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Abstract. Future scenarios for the oceans project combined developments of CO2 accumulation and global warming and their impact on marine ecosystems. The synergistic impact of both factors was addressed by studying the effect of elevated CO2 concentrations on thermal tolerance of the cold-eurythermal spider crab Hyas araneus from the population around Helgoland. Here ambient temperatures characterize the southernmost distribution limit of this species. Animals were exposed to present day normocapnia (380 ppm CO2), CO2 levels expected towards 2100 (710 ppm) and beyond (3000 ppm). Heart rate and haemolymph PO2 (PeO2) were measured during progressive short term cooling from 10 to 0°C and during warming from 10 to 25°C. An increase of PeO2 occurred during cooling, the highest values being reached at 0°C under all three CO2 levels. Heart rate increased during warming until a critical temperature (Tc) was reached. The putative Tc under normocapnia was presumably >25°C, from where it fell to 23.5°C under 710 ppm and then 21.1°C under 3000 ppm. At the same time, thermal sensitivity, as seen in the Q10 values of heart rate, rose with increasing CO2 concentration in the warmth. Our results suggest a narrowing of the thermal window of Hyas araneus under moderate increases in CO2 levels by exacerbation of the heat or cold induced oxygen and capacity limitation of thermal tolerance.


Introduction
The ongoing increase of CO 2 in the atmosphere is a key driver of global warming (IPCC, 2001(IPCC, , 2007 and causes an inrease in accumulation of CO 2 in the oceans leading to an acidification. Caldeira and Wickett (2005)

modelled
Correspondence to: K. Walther (kathleen.walther@awi.de) different future scenarios for CO 2 concentrations in the atmosphere and ocean. By 2100 they projected atmospheric and surface ocean levels of 710 ppm CO 2 and for the year 2300 much beyond depending on the continued use of fossil fuel resources. Values reached may potentially comprise 3000 ppm CO 2 . CO 2 penetrates the surface ocean by air-tosea equilibration and is distributed by ocean circulation (Orr et al., 2001). In the studied area, the German Bight, North Sea, water depth is less than 100 m (Pohlmann, 1996). In this case, the whole water body will be equilibrated with CO 2 . The ongoing and predicted increase of CO 2 levels leads to questions about the potential impact of ocean acidification on marine ecosystems in times of ocean warming (cf. Pörtner et al., 2005;Pörtner, 2008). Physiological mechanisms affected by CO 2 have been identified, however, past studies on the influence of CO 2 on crustaceans were conducted with a different focus and either used concentrations of about 10 000 ppm CO 2 (Cameron, 1978(Cameron, , 1985Wickins, 1984;Cameron and Iwama, 1987;Metzger et al., 2007;Pane and Barry, 2007) or rarely more realistic scenarios (e.g. about 800 ppm used by Spicer et al., 2007). For reliable conclusions concerning the impact of CO 2 on the physiology and fitness of crustaceans in the near future, it is necessary, however, to include realistic CO 2 concentrations in those studies, as postulated for 2100 (710 ppm) or beyond (3000 ppm), and combine them with changing temperatures.
The present study investigates the impacts of CO 2 and temperature on the physiology of Hyas araneus in the context of the thermal tolerance concept originally developed in crustaceans, namely by  for Maja squinado. The thermal tolerance window as characterized by temperature dependent haemolymph oxygen partial pressure (P e O 2 ), heart and ventilation rates comprises the temperature range of aerobic performance (or scope). The thermal optimum is the temperature where performance is maximal, supported by high haemolymph oxygen tension, and maximum scope (i.e. increase above maintenance) for heart and ventilation rates. The earliest limits of the thermal tolerance range with ecological relevance (Pörtner and Knust, 2007) are reflected by high and low pejus temperatures (T p ). Warming leads to rising oxygen demand that can initially be met by oxygen supply through enhanced ventilation and heart rate (Zainal et al., 1992;. The upper T p indicates the point, where ventilation and heart rates level off and remain constant indicating capacity limitation. Haemolymph oxygen partial pressure decreases within the subsequent pejus range, as a result of a mismatch developing between the rising oxygen demand for maintenance and the limited capacities of ventilation and circulation in oxygen supply. Beyond pejus range, a critical temperature defines the onset of anaerobic metabolism, where accumulation of L-lactate, succinate and inorganic phosphate sets in and aerobic scope vanishes Melzner et al., 2006). In the following pessimum range animal life is sustained for limited time only. Such critical temperatures can also be identified from patterns of P e O 2 or heart rate. A drop in heart rate characterizes the critical temperature as it coincides with the onset for anaerobic metabolite accumulation Melzner et al., 2006). As P e O 2 levels depend on oxygen consumption and are controlled by ventilation and heart rate the determination of critical thermal maxima in different crab species from heart rate measurements (Ahsanullah and Newell, 1971;Cuculescu et al., 1998;Stillman and Somero, 1996;Worden et al., 2006) would likely match critical thermal limits according to the concept of oxygen and capacity limited thermal tolerance.
The impact of moderate elevations in CO 2 on thermal window may be small in the thermal optimum but may exert stronger effects on thermal limits as hypothesized earlier (Pörtner et al., 2005;Pörtner and Farrell, 2008). At thermal extremes it may exacerbate the reduction in aerobic scope towards thermal extremes (cf. Metzger et al., 2007) which will decrease functional capacity and fitness and may minimize survival in the field once animals (Pörtner and Knust, 2007).
Little is known about the effect of CO 2 on temperature tolerance of cold-eurythermal invertebrates, especially in species at the border of their temperature dependent distribution range along a latitudinal gradient. The spider crab Hyas araneus (L.) was chosen as a model for a cold temperate crustacean. Hyas araneus is found in the North Atlantic from the North Sea, near Helgoland, Germany, to the Arctic around Svalbard, Norway (Christiansen, 1969). During the year the mean ambient temperature of the North Sea varies between 3 • C and 18 • C and reaches maxima of about 20 • C in summer (Wiltshire and Manly, 2004). In Svalbard waters Hyas araneus is exposed to temperatures between 0 • C and 6 • C (Svendsen et al., 2002). The species lives on stony, sandy and soft bottom from <1 down to 360 m, most commonly at depths less than 50 m. Males may reach a carapace length up to 105 mm (Christiansen, 1969).
The present study investigates the thermal window of the Hyas araneus population around Helgoland in the context of the large temperature fluctuations experienced by the species in the North Sea. One further question addressed in this study is to what extent CO 2 affects the wide thermal tolerance range and whether this effect sets in under expected CO 2 accumulation scenarios in both atmosphere and surface waters.

