Respiratory Response to Periodic Emergence in Intertidal Molluscs

SYNOPSIS: The ratio of aerial: aquatic $V˙$O₂ ($V˙$o_2(a): $V˙$o_2(W)) was computed for species of intertidal molluscs. This ratio was 1 for high littoral archeogastropods suggesting high metabolic demands in air. $V˙$O_2(a): $V˙$O_2(w) ratios were near unity for meso- and neogastropod species regardless of zonation. Littoral fringe mesogastropods had ratios <1 reflecting reduced activity on emergence. A major gastropod adaptation to increasing emergence is reduction of ctenidium surface area and formation of a mantle cavity lung. Mid- and high littoral pulmonates with both a mantle cavity lung and secondary gills have $V˙$O_2(a): $V˙$O_2(w) ratios near unity. In contrast, littoral fringe pulmonates without secondary gills are partially anaerobic in water. Emerged low and mid-littoral bivalves close the valves, are almost entirely anaerobic, have $V˙$o_2(a): $V˙$o_2(w) ratios ≤0.14:1 and conserve energy by greatly reducing metabolic demand in air. In contrast, emerged high littoral bivalves remain aerobic by periodic gaping and mantle cavity ventilation. Such behaviors support an aerobic metabolism while minimizing evaporative water loss. Aerial gas exchange prevents anaerobic end-product accumulation and, with a reduction in energy demand, allows efficient energy store utilization during prolonged emergence.