When organisms are unable to feed ad libitum they may be more susceptible to negative effects of environmental stressors such as ocean acidification and warming (OAW). We reared sea bass (Dicentrarchus labrax) at 15 or 20 °C and at ambient or high Pco 2 (650 versus 1750 µatm Pco 2 ; pH = 8.1 or 7.6) at ad libitum feeding and observed no discernible effect of Pco 2 on the size-at-age of juveniles after 277 (20 °C) and 367 (15 °C) days. Feeding trials were then conducted including a restricted ration (25% ad libitum). At 15 °C, growth rate increased with ration but was unaffected by Pco 2. At 20 °C, acidification and warming acted antagonistically and low feeding level enhanced Pco 2 effects. Differences in growth were not merely a consequence of lower food intake but also linked to changes in digestive efficiency. The specific activity of digestive enzymes (amylase, trypsin, phosphatase alkaline and aminopeptidase N) at 20 °C was lower at the higher Pco 2 level. Our study highlights the importance of incorporating restricted feeding into experimental designs examining OAW and suggests that ad libitum feeding used in the majority of the studies to date may not have been suitable to detect impacts of ecological significance.

Highlights ► Exploration of energy density sources of variability: species, season, region, size. ► Relationships between dry mass content and ED are strong but species specific. ► Larger length, mass and ED at age in the English Channel than in the Bay of Biscay. ► Sardine display larger energy reserves than anchovy. ► Larger reserves are likely in link with larger spawning or maintenance costs. ► A strong scaling of ED with size with a dome shape pattern for sardine. ► Decrease of ED with size is discussed in link with feeding and spawning behaviours.

Ocean acidification and ocean warming (OAW) are simultaneously occurring and could pose ecological challenges to marine life, particularly early life stages of fish that, although they are internal calcifiers, may have poorly developed acid-base regulation. This study assessed the effect of projected OAW on key fitness traits (growth, development and swimming ability) in European sea bass ( Dicentrarchus labrax ) larvae and juveniles. Starting at 2 days post-hatch (dph), larvae were exposed to one of three levels of P CO 2 (650, 1150, 1700 μatm; pH 8.0, 7.8, 7.6) at either a cold (15°C) or warm (20°C) temperature. Growth rate, development stage and critical swimming speed (U crit ) were repeatedly measured as sea bass grew from 0.6 to ~10.0 (cold) or ~14.0 (warm) cm body length. Exposure to different levels of P CO 2 had no significant effect on growth, development or U crit of larvae and juveniles. At the warmer temperature, larvae displayed faster growth and deeper bodies. Notochord flexion occurred at 0.8 and 1.2 cm and metamorphosis was completed at an age of ~45 and ~60 days post-hatch for sea bass in the warm and cold treatments, respectively. Swimming performance increased rapidly with larval development but better swimmers were observed in the cold treatment, reflecting a potential trade-off between fast grow and swimming ability. A comparison of the results of this and other studies on marine fish indicates that the effects of OAW on the growth, development and swimming ability of early life stages are species-specific and that generalizing the impacts of climate-driven warming or ocean acidification is not warranted.

