"Body size is a primary determinant of key life-history traits such as fecundity, longevity and dispersal, thus governing the way organisms interact with each other and their environment. Given the large consequences of body size there is great interest in why do organisms differ in size.Temperature is a major corollary of body size. At higher temperatures, individuals grow faster but to a smaller final size. This temperature size rule (hereafter termed TSR) has been documented widely and is one of the strongest patterns in ecology, yet it remains poorly understood. It is a life-history puzzle, because altered resource supply (e.g. food) usually has parallel effects on both growth rate and adult body size. During the life-history, resources can be allocated towards either maturation or growth, giving rise to the well-known trade-off between early maturation at a small size or late maturation at a larger size. One explanation for the TSR could be that higher temperatures shift this trade-off towards early maturation. Another explanation is related not to energy expenditure, but its acquisition. Oxygen is used to metabolize food and generate energy and is thus central to an organisms’ scope for growth. Breathing underwater is challenging, as the availability of oxygen is orders of magnitude lower than in air. Respiratory surfaces are relatively smaller in larger organisms (lower surface area to volume ratio). Hence, oxygen could be the key resource limiting growth in aquatic ectotherms.The SCOPE project will test the hypothesis that for aquatic ectotherms, oxygen limitation underlies temperature effects on growth and size at maturity. A key aim is to integrate life-history theory and respiration physiology to understand and predict the ecological consequences of temperature, through its effect on metabolism, growth, reproduction and survival in aquatic ectotherms."