FLY, E.K.**; BURNAFORD, J.L.; TULLIS, A.; University of Puget Sound, Tacoma, WA; University of Puget Sound, Tacoma, WA; University of Puget Sound, Tacoma, WA: Temperature effects on the metabolism and microhabitat choice of the keystone predator Pisaster ochraceus

The sea star Pisaster ochraceus strongly impacts the distribution and abundance of its prey species in the rocky intertidal zone of the Pacific Northwest. During summertime low tides in the San Juan Islands, Washington, we observed that Pisaster were found more often in shaded areas than in sunny microhabitats. Temperatures in these shaded microhabitats (in rock crevices or under kelp canopies) were up to 18°C cooler than areas in the sun. Further observations showed that temperatures of Pisaster in intertidal pools at low tide ranged from 0 to 10°C above sea surface temperatures, depending on pool volume. Therefore, during emersion at low tide, Pisaster in different microhabitats experience substantially different abiotic conditions. To determine the effect of ambient temperatures on Pisasterís metabolism, we measured oxygen consumption rates in water and air. We tested 30 Pisaster at 3 water temperatures (10, 15, and 20°C) and 34 Pisaster at 4 air temperatures (10, 15, 20, and 25°C). In both media, mean oxygen consumption rates doubled between 10 and 15°C, and increased further at higher temperatures. Mean oxygen consumption rates in water were at least double those in air for all temperatures examined. Our data reveal that temperature has a large impact on sea star metabolic rate in both air and water. Pisaster that select cooler microhabitats at low tide may benefit by lowering their overall metabolic cost, possibly resulting in greater food assimilation rates or reduced energy requirements.