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Rapid ocean warming may alter habitat suitability and population fitness for marine ectotherms. Susceptibility to thermal perturbations will depend in part on plasticity of a species' upper thermal limits of performance (CTmax). However, we currently lack data regarding CTmax plasticity for several major marine taxa, including nudibranch mollusks, thus limiting predictive responses to habitat warming for these species. In order to determine relative sensitivity to future warming, we investigated heat tolerance limits (CTmax), heat tolerance plasticity (acclimation response ratio), thermal safety margins, temperature sensitivity of metabolism, and metabolic cost of heat shock in nine species of nudibranchs collected across a thermal gradient along the northeastern Pacific coast of California and held at ambient and elevated temperature for thermal acclimation. Heat tolerance differed significantly among species, ranging from 25.4 degrees +/- 0.5 degrees Cto 32.2 degrees +/- 1.8 degrees C(xoer bar +/- SD ), but did not ary with collection site within species. Thermal plasticity was generally high (0.52 +/- 0.06 , x oer bar +/- SE ) and was strongly negatiely correlated with CTmax in accordance with the trade-off hypothesis of thermal adaptation. Metabolic costs of thermal challenge were low, with no significant alteration in respiration rate of any species 1 h after exposure to acute heat shock. Thermal safety margins, calculated against maximum habitat temperatures, were negatie for nearly all species examined (-8.5 degrees +/- 5.3 degrees C , x oer bar +/- CI [confidence interal]). From these data, we conclude that warm adaptation in intertidal nudibranchs constrains plastic responses to acute thermal challenge and that southern warm-adapted species are likely most ulnerable to future warming.