Thermal boundaries of survival: A case study in a marine invertebrate

Understanding the physiological mechanisms that define species' thermal tolerance is critical for predicting the impact of ocean warming on marine biodiversity. We investigated multiple thermal threshold parameters in the Jonah crab, Cancer borealis, integrating laboratory-derived physiological and molecular data with field-based ecological distribution modeling. Crabs were subjected to acute and chronic temperature challenges while monitoring heart rate, ventilation rate, hemolymph oxygenation, motor activity, and lactate accumulation. Gene expression of heat shock proteins (HSP70, HSP90A, HSP90B) and transient receptor potential (TRP) ion channel subfamilies were quantified via qPCR. These physiological and molecular thresholds were compared with generalized additive models (GAMs) relating C. borealis presence to bottom temperatures across the U.S. Northeast Shelf. Heart rate increased steadily to 29 °C before failure, while ventilation showed distinct breakpoints at 14 °C and 25.5 °C. Hemolymph oxygen levels peaked between 7 °C and 20 °C, coinciding with optimal motor activity and minimal lactate accumulation, indicating a transition from aerobic to anaerobic metabolism above 20 °C. Gene expression of HSPs and TRPs exhibited tissue-specific, temperature-dependent trends with high variability. Ecological modeling revealed peak C. borealis occurrence at 11 °C-14 °C with declining probability above 15 °C, matching some experimentally derived physiological optima. Our integrative analysis demonstrates that no single physiological parameter fully defines the species' thermal limits; however, hemolymph oxygenation and motor activity most closely align with field-based distribution patterns. These results are somewhat consistent with the oxygen- and capacity-limited thermal tolerance (OCLTT) framework but highlight the utility of a multi-parameter approach consistent with the Multiple Performances-Multiple Optima (MPMO) concept.