Fig. 1. Relative humidity (RH) and temperature conditions at sites in the Strait of Juan de Fuca. (A) Monitored sites and study collection site (French Beach, BC, Canada). (B) RH (%) at each site (CC = Colin’s Cove, OP = Observatory Point, TI = Tatoosh Island, Washington; N = 3865). The diamonds in the center of each plot are the mean. (C) Relationship between temperature and RH at each site (red = CC, green = OP, blue = TI). Lines are fitted linear regressions. Maps were produced using the R packages ’ggplot2’ v3.5.2 (Wickham et al. 2025) and ’ggspatial’ v1.1.9 (Dunnington 2023), and a base map of North America from the US Department of State (US Department of State, Office of the Geographer 2013)
ABSTRACT: Extreme temperature events have cascading impacts on biological systems, from physiological damage to individuals to the collapse of species interactions. In cnidarians, mutualistic symbioses are vulnerable to high temperatures, but the effects of extremes like heatwaves and cold snaps on temperate host–symbiont pairs are poorly understood. This research aimed to determine the upper and lower thermal tolerance thresholds of an abundant intertidal symbiotic system, the aggregating anemone Anthopleura elegantissima, and identify factors influencing survival and bleaching (loss of symbiotic algae) at extreme emersion temperatures. We exposed anemones to a range of air temperatures and 2 levels of relative humidity during simulated low tides, recording upper and lower median lethal temperatures (ULT50 and LLT50) and symbiont retention. Aggregating anemones had nearly identical ULT50s of 33°C across humidity levels. Larger individuals had ULT50s nearly 3°C higher and LLT50s 1.5°C lower than smaller individuals. High intertidal anemones had ULT50s almost 1°C higher than those from lower zones. Anemones experienced severe bleaching (over 90 % symbiont loss) after extreme cold exposure, with a bleaching threshold coinciding with freezing (–6 to –7°C). However, anemones maintained moderate symbiont densities at high temperatures (30–36°C). These results suggest that extreme cold may be a key driver of symbiotic breakdown in this symbiotic assemblage. Defining environmental thresholds for host and symbiont performance — and identifying physiological and environmental drivers (e.g. body size, thermal history) — is essential to predicting shifts in interspecific interactions under increasingly frequent thermal extremes