. Aerial gas exchange prevents anaerobic end-product accumulationand, with a reduction in energy demand, allows efficient energystore utilization during prolonged emergence.