World's largest chiton (Cryptochiton stelleri) is an inefficient thermoregulator

Rocky intertidal zones are some of the most thermally stressful environments on earth, where ectotherms deal with tidally driven fluctuations in air and water temperatures that can exceed their maximum thermal tolerance. However, not all intertidal ectotherms face the same exposure risk. In northern regions of the eastern Pacific, summertime low tides occur during midday, exposing ectotherms to potentially stressful temperatures, whereas cooler pre-dawn low tides in southern regions buffer ectotherms from thermal stress. Gumboot chitons Cryptochiton stelleri are thermally sensitive intertidal grazers that range from southern California to Alaska, exposing them to a mosaic of thermal stresses. We quantified chiton thermal performance limits in the laboratory by testing the effects of elevated air and water temperatures on grazing. We also compared the thermoregulation efficiency of chitons from thermally benign northern California sites with those from thermally stressful San Juan Island, Washington sites, using 3 components: (1) biomimetic thermal models deployed intertidally, (2) chiton body temperatures in the field, and (3) chiton thermal preference in a laboratory-based thermal gradient. We found that chiton grazing performance was greatly reduced at 18 degrees C in water, and they reached their grazing performance limit after exposure to 20 degrees C in air, confirming previous work documenting thermal limits on chiton respiration. Chitons preferred body temperatures within 3 degrees C of their thermal performance limits, but they rarely achieved body temperatures that would maximize grazing in the field. This suggests that chitons are not thermoregulating efficiently with respect to maximizing grazing performance, but instead are minimizing exposure to temperatures that would be detrimental to their performance.