• As climate change accelerates, increasing ocean temperatures and altered herbivory pressure are reshaping temperate marine ecosystems dominated by macrophyte foundation species. Despite the importance of these processes, it remains unclear how reduced above-ground biomass, due to anticipated in-creased herbivory, will affect the thermal performance of macrophytes. • We simulated herbivory by clipping the leaf length of two temperate seagrass species, Posidonia australis and Heterozostera tasmanica, and grew them at temperatures spanning their thermal range (6-32 °C). We assessed the effects of leaf size on thermal performance by quantifying growth, photosynthetic rates, leaf nutrient content and pigment concentrations. • Responses to clipping treatment and growing temperature were species-specific. Highly clipped P. australis showed higher photosynthetic rates and nutrient concentrations at 32 °C, relative to low-clipped and control plants, while growth did not differ between clipping treatments and survival was high across treatments. In H. tasmanica, clipping lowered optimal growth temperatures, and survival declined sharply at 32 °C across treatments. • Our results suggest that smaller leaf size can increase thermal resilience in P. australis, whereas high herbivory of H. tasmanica is likely to increase its sensitivity to thermal stress. Increased herbivory pressure under climate change may therefore have positive implications for some seagrasses, highlighting the importance of considering species-specific responses when predicting the future resilience of seagrass ecosystems.

Phenotypic plasticity can allow individuals to compensate for potentially negative changes in their thermal environment. It is important, therefore, to understand the impacts of changes in both mean and fluctuations in temperature on plastic responses. Our aim was to establish the current state-of-knowledge regarding the influence of thermal variability on the capacity for phenotypic plasticity in ectothermic vertebrates and invertebrates. We conducted a quantitative synthesis of 44 studies (212 effect sizes across 40 species) to compare the effects of constant and fluctuating temperatures with the same mean on plasticity in biological responses, across different ecosystems (terrestrial and aquatic) and type of phenotypic plasticity (acclimation and developmental plasticity). We found that most studies implemented diel temperature fluctuations, and that phenotypic plasticity does not differ between constant and fluctuating thermal environments. We conclude that plasticity and its attendant compensation for thermal variability is driven by changes in longer-term mean temperatures.

Transmission of environmentally induced variation from parents to offspring via inter- and trans-generational phenotypic plasticity is a major source of phenotypic variation across generations. In contrast to the slower process of genetic adaptation, epigenetic mechanisms such as altered DNA methylation patterns can change phenotypes between generations and may buffer offspring from environmental stress. Epigenetics thereby has the potential to promote future population persistence. However, epigenetic changes are not always beneficial. Similar to the adaptationist programme described in the classic paper by Gould and Lewontin (1979), a Panglossian approach has been adopted in recent years: transgenerational phenotypic plasticity is often assumed to be beneficial and to enhance offspring fitness. Yet, epigenetic transfer of information can also induce offspring phenotypes that are neutral or detrimental. Here, we challenge the implicit assumption that shifts in offspring phenotype in response to changed parent environments are necessarily adaptive. We instead advocate for the concept of “epigenetic drift” as the most parsimonious null-hypothesis. We propose a quantitative genetics framework to assess the fitness consequences of inter- and transgenerational phenotypic plasticity. We use worked examples to demonstrate how selection analysis can provide standardised estimates of selection to assess the fitness benefits of the transmission of epigenetic information across generations.