Journal of Experimental Zoology Part A: Ecological and Integrative Physiology

In natural environments two or more abiotic parameters often vary simultaneously, and interactions between covarying parameters frequently result in unpredictable, non-additive biological responses. To better understand the mechanisms and consequences of interactions between multiple stressors it is important to study their effects on both survival and performance. The splashpool copepod Tigriopus californicus tolerates extremely variable abiotic conditions and exhibits a non-additive, antagonistic interaction resulting in higher survival when simultaneously exposed to high salinity and acute heat stress. Here, we investigated T. californicus response in activity and oxygen consumption under simultaneous manipulation of salinity and temperature to identify if this interaction also arises in these sublethal measures of performance. Oxygen consumption and activity rates decreased with increasing assay salinity. Oxygen consumption also sharply increased in response to acute transfer to lower salinities, an effect that was absent upon transfer to higher salinities. Elevated temperature led to reduced rates of activity overall, resulting in no discernible impact of increased temperature on routine metabolic rates. This suggests that swimming activity has a non-negligible effect on copepods metabolic rates and must be accounted for in metabolic studies. Temperature also interacted with assay salinity to affect activity and with acclimation salinity to affect routine metabolic rates upon acute salinity transfer, implying that the sublethal impacts of these co-varying factors are also not predictable from experiments that study them in isolation. Summary StatementTemperature and salinity interact to affect metabolic rate in the copepod Tigriopus californicus, but the stressors individual effects and their interaction are complicated by concurrent changes in activity.

Pacific Oysters (Magallana/Crassostrea gigas) are marine bivalves that are widely cultivated but increasingly experience summer mortality due to interacting stressors. Two major concerns are (1) the rising severity and frequency of marine heat waves and (2) disease outbreaks (e.g., OsHV-1). To better understand how multiple stressors influence oyster resilience and whether stress priming can improve survival, we tested the effects of parental immune challenge on offspring performance. We exposed broodstock to a Poly(I:C) immune challenge, reared their offspring to the seed stage, and assessed survival, growth, and metabolic responses under thermal stress in the lab. Offspring of immune-challenged parents showed higher growth rates during development. Under elevated temperatures, these offspring had higher survival than controls at 40{degrees}C, but lower survival at 42{degrees}C, suggesting thermal limits to priming benefits. Metabolic assays further revealed that at moderately elevated temperature (36{degrees}C), primed offspring had higher metabolic activity, whereas at higher temperature (40{degrees}C), they exhibited lower metabolic activity than controls. This pattern indicates that parental immune challenge may influence offspring metabolic flexibility, potentially enhancing thermal tolerance through an increased capacity for metabolic depression at extreme temperatures. Together, our results highlight cross-generational links between immune priming and thermal tolerance.

From a conservation perspective, it is important to identify when sub-lethal temperatures begin to adversely impact an organism. However, it is unclear whether, during acute exposures, these cellular thresholds occur at similar temperatures to other physiological or behavioural changes. To test this, we estimated temperature preference (15.1 {+/-} 1.1 {degrees}C) using a shuttle box, thermal optima for aerobic scope (10-15 {degrees}C) using respirometry, agitation temperature (22.0 {+/-} 1.4 {degrees}C) as the point where a fish exhibits a behavioural avoidance response and the CTmax (28.2 {+/-} 0.4 {degrees}C) as the upper thermal limit for 1 yr old Brook Trout (Salvelinus fontinalis) acclimated to 10 {degrees}C. We then acutely exposed a different subset of fish to these temperatures and sampled tissues when they reached the target temperature or after 60 min of recovery at 10 {degrees}C. We used qPCR to estimate mRNA transcript levels of genes associated with heat shock proteins, oxidative stress, apoptosis, and inducible transcription factors. A major shift in the transcriptome response occurred near the agitation temperature, which may identify a link between the cellular stress response and the behavioural avoidance response.

