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

¹ Department of Biology, University of Texas at Arlington, Arlington, Texas 76019; Department of Biology, Syracuse University, Syracuse, New York 13210; and Marine Biological Laboratory, Woods Hole, Massachusetts 02543

Aerial and aquatic rates of oxygen consumption were determined over a range of 5° to 45° C at 5° C intervals for six species of marine littoral snails: including the sublittoral species, Acmaca 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° to 35°. 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° C and 30° 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° C, representing a reversible torpor up to its thermal maximum at 44° 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 near-terrestrial 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.