Shore-level size gradients and thermal refuge use in the predatory sea star Pisaster ochraceus: the role of environmental stressors

The body size of a mobile intertidal invertebrate can determine its access to different microhabitats, and thus alter its exposure to environmental stressors. We surveyed a rocky intertidal keystone predator, the sea star Pisaster ochraceus, and characterized size-dependent distribution, defined by individuals' shore level and refuge use. At 2 field sites (in California and Oregon, USA) we examined temporal and geographical variability in habitat selection. We evaluated the hypothesis that environmental drivers measured on-site and body temperatures measured using biomimetic sensors (i.e. 'robo-sea stars'), explained the observed distribution patterns, including shore-level size gradients, with larger animals lower on the shore. We tested the effect of size on animals' thermo-and desiccation-tolerance. Using robo-sea star data, thermal performance curves and critical temperatures of different size classes, we investigated potential physiological and survival consequences of microhabitat use (shaded vs. sun-exposed). Pisaster is mostly found in thermal refugia during low tide even when the risk of thermal stress in sun-exposed areas is low, suggesting a risk-avoidance strategy. In sheltered microhabitats, Pisaster exhibited shore-level size gradients; in sun-exposed microhabitats, this pattern disappeared. The proportion of individuals found in shaded microhabitats increased with air temperature, solar radiation, and body temperature. Size-dependent sensitivity to stressful temperatures and wind speed did not explain the observed distribution patterns, suggesting that in the field, size constraints prevent larger animals from occupying refuges that small individuals can use. Our data reveal that, despite generally mild conditions, Pisaster risk-avoidance strategy buffers against rare but potentially highly stressful events.