Short-term, acute warming events are increasing in frequency across the worlds oceans. For short-lived species like most copepods, these extreme events can occur over both within- and between-generational time scales. Yet, it is unclear whether exposure to acute warming during early life stages of copepods can cause lingering effects on metabolism through development, even after the event has ended. These lingering effects would reduce the amount of energy devoted to growth and affect copepod population dynamics. We exposed nauplii of an ecologically important coastal species, Acartia tonsa, to a 24-hour warming event (control: 18{degrees}C; treatment: 28{degrees}C), and then tracked individual respiration rate, body length, and stage duration through development. As expected, we observed a decrease in massspecific respiration rates as individuals developed. However, exposure to acute warming had no effect on the ontogenetic patterns in per-capita or mass-specific respiration rates, body length, or development time. The lack of these carryover effects through ontogeny suggests within-generational resilience to acute warming in this copepod species.

Photosynthetic animals produce oxygen internally, providing an ideal lens for studying how oxygen dynamics influence thermal sensitivity. The sea slug, Elysia viridis, can retain functional chloroplasts from its food alga Bryopsis plumosa for months, but retention is limited when fed Chaetomorpha sp., limiting potential oxygenic benefits. We fed slugs each alga and exposed them to 17{degrees}C (their current yearly maximum temperature) and 22{degrees}C (the increase predicted for 2100), to examine plasticity in thermal tolerance and changes in oxygen uptake when fed and starving. We also examined slugs under increased illumination to examine a potential tradeoff between increased oxygen production, and a faster rate of chloroplast degradation. Following exposure to these conditions, we performed ramping trials, subjecting them to acute thermal stress to determine their thermal tolerance. We also measured oxygen uptake before and after ramping. We observed increases in thermal tolerance for specimens exposed to 22{degrees}C, indicating they acclimated to temperatures higher than they naturally experience. Fed slugs exhibited higher rates of oxygen consumption before exposure to acute thermal stress, and suppressed their oxygen uptake more after it, than starved slugs. Under higher light, slugs exhibited improved thermal tolerance, possibly because increased oxygen production alleviated host oxygen limitation. Accordingly, this advantage disappeared later in the starvation period when photosynthesis ceased due to chloroplast digestion. In conclusion, E. viridis can suppress metabolism to cope with heat waves, however, starvation influences a slugs thermal tolerance and oxygen uptake, so continuous access to algal food for chloroplast retention is critical when facing thermal stress. Summary StatementOxygen has been implicated in determining an ectotherms thermal sensitivity. Examining photosynthetic (and therefore oxygen-producing) sea slugs under various conditions helps elucidate how oxygen and other factors impact thermal tolerance.

Neurobiological research focuses on a small number of model organisms, broadening the pool of animals used in research may lead to important insights into the evolution of nervous systems. The ctenophore is emerging as a promising model, but we are currently lacking an understanding into the relationship between behaviour and environment which is in part due to a lack of a standardised long-term laboratory husbandry system. We established a collection and husbandry system for wild caught Pleurobrachia pileus. We examined the behavioural profile of the animals over time in this controlled environment. We could reliably catch them on a seasonal basis, and we could keep the animals alive in our specialised aquarium system for months at a time. P. pileus spends most of the time in an inactive drifting state which is interspersed with periods of one of 5 active behaviours. The most common active behaviours are tentacle resetting and feeding. The longest duration behaviours include swimming up or down. Time of day does not appear to alter their behavioural profile. Gaining a better understanding of the behaviour of these animals has important implications for systems and evolutionary neuroscience.

