Measuring critical thermal maximum in aquatic ectotherms: a practical guide

Critical thermal limits, commonly quantified as CTmax (maximum) or CTmin (minimum), are core metrics in the thermal biology of aquatic ectotherms. CTmax, in particular, has recently surged in popularity due to its various applications, including understanding and predicting the responses of animals to climate warming. Despite its growing popularity, there is a limited literature aimed at establishing best practices for designing, running, and reporting CTmax experiments. This lack of standardisation and insufficiently detailed reporting in the literature creates challenges when designing CTmax studies or comparing results across studies. Here, we provide a comprehensive, practical guide for designing and conducting experiments to measure critical thermal limits, with an emphasis on CTmax. Our recommendations cover 12 topic areas including apparatus design, masking (blinding), warming rates, endpoints, replication, and reporting. We include diagrams and photos for designing and building critical thermal limit arenas for field or lab applications. We also provide a reporting checklist as a reference for researchers when carrying out experiments and preparing manuscripts. Future studies incorporating critical thermal limits would benefit from transparent reporting of warming/cooling rates (raw data, supplementary graphs) and photo/video evidence showing arena designs and critical thermal limit endpoints. We also provide directions for empirical research that will help further inform the measurement of critical thermal limits, including on biotic factors like stress and digestion, warming/cooling rates, the effects of body mass on heat transfer, and the physiological mechanisms underlying thermal tolerance.