Preference versus performance: body temperature of the intertidal snail Chlorostoma funebralis

Evolutionary theory predicts that, in variable environments, it is advantageous for ectothermic organisms to prefer a body temperature slightly below the physiological optimum. This theory works well for many terrestrial organisms but has not been tested for animals inhabiting the hypervariable physical environment of intertidal shores. In laboratory experiments, we allowed the intertidal snail Chlorostoma funebralis to position itself on a temperature gradient, then measured its thermal preference and determined an index of how its performance varied with temperature. Snails performed a biased random walk along the temperature gradient, which, contrary to expectations, caused them to aggregate where body temperature was 15 to 17 °C below their temperature of optimum performance and near the species' lower thermal limit. This "cold-biased" behavioral response may guide snails to refuges in shaded cracks and crevices, but potentially precludes C. funebralis from taking full advantage of its physiological capabilities.