Behavioral metabolic suppression confounds thermal performance estimates and climate vulnerability assessments in a marine ectotherm

Abstract

O_LIThermal performance curves (TPCs) predict species vulnerability to climate change, but standard respirometry assumes that measured oxygen consumption reflects physiological state. Sessile invertebrates that retract their tentacles and contract under thermal stress violate this assumption, with unmeasured consequences for thermal limit estimates. C_LIO_LIWe tested this behavioral confound in an undescribed cold-water intertidal anemone (Urticina sp.) in the Northwest Atlantic by integrating a negative binomial encounter-rate regression, a maximum entropy species distribution model (both from effort-corrected iNaturalist data), and closed-chamber respirometry across seven temperatures (1-30{degrees}C). C_LIO_LIThe strongest distributional predictors were cloud cover and coastal urbanization, with a weaker association with winter minimum SST; direct evidence for warm-edge thermal limitation came from the experiment. Anemone expansion state was variable and without a clear trend across the coldest treatments, declining above 20{degrees}C, and collapsing at the 30{degrees}C treatment, which proved lethal. C_LIO_LIStandard TPC models extrapolated the thermal maximum far beyond the lethal bracket. A Bayesian multiplicative model that separated physiology from behavior showed that physiology continued to track temperature while expansion state declined above 20{degrees}C; a fully expanded anemone respired about twice as fast as a fully closed one at the same temperature. The decline in measured respiration is therefore both behavioral and physiological, and disentangling the two requires recording expansion state alongside oxygen consumption. C_LIO_LIBecause a closed anemone cannot feed or exchange gases, the ecologically relevant thermal limit is the temperature at which the animal can no longer maintain its normal expanded posture, not a curve-fitted thermal maximum. That behavioral threshold leaves warm-edge populations within a few degrees of functional thermal failure. C_LIO_LIFuture thermal physiology studies of organisms capable of modulating oxygen consumption through behavior should incorporate quantitative behavioral covariates to separate physiological from behavioral components of the metabolic response. C_LI ResumeO_LILes courbes de performance thermique (CPT) predisent la vulnerabilite des especes au changement climatique, mais la respirometrie classique postule que la consommation doxygene mesuree reflete letat physiologique. Les invertebres sessiles qui retractent leurs tentacules et se contractent sous stress thermique ne satisfont pas cette hypothese, avec des consequences non quantifiees sur lestimation des limites thermiques. C_LIO_LINous avons teste ce facteur de confusion comportemental chez une anemone intertidale deau froide non decrite (Urticina sp.) de lAtlantique Nord-Ouest, en combinant une regression binomiale negative du taux de rencontre, un modele de distribution despeces a maximum dentropie (tous deux sur donnees iNaturalist corrigees de leffort) et une respirometrie en chambre close a sept temperatures (1 a 30 {degrees}C). C_LIO_LILes principaux predicteurs etaient la couverture nuageuse et lurbanisation cotiere, lassociation au minimum hivernal de temperature de surface de la mer etant plus faible. La preuve directe dune limitation thermique en bordure chaude provient de lexperience. Letat dexpansion variait sans tendance nette aux traitements les plus froids, diminuait au-dela de 20 {degrees}C et seffondrait a 30 {degrees}C, traitement qui sest revele letal. C_LIO_LILes CPT classiques ont extrapole le maximum thermique bien au-dela de lintervalle letal. Un modele bayesien multiplicatif separant physiologie et comportement a montre que la physiologie suivait toujours la temperature alors que letat dexpansion diminuait au-dela de 20 {degrees}C; a temperature egale, une anemone pleinement epanouie respirait environ deux fois plus quune anemone pleinement fermee. La baisse mesuree est donc a la fois comportementale et physiologique, et les dissocier exige denregistrer letat dexpansion avec la consommation doxygene. C_LIO_LIUne anemone fermee ne pouvant ni se nourrir ni assurer ses echanges gazeux, la limite thermique ecologiquement pertinente est la temperature a laquelle lanimal ne peut plus maintenir son expansion normale, et non un maximum issu dun ajustement de courbe. Ce seuil comportemental place les populations de bordure chaude a quelques degres de la defaillance thermique fonctionnelle. C_LIO_LILes futures etudes de physiologie thermique sur des organismes capables de moduler leur consommation doxygene par leur comportement devraient integrer des covariables comportementales quantitatives pour separer les composantes physiologique et comportementale de la reponse metabolique. C_LI