Physiological adaptations to high intertidal life involve improved water conservation abilities and metabolic rate depression in Littorina saxatilis

Mortality, rates of water loss, aerobic and anaerobic metabolic rates and changes in the parameters of cellular energy status were monitored in the intertidal gastropod Littorina saxatilis from different latitudes and shore levels during prolonged air exposure at elevated temperatures (30°C). Prolonged air exposure, imitating environmental conditions during summer low tide at highshore levels, resulted in the onset of anaerobic metabolism, as indicated by the significant accumulation of succinate and alanine and a considerable depletion of high-energy phosphates (ATP and phospho-L-arginine, PLA). However, anaerobic metabolism accounted for only 1 to 2% of total ATP turnover in all groups, the remainder being supplied by aerobic metabolism. Snails from the White Sea population, which were more resistant to prolonged air exposure at 30°C than North Sea specimens, demonstrated higher anaerobic capacity and a depression of overall metabolic rate by ca 20% in air, whereas the less-resistant North Sea animals displayed very low anaerobic metabolic rates and elevated aerial oxygen consumption rates (110 to 125% of the rate observed in water). Within the White Sea population, high-shore periwinkles demonstrated higher resistance to prolonged air exposure than low-shore specimens associated with lower evaporation water loss, reduced rates of anaerobic ATP turnover, higher stores of L-aspartate and lower defended values of ATP and PLA. In contrast, no differences were found between highand low-shore snails in the North Sea. This may reflect environmental conditions at the 2 study sites, with stronger contrasts between highand low-shore habitats in the White Sea than in the North Sea. In general, our data show that adaptation to prolonged air exposure at elevated temperatures, which may occur at high-shore levels during summer low tide, does not necessarily involve increased anaerobic capacity in L. saxatilis. Instead, enhanced abilities for water conservation and metabolic rate depression combined with increased stores of fermentable substrate seem to be crucial for survival during prolonged periods of emersion.