Aquatic ectotherms are vulnerable to heatwave-induced physiological stress, which arises from increased energy demands and reduced dissolved oxygen content in warmer waters. Understanding thermal physiology is critical for predicting how commercially and ecologically important populations could be affected by the increasing risk of rising temperatures. Heatwave risk assessments often examine extremities of time scales: immediate impacts or long-term consequences. However, little is known about how consistently increasing mid-term thermal stress shapes aerobic performance in commercially important species such as Atlantic salmon (Salmo salar), which may face heat stress in rivers, especially at the juvenile life stage. By measuring how salmon juveniles manage their aerobic capacity at 16, 19 and 22{degrees}C using intermittent respirometry, we test if their thermal performance curve shows a decline at temperatures commonly occurring during heatwaves. Whole-animal metabolism was measured from control individuals kept at 16 {degrees}C before and after the heatwave, and after 4-5 days exposure at 19 and 22{degrees}C during the heatwave. We show that standard metabolic rate increases with temperature, but maximum metabolic rate and aerobic scope do not change between these temperatures. These findings suggest that juvenile Atlantic salmon may have limited capacity to increase aerobic performance during moderate heatwaves, leaving them vulnerable to cumulative effects of oxygen limitation to vital functions such as growth and stress responses. As climate change intensifies, incorporating thermal performance curves into conservation strategies can be utilized for predicting population resilience and informing effective management. Lay SummaryIn juvenile Atlantic salmon, standard metabolic rate increases but there is no difference in maximum metabolic rate and aerobic scope with increasing temperatures. Considering the increased metabolic costs of activities at higher temperatures, our results show that juvenile salmon are vulnerable to deteriorating performance at temperatures commonly experienced in present-day heatwaves.

Correlations between variation in opsin expression and variation in vision are often assumed but rarely tested. We exposed starry flounder (Platichthys stellatus) to either broad spectrum sunlight or green-filtered light in outdoor aquaria for seven weeks and then combined digital-PCR and camouflage experiments to test two hypotheses: i) short-wavelength sensitive opsin expression decreases in a green light environment, and ii) if observed, this change in opsin expression influences colour vision as estimated using a camouflage-based behavioural assay. Of the eight visual opsins measured, Sws1 (UV sensitive) and Sws2B (blue sensitive) expression was significantly lower in fish exposed to green light. However, opsin expression in fish transferred to an arena illuminated with white LED light for three hours after the green light treatment did not differ from broad spectrum controls. Changes in opsin expression in response to artificial light environments have been reported before, but rapid changes over three hours rather than days or weeks is unprecedented. We did not observe a significant difference in a flounders camouflage response based on light environment, although broad spectrum fish increased and green-filter fish decreased the pattern contrast when on the blue-green substrate, and this difference approached significance. This pattern is intriguing considering green-filter fish expressed fewer UV and blue opsins and we recommend increased statistical power for future experiments. Together, our results show that starry flounder opsin expression changes rapidly in response to changes in light environment, however, there is no apparent effect on their visually mediated camouflage.

The physiology of tropical birds is poorly understood, particularly in how it relates to local climate and changes between seasons. This is particularly true of tropical montane species, which may have sensitive thermal tolerances to local microclimates. We studied metabolic rates (using open flow respirometry), body mass and haemoglobin concentrations of five sedentary Mesoamerican songbirds between the summer and winter at two elevations (1550 m and 1950 m, respectively). We asked whether there were uniform seasonal shifts in physiological traits across species, and whether higher elevation species displayed evidence for cold tolerance. Seasonal shifts in metabolic rates differed between the three species for which data were collected. Basal metabolic rates in one species - black-headed nightingalethrushes Catharus mexicanus - were up-regulated in summer (~19% increase of winter metabolism), however two other species displayed no seasonal regulation. No species exhibited shifts in haemoglobin concentrations across season or across elevation, whereas body mass in two species was significantly higher in the summer. One species restricted to higher elevations - ruddy-capped nightingale-thrushes C. frantzii - displayed physiological traits indicative of cold-tolerance. Although only summer data were available for this species (C. frantzii), metabolic rates were constant across temperatures tested (5-34{degrees}C) and haemoglobin concentrations were significantly higher compared to the other four species. Our results suggest that seasonal acclimatisation in physiological traits is variable between species and appear unrelated to changes in local climate. As such, the distinct physiological traits observed in ruddy-capped nightingale-thrushes likely relate to historic isolation and conserved physiological traits rather than contemporary climatic adaption.

