Lydia Walton

Topic

Physiological and behavioural responses of a keystone intertidal predator, Pisaster ochraceus, to periodic aerial heat stress

Department of Biology

Date & location

• Monday, April 14, 2025
• 10:00 A.M.
• Clearihue Building, Room B009

Examining Committee

Supervisory Committee

• Dr. Amanda Bates, Department of Biology, University of Victoria (Supervisor)
• Dr. Rana El-Sabaawi, Department of Biology, UVic (Member)
• Dr. Iain McGaw, Department of Ocean Sciences, Memorial University of Newfoundland (Outside Member)

External Examiner

• Dr. Matthew Bracken, School of Biological Sciences, University of California, Irvine

Chair of Oral Examination

• Dr. Todd Milford, Department of Curriculum and Instruction, UVic

Abstract

Extreme heat events are becoming more frequent and intense, requiring a better understanding of how temperature variability influences species performance and interactions. When temperatures approach critical thermal limits, disruptions to individual behaviour and physiology, such as reduced prey consumption and suppressed metabolic activity, can have cascading effects on ecosystem functioning. These effects can be especially widespread when keystone predators are negatively impacted, as they exert a disproportionate effect on their community relative to their biomass. My thesis investigates how periodic aerial heat stress relates to mortality, feeding, metabolism, and body temperature in juvenile Pisaster ochraceus (ochre sea star, hereafter Pisaster), a keystone intertidal predator. Through controlled laboratory experiments, I show that exposure to extreme aerial temperatures (~30°C) increases mortality by 33% and reduces mussel consumption to half of that observed in animals experiencing typical aerial temperatures (~20°C or 25°C). Furthermore, cooler seawater (~15°C) exacerbates these adverse effects, leading to a 17% increase in mortality, and ~50% reductions in feeding and metabolic rates compared to individuals exposed to high aerial temperatures (~30°C) under warmer seawater (~20°C). To further assess the effects of temperature variability, I use infrared thermography (IRT) to examine the surface body temperatures of Pisaster (n = 738) from eight beaches during summer low tides on Vancouver Island and relate these temperatures to biological factors and environmental conditions. I find that Pisaster use several strategies to minimize thermal stress in the field. Specifically, the selection of shaded microhabitats and the use of evaporative cooling allow Pisaster to maintain body temperatures up to 14°C degrees cooler than the ambient air and up to 6°C cooler than ambient sea surface temperatures. Even so, some individuals were found in sun exposed microhabitats, and in those rare cases, body temperatures exceeded ambient air temperatures by 2°C. The integration of manipulated experiments with field monitoring reveals that increasing air-sea surface temperature disparities could reshape community dynamics through shifts in keystone predation, however, the severity of thermal stress may be reduced through behavioural and physiological buffering strategies.