statement: Mitochondria of juvenile European sea bass hearts are impaired by acute 17 warming, but seem to benefit from acclimation to warmer temperatures, they are only marginally 18 impacted by ocean acidification. 19 20 CII -Complex II of the electron transport system 48 CIV -Complex IV of the electron transport system 49 dph -Days post hatch 50 IPCC -Intergovernmental Panel on Climate Change 51 Leak-CI -State 4 respiration of complex I 52 MS-222 -Tricaine methane sulfonate 53 OA -Ocean acidification 54 OAW -Ocean acidification and warming 55 OW -Ocean warming 56 OXPHOS -Full state 3 respiration of the electron transport system 57 OXPHOS-CI -State 3 respiration of complex I 58 OXPHOS-CII -State 3 respiration of complex II 59 OXPHOS-cytC -State 3 respiration after addition of cytochrome c 60 PCO 2 -Partial pressure of CO 2 61 RCP -Representative concentration pathway 62 RCR o -Respiratory control ratio 63 State 4 o -State 4 respiration after addition of oligomycin 64 W -Warm life conditioned group 65 66 the other hand, compensational processes after long-term and developmental thermal acclimation 130 could include changes in mitochondrial membrane properties, which would reduce leak respiration 131rates and consequently restore RCR. Additionally, we wanted to fathom the capacities of seabass 132 mitochondria to cope with OA, especially when combined with OW. We hypothesized that the 133 changes in intracellular PCO 2 and bicarbonate concentration elicited by OA would affect 134 mitochondrial metabolism, putting further pressure on the cellular energy metabolism. 135 136 157 conditions were applied directly after division into the experimental tanks. Starting at 7 dph (mouth 158 opening), larvae were fed with live artemia, hatched from High HUFA Premium artemia cysts (Catvis, 159 AE 's-Hertogenbosch, Netherlands). Until 33 dph the artemia were fed to the larvae 24h after rearing 160 cysts in sea water, afterwards the artemia nauplii themselves were fed with cod liver oil and dry 161 yeast after 24 h and fed to the larvae after 48 h. The artemia were transferred to the larval rearing 162 tanks from two storage tanks (one for each temperature) with peristaltic pumps, their concentration 163 in the tanks was maintained high during the day, to allow ad libitum feeding, excess artemia left the 164 tank via the waste water outflow. The 15 h photoperiod in the larval rearing room lasted from 7 am Materials and Methods

The world's oceans are acidifying and warming as a result of increasing atmospheric CO 2 concentrations. The thermal tolerance of fish greatly depends on the cardiovascular ability to supply the tissues with oxygen. The highly oxygen-dependent heart mitochondria thus might play a key role in shaping an organism's tolerance to temperature. The present study aimed to investigate the effects of acute and chronic warming on the respiratory capacity of European sea bass (Dicentrarchus labrax L.) heart mitochondria. We hypothesized that acute warming would impair mitochondrial respiratory capacity, but be compensated for by life-time conditioning. Increasing P CO2 may additionally cause shifts in metabolic pathways by inhibiting several enzymes of the cellular energy metabolism. Among other shifts in metabolic pathways, acute warming of heart mitochondria of cold lifeconditioned fish increased leak respiration rate, suggesting a lower aerobic capacity to synthesize ATP with acute warming. However, thermal conditioning increased mitochondrial functionality, e.g. higher respiratory control ratios in heart mitochondria of warm life-conditioned compared with cold life-conditioned fish. Exposure to high P CO2 synergistically amplified the effects of acute and long-term warming, but did not result in changes by itself. This high ability to maintain mitochondrial function under ocean acidification can be explained by the fact that seabass are generally able to acclimate to a variety of environmental conditions. Improved mitochondrial energy metabolism after warm conditioning could be due to the origin of this species in the warm waters of the Mediterranean. Our results also indicate that seabass are not yet fully adapted to the colder temperatures in their northern distribution range and might benefit from warmer temperatures in these latitudes. Figure S1 Ratio of CI respiration over OXPHOS respiration of permeabilized heart ventricle fibers of European seabass (expressed as percentage). Shown are lsmeans ± s.e.m. Different letters indicate significant differences (LME, p<0.05); blue: cold life conditioned fish (C), orange: warm life conditioned fish (W), light color: cold assay temperature, dark color: warm assay temperature, Journal of Experimental Biology • Supplementary information Figure S2 Ratio of CII respiration over OXPHOS respiration of permeabilized heart ventricle fibers of European seabass (expressed as percentage). Shown are lsmeans ± s.e.m. Different letters indicate significant differences (LME, p<0.05); blue: cold life conditioned fish (C), orange: warm life conditioned fish (W), light color: cold assay temperature, dark color: warm assay temperature, Journal of Experimental Biology • Supplementary information Figure S3 Ratio of CIV capacities over OXPHOS respiration of permeabilized heart ventricle fibers of European seabass (expressed as percentage). Shown are lsmeans ± s.e.m. Different letters indicate significant differences (LME, p<0.05); blue: cold life conditioned fish (C), orange: warm life conditione...