Climate warming alters the thermal environment experienced by ectotherms, whose physiological performance and fitness are constrained by temperature. Early life stages are often the temperature-sensitive phases of the life cycle, with potential consequences for population persistence, particularly in freshwater stenotherms such as the Arctic charr (Salvelinus alpinus). The persistence of populations will partly depend on the adaptive potential of critical life stages to environmental changes. In this study, we used a common garden approach to compare the response and phenotypic plasticity of four charr populations to warmer conditions. These populations inhabit thermally contrasted lakes and differ in origin (native/introduced) and management history. We reared embryos at either an optimal (5{degrees}C) temperature for larval development or a warmer but realistic (8.5 {degrees}C) temperature. We tested adaptive divergence among populations in four traits (survival, incubation duration, body length and yolk sac volume), using Qst - Fst comparisons. We report negative effects of temperature on body size, survival and earlier hatching. Thermal reaction norms differed among populations, indicating adaptive divergence. Contrary to expectations, populations originating from warmer environments did not consistently exhibit higher trait values under elevated temperatures. In contrast, the unmanaged and colder high-altitude population exhibited higher survival rates and lower yolk reserves for a given size under heat stress than the other populations. Our results suggested that evolutionary trajectories specific to each population are shaped by factors related to the populations history, including introductions, demographic fluctuations and long-term repopulation practices, which can jointly influence the potential for adaptation to heat stress.

Air temperatures are warming at faster rates than ocean temperatures, and this "land-sea warming contrast" may create reprieves from thermal stress by providing cool underwater refugia during extreme heat events. Here we tested the impacts of the "land-sea warming contrast" on physiology (metabolism) and behaviour (feeding) in the juvenile life stage of a keystone intertidal predator, Pisaster ochraceus, by experimentally manipulating air ([~]20, 25, 30) and water ([~]15, 20) temperatures (at independent rates) representing early summer, late summer, and heatwave conditions in Barkley Sound (British Columbia, Canada). We further made observations of air temperatures, sea surface temperatures, and Pisaster moribundity at our study location to support interpretation of our results. We predicted metabolism and feeding would increase with early and late summer temperatures, but decrease during heatwave conditions as animals surpass their thermal optimum. We observed the greatest mortality and lowest feeding in juvenile Pisaster exposed to cool ocean temperatures ([~]15) and high aerial temperatures typical of extreme heat events ([~]30). Feeding rates increased with heat stress duration, indicating animals may be compensating for elevated metabolism. Metabolic rates did not differ between air temperatures, but oxygen consumption was higher in animals with access to mussels than for Pisaster that were fasted. The highest levels of experimental and field moribundity were observed in August, indicating Pisaster may have accumulated physiological stress damage following elevated air and ocean temperatures throughout the summer. Our research implicates shifts in community dynamics due to the loss of this keystone species as air temperatures warm. Summary StatementCooler ocean temperatures, rather than creating thermal refugia, may cause physiological stress for juvenile Pisaster ochraceus exposed to warm air during low tide.

Life history theory suggests that aging is one of the costs of reproduction. Accordingly, a higher reproductive allocation is expected to increase the deterioration of both the somatic and the germinal lines through enhanced telomere attrition. In most species, males reproductive allocation mainly regards traits that increase mating and fertilization success, i.e. sexually selected traits. In the current study, we tested the hypothesis that a higher investment in sexually selected traits is associated with a reduced telomere length in the guppy (Poecilia reticulata), an ectotherm species characterized by strong pre- and postcopulatory sexual selection. We first measured telomere length in both the soma and the sperm over the course of guppys lifespan to see if there was any variation in telomere length associated with age. Secondly, we investigated whether a greater expression of pre- and postcopulatory sexually selected traits is linked to shorter telomere length in both the somatic and the sperm germinal lines, and in young and old males. We found that telomeres lengthened with age in the somatic tissue, but there was no age-dependent variation in telomere length in the sperm cells. Telomere length in guppies was significantly and negatively correlated with sperm production in both tissues and life stages considered in this study. Our findings indicate that telomere erosion in male guppies is more strongly associated with their reproductive investment (sperm production) rather than their age, suggesting a trade-off between reproduction and maintenance is occurring at each stage of males life in this species.

