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Aerial and aquatic rates of oxygen consumption were determined over a range of 5 degrees to 45 degrees C at 5 degrees C intervals for six species of marine littoral snails: including the sublittoral species, Acmaea testudinalis, Mitrella lunata, and Lacuna vincta; and the truly intertidal species, Littorina obtusata, L. littorea, and L. saxatilis. Polarographic oxygen electrodes were used with normally active snails collected from populations on Nobska and Manomet Points, Massachusetts. Three subtidal species, A. testudinalis, Lacuna vincta, and M. lunata, do not display any metabolic adjustment to increasing temperature, with thermal limits reached at 30 degrees to 35 degrees C. Aerial respiration in A. testudinalis is similar to aquatic O2 uptake, but rates average only 36.4% of aquatic rates. The intertidal congeners, Littorina obtusata, L. littorea and L. saxatilis, have varying degrees of aerial and aquatic metabolic regulation with increasing temperature. L. obtusata, a low intertidal snail exposed to air for 15% to 45% of the tidal cycle, displays a respiratory pattern of "passive endurance" to high temperatures both in air and in water. L. littorea, the dominant snail of the midlittoral region, remains active when exposed to air (30% to 75% of the tidal cycle) and has a zone of metabolic regulation between 20 degrees C and 30 degrees C. Over this, the normal ambient temperature range, the Q10 closely approximates one, and nearly equivalent O2 uptake rates occur in air and in water. L. saxatilis from the upper littoral region is exposed to air for 70% to 95% of the tidal cycle and is characterized by reduced aerial and aquatic O2 uptake rates above 25 degrees C, representing a reversible torpor up to its thermal maximum at 44 degrees C. For these six snail species, respiratory responses to increasing temperature are thus directly related to the pattern of vertical distribution in the intertidal environment. Discussion of this relationship stresses that the evolution of other nearterrestrial structures and functions in littoral snails has proceeded in a discontinuous fashion. Despite this, the temperature responses in respiration parallel the functional morphology of the pallial structures and the physiological patterns of response to low oxygen stress, as well as adaptive features of reproduction, larval development, water-control, and nitrogenous excretion.

TEMPERATURE RELATIONS OF AERIAL AND AQUATIC RESPIRATION IN SIX LITTORAL SNAILS IN RELATION TO THEIR VERTICAL ZONATION