Beating the heat at low tide: infrared thermography suggests persistent evaporative cooling in a keystone predator (Pisaster ochraceus).

Climate change is increasing temperature variability, driving more frequent and intense heat events that risk pushing species beyond their thermal limits. Ectotherms, which rely on external sources to regulate their body temperature, are especially vulnerable to heat stress. Understanding the range of body temperatures ectotherms experience in the field is essential for assessing how they respond to changing thermal regimes. Yet, point-source temperature loggers often fail to capture the complexity of thermoregulation in living organisms. Infrared thermography (IRT) offers a portable, cost-effective alternative for measuring body surface temperatures of terrestrial and intertidal species. Using IRT, we investigated how biological factors (size, aggregation) and environmental conditions (air, sea surface (SST), and substrate temperatures; humidity, wind, and microhabitat) influenced body surface temperatures of Pisaster ochraceus during summer low tides on Vancouver Island, Canada. We found that body size, aggregation, and wind speed did not predict body surface temperature. We further measured body surface temperatures which differed from ambient air (-6.2 ± 2.5 °C), SST (-1.4 ± 1.2 °C), and adjacent rocks (substrate temperature; -0.6 ± 1.2 °C), particularly where animals occupied heat-protected microhabitats on low humidity days (mean ± SD). Thus, sea star temperatures deviated significantly from environmental temperatures, with most animals maintaining low temperatures in the field, even on hot days. These findings demonstrate the limitations of using air and SST as proxies for body temperature, highlight evaporative cooling as an important thermoregulatory mechanism in intertidal animals, and underscore the value of IRT for thermal biology.