Heat tolerance is critical for ectotherms facing environmental temperature variability, yet how it varies across life stages, and whether trade-offs occur between temperature-induced developmental plasticity and heat tolerance, remain unclear, particularly in organisms undergoing metamorphosis which represent 95% of all animal species. We examined how early-life thermal conditions shape growth, development, survival, acclimation capacity, heat tolerance, and energy allocation across ontogeny in the African clawed frog (Xenopus laevis), reared at six constant temperatures (17-32{degrees}C). We tested whether higher developmental temperatures generate trade-offs between accelerated growth and heat tolerance, and the consequences for post-metamorphic resilience to extreme heat. Rearing at 32{degrees}C was lethal before metamorphosis. At non-lethal warm temperatures (17-29{degrees}C), larvae and juveniles simultaneously accelerated development, maintained growth, and enhanced heat tolerance. However, juveniles reared at 29{degrees}C showed reduced survival, elevated corticosterone responses to acute stress, and diminished acclimation capacity, indicating increased energetic demands and constrained metabolic flexibility. These findings show that amphibians can integrate developmental plasticity with plastic adjustments in heat tolerance, but that such strategies incur cumulative physiological costs. By adopting an across-life-stage approach, our study highlights energy-allocation constraints that may limit population persistence under climate warming in species with complex life cycles.

O_LIGlobal warming is reducing prey availability in many aquatic systems, raising questions about the combined effects of higher temperatures and lower food availability on fish life histories and reproductive output. C_LIO_LIIn ectotherms, higher temperatures accelerate growth and promote an earlier onset of reproduction. However, when fish have less food during development, resource depletion might constrain these temperature-driven processes. C_LIO_LIWe manipulated water temperature (24 or 28{degrees}C) and early-life food availability (control or restricted) for female guppies (Poecilia reticulata). We measured how both factors affected key life history traits (growth, reproduction, survival, self-maintenance). C_LIO_LIHigher temperature significantly affected female life histories. Females at 28{degrees}C matured at a larger size, but then grew more slowly and produced fewer, smaller offspring than females at 24{degrees}C. The effect of temperature on reproduction persisted even after controlling for body size, suggesting there was a shift in the fecundity-size relationship. C_LIO_LIAdult mortality was greater at 28{degrees}C. Higher temperature also resulted in a longer gut, potentially enhancing resource acquisition, but a higher temperature did not affect immunity or telomere length of the surviving females. C_LIO_LIEarly-life food shortage affected very few traits, except for a weak interaction with temperature that affected total fecundity. At 28{degrees}C, females that experienced early-life food restriction produced fewer offspring than females with continual food supply. No such diet effect occurred at 24{degrees}C. C_LIO_LIOur results suggest that tropical fish may be severely impacted by increased temperatures (i.e., decreased reproduction with increased morality), but are likely to be resilient to brief periods of food limitations during early development. C_LIO_LIInterestingly, early-life food shortage caused a reduction in total offspring number but only at 28{degrees}C, suggesting that global prey decline might exacerbate the negative effects of a warming climate on stock recruitment of tropical fish. C_LI

