The role of temperature and desiccation stress in limiting the local-scale distribution of the owl limpet, Lottia gigantea

P>Small-scale distributions of rocky intertidal organisms may be determined in part by temperature and desiccation stress during low tide and the ability of organisms to resist such stresses. Biophysical modelling techniques, coupled with data on the physiological tolerance of organisms can be used to predict the frequency and severity of abiotic stress events. The limpet Lottia gigantea is a major competitor for primary space on rocky shores along the west coast of North America. The goal of this study was to quantify the thermal and desiccation tolerance of this species and to predict the frequency of stressful events at Hopkins Marine Station (HMS) in Pacific Grove, CA, USA. Lottia gigantea were exposed to thermal stress regimes in the laboratory using protocols designed to mimic prolonged exposures in the field. The production of heat shock protein 70 (Hsp70) was measured as an indicator of sub-lethal stress, and mortality rates were measured. Peak expression of Hsp70 occurred during exposures at 32 degrees C, while the lethal limits of the limpets ranged between 30 and 42 degrees C, depending on the exposure regime. Lethal temperatures were approximately 5 degrees C lower in 50-60% relative humidity conditions compared to 100% relative humidity trials. Results from the physiology experiments were compared to field surveys of the upper limits of L. gigantea at HMS and with heat-budget model predictions of limpet body temperatures on a variety of substratum orientations. The highest frequency of sub-lethal and lethal stress events was predicted to occur on high-shore westerly and southerly slopes and horizontal surfaces, microhabitats where L. gigantea is comparatively rare at HMS. Predicted stress and mortality events were rare at low-shore sites, vertical surfaces, and sloping rock faces oriented towards the northeast and northwest. The combination of direct measurements of sub-lethal and lethal temperature thresholds using realistic stress exposures and predictions from a heat-budget model give insight into the role of thermal disturbance in affecting the local-scale distributions of intertidal organisms. These methods hold great promise for enhancing our understanding of the role of abiotic factors in limiting species distributions and making predictions about responses to future climatic conditions.