Parental environment modulates offspring thermal tolerance in a foundational intertidal seaweed

Abstract Thermotolerance acquisition is an important ecophysiological trait under global warming scenarios because it can allow organisms and populations to adapt, particularly during the most sensitive early stages of a life cycle. Here we used seasonality as a natural scenario to explore whether parental thermal histories can modulate thermotolerance of recruits of a canopy-forming intertidal seaweed (Fucus guiryi) across an ecologically relevant thermal gradient (15–28°C). For this purpose, we harvested embryos from parents after the periods of maximum and minimum accumulated heat exposure (late summer and late winter), and at the onset of summer. During early ontogeny we followed initial embryo size, internal nutrient content, survival, growth and developmental stages as performance metrics to address whether parental acclimation modulates thermal tolerance via provisioning or parental effects. Late winter recruits of F. guiryi exhibited the greatest thermotolerance, showing a broader range of optimal temperatures and higher upper thermal limits for growth and survival, probably associated with better provisioning from parental thalli. Physiological fitness of recruits decreased above 25°C, showing arrested growth, impaired development and dropping survival rates, but functional loss was more abrupt in early summer. Late summer responses confirmed that heat hardening occurs in natural populations, but at the seasonal scale the adaptive significance of this increased thermotolerance is much lower than that induced by winter parental provisioning. Heat-induced thermotolerance occurred from early to late summer due to parental exposure to warming. However, winter provisioning promoted greater thermotolerance acquisition. Exposure to moderate thermal stress at the onset of summer without prior seasonal acclimation resulted in minimum levels of thermal tolerance and loss of offspring fitness. While warmer winters might be neutral or benefit early development, increasing temperatures and poor nutritional conditions at the onset of the summer season may reduce survival and hamper population recruitment. Highlights Thermotolerance of Fucus guiryi increased towards the late cold season. Early summer recruits had the lowest survival under warming. Seasonal performance of recruits might be driven by parental environment.