Mounting evidence suggests that temperature seasonality plays a pivotal role in shaping the thermal biology of ectotherms. However, we still have a limited understanding of how amphibians maintain thermal balance in the face of varying temperatures, especially in fossorial species. Due to thermal buffering underground, theory predicts relaxed selection pressure over thermoregulation in fossorial ectotherms. As a result, fossorial ectotherms typically show low thermoregulatory precision and low evidence of thermotactic behaviours when tested in laboratory thermal gradients. In this study, we evaluated how temperature selection (Tsel) and behavioural thermoregulation differed between seasons in the fossorial Spotted Salamander (Ambystoma maculatum). By comparing thermoregulatory parameters between the activity and overwintering seasons, we provide evidence that A. maculatum engages in active behavioural thermoregulation despite its fossorial habit. In both seasons, we found Tsel to be consistently offset higher than prevailing thermal conditions. Thermoregulation differed between seasons, with salamanders having higher Tsel and showing greater evidence of thermophilic behaviours in the active season compared to the overwintering season. Our study highlights that the combination of behavioural and thermal biology measurements is a necessary step to better understand the mechanisms that underlie body temperature control in amphibians. Ultimately, our study provides a broader understanding of thermoregulation in amphibians, particularly in the context of behavioural responses to seasonality in fossorial species. Summary statementBy comparing thermoregulatory parameters between seasons, we demonstrate that the Spotted Salamander engages in active behavioural thermoregulation despite being fossorial.

The Mediterranean Sea is undergoing significant environmental changes due to climate change and the introduction of non-native species, impacting biodiversity and ecosystem dynamics. Ocypode cursor (tufted ghost crab) has expanded its range, likely in response to changing thermal conditions.This study investigates the metabolic scaling of O. cursor across three active months (July, August, and September) under air- and water-breathing conditions during both daytime and nighttime. The results revealed seasonal variation in metabolic scaling, with significant differences in intercept values and scaling slopes among months. During daytime air-breathing conditions, metabolic rates increased in August and September regardless of body size, while at night in August, larger crabs exhibited higher metabolic rates. Under water-breathing conditions, smaller crabs showed greater metabolic responses in August during the day, whereas nighttime activity remained stable across months. These results indicate that temperature, diel cycle, and respiratory mode shape metabolic scaling as a comparative indicator of thermal performance rather than a direct proxy for fitness. Understanding these responses provides insight into O. cursors ecological flexibility and contributes to assessing ectotherm responses to ongoing Mediterranean warming.

1.The aim of this study was to investigate the effect of ocean acidification (OA) and warming (OW) as well as the transgenerational effect of OA on larval and juvenile growth and metabolism of a large economically important fish species with a long generation time. Therefore we incubated European sea bass from Brittany (France) for two generations (>5 years in total) under current and predicted OA conditions (PCO2: 650 and 1700 {micro}atm). In the F1 generation both OA condition were crossed with OW (temperature: 15-18{degrees}C and 20-23{degrees}C). We found that OA alone did not affect larval or juvenile growth and OW increased developmental time and growth rates, but OAW decreased larval size at metamorphosis. Larval routine metabolic rate (RMR) and juvenile standard metabolic rate (SMR) were significantly lower in cold compared to warm conditioned fish and also lower in F0 compared to F1 fish. We did not find any effect of OA on RMR or SMR. Juvenile PO2crit was not affected by OA, OW or OAW in both generations. We discuss the potential underlying mechanisms resulting in beneficial effects of OW on F1 larval growth and RMR and in resilience of F0 and F1 larvae and juveniles to OA, but on the other hand resulting in vulnerability of F1, but not F0 larvae to OAW.. With regard to the ecological perspective, we conclude that recruitment of larvae and early juveniles to nursery areas might decrease under OAW conditions but individuals reaching juvenile phase might benefit from increased performance at higher temperatures. Summary statementWe found that OA did not affect developmental time, growth, RMR and SMR, while OW increased these traits. OAW decreased larval size at metamorphosis. We discuss underlying mechanisms and the ecological perspective resulting from these results and conclude that recruitment to nursery areas might decrease under OAW conditions but individuals reaching juvenile phase might benefit from increased performance at higher temperatures in Atlantic waters.

