Both resting and maximal metabolic rate increased with temperature (4 to 14°C), with the increase in resting metabolic rate being more pronounced for all three species, i.e. 171% (max: 54%), 135% (max: 45%) and 205% (max: 43%) for the shorthorn, Arctic and Arctic staghorn sculpins, respectively (). In shorthorn and Arctic sculpins this meant that the aerobic scope leveled off as temperature increased, resulting in bell-shaped scope curves for these two species. This is similar to a previous hypothesis that states that there is an optimal temperature7, below and above which there are limitations in e.g. growth and reproduction. Nevertheless, maximal metabolic scopes for shorthorn and Arctic scuplins were reached at 9°C (91±13 and 95±7 mg O2 kg−1 h−1, respectively) (), which was the recorded SST at the time of the experiment. For Arctic staghorn sculpin, however, metabolic scope decreased from its peak at 4°C (57±5 mg O2 kg−1 h−1) and was much reduced at 14°C (22±11 mg O2 kg−1 h−1), probably limiting the capacity for metabolically demanding activities beyond basic maintenance. Overall, Arctic staghorn sculpin had a significantly lower metabolic scope at all temperatures compared to the other two species (). Thus, this species may already have exceeded its optimal temperature during the warm summer of 2009. The available temperature data () shows that there has been a rapid increase in temperature over the last years, while the distribution () of this species has been restricted to the arctic region. Therefore, although the exact temperature range of the Arctic staghorn sculpin is unknown, it is reasonable to speculate that its optimal temperature is at or below 4°C.
Some ectotherms can maintain a low resting metabolism by actively avoiding high temperatures and thereby avoid negative influences on metabolic scope, e.g. as observed in Atlantic cod (Gadus morhua
) when swimming in a thermally stratified water column 
. However, this may not be an option for the benthic, stationary sculpins studied here. An alternative strategy is to reduce the temperature dependence of metabolism per se,
maintaining metabolic rate despite an increase in temperature, i.e. acclimation. This is a common strategy for fish to cope with environmental challenges such as hypoxia 
and is also seen in the gadiod Lota lota
in response to high summer temperatures 
. Our preliminary, unpublished data support the existence of a metabolic depression in association with warm-acclimation in sculpins. In gadiods, such acclimation depends on rapid down-regulation of enzymes involved in aerobic metabolism 
. Although the suppression of metabolic rate could be a long-term process, none of the sculpins maintained the resting metabolism seen at 4°C when exposed to 9°C, despite the latter being the prevailing temperature when they were caught. Hence, the sculpins would have had time to lower the metabolism to maintain a lower resting metabolism at 9°C. Given the plasticity of thermal acclimation, the resting metabolism seen at 4°C could have increased if the fish were allowed more time to acclimate to this temperature (as during winter). Such cold adaptation, although a matter of debate, was demonstrated by Precht et al. 
and could depend on mechanisms at the whole animal level, as well as the enzyme level 
. The physiological responses to acute temperature increases are probably of high ecological importance in these fish and may be particularly important in their coastal habitats, e.g. as a result of diurnal and/or tidal cycles 
. In fact, several studies indicate that short-term temperature fluctuations and heat spells will increase in magnitude and frequency as a consequence of global warming 
, and studies on e.g. amphibians 
indicate that it is not the increase in average temperature per se
, but rather the greater short-term temperature variability that will be most detrimental for many populations. However, few studies in fish have addressed this question 
The results of the behavioral aspect of the study were partly consistent with our second hypothesis that metabolic scope can be used to predict competitive ability. Arctic staghorn sculpin, the species with the lowest metabolic scope at 9°C, was consistently outcompeted by shorthorn sculpin for access to cover (), whereas Arctic sculpin outcompeted the shorthorn sculpin despite a similar metabolic scope (–). The fact that Arctic staghorn sculpin, also had the lowest competitive ability of the three species supports the metabolic theory suggested by Brown 
, stating that metabolism, and metabolic rate in particular, shape population dynamics and predator/prey interactions.
The common resource was a protective cover since marine sculpins use a cover (i.e. shelter) when available 
and the performance of the sculpins was monitored after a simulated attack, which should increase the motivation of the fish to regain and maintain a cover 
. Numerous species of fish increase their use of a cover/shelter by up to 40% when the risk of predation is high 
, and in a pilot study it was apparent that a simulated attack increased cover/shelter use in all three species. The cover was not added to the arena until just before the simulated attack. This was done in order to exclude a “resident effect” whereby the prior resident may win due to higher motivation to fight, i.e. the value asymmetry hypothesis 
. It should be pointed out that we could not control for potential species differences in the innate motivation to seek cover. A higher motivation could potentially enhance the competitive ability when the physiological capacity (metabolic scope) is equal. For example, the less developed body armour of Arctic sculpins could make them more vulnerable to some (e.g. smaller sized) predators compared to the shorthorn sculpin, thereby increasing the motivation to seek cover 
. This might explain why Arctic sculpin outcompeted shorthorn sculpin in competition for cover, despite a similar metabolic scope at 9°C.
Previous studies on shorthorn sculpin have shown that the ability to both capture prey and to escape predators is affected by acclimation temperature 
. These performances were improved during acclimation to higher temperatures (15°C), while cold acclimation (5°C) had the opposite effect. While these studies did not report on metabolic scope, it is possible that the sculpin were able to maintain or even elevate their metabolic capacity when acclimated to the higher temperature, suggesting considerable potential for phenotypic plasticity in terms of metabolic capacity when exposed to more long-term thermal challenges. Interestingly, the escape behavior of another related sculpin species, the long-spined sea scorpion (Taurulus bubalis
), which has narrower geographical distribution and does not occur as far south as the shorthorn sculpin, was not improved by acclimation to higher temperatures 
. Instead, acclimation to low temperatures was advantageous for this species when performing in cold water. This indicates that temperature acclimation patterns may differ even between closely related species. Such differences may be of major importance for interactions among species coexisting in habitats with large seasonal fluctuations in temperature, a fact that needs to be considered when interpreting results from only one acclimation temperature.
Climate change is a driving force for evolution and the influence of temperature on metabolism is an important mechanism linking physiology and behavior with fitness in ectothermic animals 
. Learning more about thermal tolerance and its physiological, behavioral and ecological consequences will aid in understanding the possible effects of climate change. While the future of many species will depend on their capacity to find new habitats and/or to adapt to changes within their present habitats 
, the capacity to do so may be limited in species like the Arctic staghorn sculpin that is already pushed to the higher end of its physiological thermal optimum as indicated in this study. Thus, in due course there will likely be a shift in the species distribution, abundance and migratory patterns as our oceans warm