Novel contributions to marine heatwave ecology: Identifying and addressing knowledge gaps in ecological impacts.

As the climate crisis unfolds, marine heatwaves (MHWs), defined as discrete periods of anomalously high seawater temperatures, are emerging as one of the most pervasive threats to marine biodiversity worldwide. From coastal shallow waters to the deep sea, increasingly frequent and intense MHWs are reshaping ocean life at all levels of ecological organisation, undermining ecosystem resilience and compromising the provision of essential ecosystem services to human societies. This growing environmental challenge has rendered a new scientific discipline-marine heatwave ecology-which aims to advance our understanding, forecasting capacity and mitigation of MHW impacts on ecological systems. These priorities are central to marine science and conservation. Yet, despite increasing scientific attention, many critical research questions remain unresolved. In this cross-journal Special Feature, published across the Journal of Animal Ecology, Journal of Ecology and Functional Ecology, we present a collection of 13 studies that address some of the most pressing knowledge gaps in MHW ecology. These studies were conducted across diverse ocean basins and encompass a wide range of marine taxa, such as corals, macroalgae, seagrasses, molluscs, fish and plankton, among others. They span multiple levels of ecological organisation, from individual organisms to entire ecosystems, and employ a variety of methodologies and approaches. Collectively, the contributions to this special feature demonstrate how MHWs erode ecosystem resilience, reveal previously hidden biological and ecological impacts and show that vulnerability not only depends on thermal tolerance but also on environmental context. The studies also explore how MHW effects cascade up and down across levels of ecological organisation and reinforce the importance of applying ecological frameworks to better categorise and understand MHW dynamics. Finally, we have identified remaining knowledge gaps to guide future research, essential to further develop the MHW ecology field and to inform more effective conservation and management strategies. This will be especially urgent in the context of a rapidly warming ocean, where strong and recurrent MHWs are becoming the new normal.