Climate change causes warming, decreased O2, and increased CO2 in marine systems and responses of organisms will depend on interactive effects between these factors. We provide the first experimental assessment of the interactive effects of warming (14 to 22{degrees}C), reduced O2 ([~]3 - 21 kPa O2), and increased CO2 ([~]400 or [~]1000 {micro}atm ambient CO2) on four indicators of aerobic performance (standard metabolic rate, SMR, maximum metabolic rate, MMR, aerobic scope, and hypoxia tolerance, O2crit), blood chemistry, and O2 transport (P50) of a marine fish, the European sea bass (Dicentrarchus labrax). Warming increased SMR and O2crit (i.e. reduced hypoxia tolerance) as well as MMR in normoxia but there was an interactive effect with O2 so that hypoxia caused larger reductions in MMR and aerobic scope at higher temperatures. Increasing CO2 had minimal effects on SMR, MMR and O2crit and did not show interactive effects with temperature or O2 for any measured variables. Aerobic performance was not linked to changes in blood chemistry or P50. Despite lack of effects of CO2 on aerobic performance, increased CO2 induced 30% mortality of fish exercised in low O2 at 22{degrees}C indicating important threshold effects independent of aerobic performance. Overall, our results show temperature and O2, but not CO2, interact to affect aerobic performance of sea bass, disagreeing with predictions of the oxygen- and capacity-limited thermal tolerance hypothesis.

Increasing ocean temperatures are causing dysbiosis between coral hosts and their symbionts. Previous work suggests that coral host gene expression responds more strongly to environmental stress compared to their intracellular symbionts; however, the causes and consequences of this phenomenon remain untested. We hypothesized that symbionts are less responsive because hosts modulate symbiont environments to buffer stress. To test this hypothesis, we leveraged the facultative symbiosis between the scleractinian coral Oculina arbuscula and its symbiont Breviolum psygmophilum to characterize gene expression responses of both symbiotic partners in and ex hospite under thermal challenges. To characterize host and in hospite symbiont responses, symbiotic and aposymbiotic O. arbuscula were exposed to three treatments: 1) control (18{degrees}C), 2) heat (32{degrees}C), and 3) cold (6{degrees}C). This experiment was replicated with B. psygmophilum cultured from O. arbuscula to characterize ex hospite symbiont responses. Both thermal challenges elicited classic environmental stress responses (ESRs) in O. arbuscula regardless of symbiotic state, with hosts responding more strongly to cold challenge. Hosts also exhibited stronger responses than in hospite symbionts. In and ex hospite B. psygmophilum both downregulated genes associated with photosynthesis under thermal challenge; however, ex hospite symbionts exhibited greater gene expression plasticity and differential expression of genes associated with ESRs. Taken together, these findings suggest that O. arbuscula hosts may buffer environments of B. psygmophilum symbionts; however, we outline the future work needed to confirm this hypothesis.

Global climate change is increasing thermal variability in coastal marine environments and the frequency, intensity, and duration of marine heatwaves. At the same time, nutritional resources are being altered by anthropogenic environmental changes. Marine ectotherms often cope with changes in temperature through physiological acclimation, which can take several weeks to occur and is a nutritionally demanding process. Here, we tested the hypothesis that different ecologically relevant diets (omnivorous, herbivorous, carnivorous) can impact thermal acclimation rate and capacity, using a temperate omnivorous fish as a model (opaleye; Girella nigricans). We measured acute thermal performance curves for maximum heart rate because cardiac function has been observed to set upper thermal limits in ectotherms. Opaleye acclimated rapidly after warming, but their thermal limits and acclimation rate were not affected by diet. However, the fishs acclimation capacity for maximum heart rate was sensitive to diet, with fish in the herbivorous treatment displaying the smallest change in heart rate throughout acclimation. Mechanistically, ventricle fatty acid composition differed with diet treatment and was significantly related to cardiac performance in ways consistent with homoviscous adaptation. Our results suggest that diet is an important, but often overlooked, determinant of thermal performance in ectotherms on environmentally relevant timescales.