Animals
Adult Hyas araneus (L.), including males and females with similar sizes (carapace length: 68.8±2.8 mm) were caught between August and October 2007 around Helgoland, Germany. The females used were all in the same reproductive stage and were not carrying egg masses. The animals were held in tanks with aerated re-circulating natural seawater at 10±0.2 • C, 32-33‰ salinity, pH 8.0 and a 12 h light cycle at the Alfred-Wegener-Institute in Bremerhaven, Germany, for at least 4 weeks before the beginning of the experiments. The animals were fed twice a week with pieces of mussels (Mytilus edulis). A thermostat (Lauda, T1200) ensured tight temperature control in the experimental setup tank. The temperature ramp starting from a control temperature of 10 • C was coded using the wintherm plus program (Version 2.2) of the thermostat. The water was cooled from 10 • C to 0 • C and warmed continuously from 10 • C to 25 • C at a rate of 1 • C per h. The accuracy of the temperature ramp was 1±0.2 • C/h for cooling and 1±0.1 • C/h for warming protocols.

Surgical procedures
Prior to experimentation animals were prepared for continuous simultaneous measurements of arterial haemolymph oxygen partial pressure (P e O 2 ) and of heart rate. Briefly, two holes were drilled through the carapace, one directly over the heart, avoiding injury to the hypodermis. This hole was covered with latex dam to prevent haemolymph loss. The sleeve of an inflexible venipuncture needle (after Strauss, BRAUN, Germany) was used as an adapter for fixation of the oxygen optode. This adapter was fixed with dental wax over the drilled hole. A second hole was drilled behind the optode preparation for fixation of the Doppler probe used for heart rate measurements.

Oxygen measurements
Measurements of arterial P e O 2 were carried out with microoptodes (NTH-PSt1-L5lTF-PC3,1-NS 35x1,20-YOP, PreSens GmbH, 93053 Regensburg, Germany). Data were recorded on-line by use of temperature compensation via TX2-A oxygen monitors and software (Oxy View TX2 C 4.02) (PreSens Regensburg, Germany). Optodes were calibrated in air-saturated millipore water (100%) and in oxygenfree seawater, using sodium disulfide (0%). Haemolymph clotting around the oxygen probes was prevented by rinsing the probes in a heparin solution (5000 U/ml) prior to use. The tip of the optode was inserted through the adapter and latex dam into the pericardial sinus and fixed with parafilm. Oxygen values were recorded as % air saturation and converted to P e O 2 .

Heart rate measurements
Measurements of heart rate were carried out with a noninvasive laser Doppler perfusion monitor (LDPM PeriFlux System 5000, Perimed AB, Järfälla, Sweden) as described by Lannig et al. (2008). Prior to the experiments, the probe was two-point calibrated. The laser Doppler signal was monitored by chart 5 (AD instruments). Heart rates (beats/min) were derived from regular changes in the laser Doppler signal caused by fluctuating haemolymph flow. Laser Doppler values were averaged for individual temperature steps (0.1 • C for the warming ramp, 0.2 • C for the cooling ramp).

CO 2 incubations
After the implantation of the sensors, animals were allowed to recover for 24 h in 75-l seawater tanks at 10 • C. During the experiments animals were exposed to different CO 2 concentrations (normocapnia, 710 ppm, 3000 ppm) in the seawater. For normocapnic conditions (380 ppm CO 2 ) seawater was bubbled with air. For exposure to different CO 2 concentrations Wösthoff gas mixing pumps (Typ 2M303/a-F-T, 5kM303/a-F, 5kM402-F) were used to mix CO 2 -free air with CO 2 . During exposure to 3000 ppm CO 2 water, water pH dropped from 8.0 to 7.3 (expected pH value calculated as 7.29). Equilibration with 710 ppm CO 2 concentration caused a pH decrease from 8.0 to 7.8 (expected value calculated as pH 7.80). Prior to exposure to the temperature ramp animals were exposed for 24 h to 10 • C at each particular CO 2 concentration. New acid-base equilibria were reached in body fluids of the crabs within 24 h (Truchot, 1984). All animals survived experimentation.

Data analysis
Data are presented as means ± SE. Statistical significance was tested using one-way ANOVA and post hoc Tukey tests (GraphPad Software, Prism 4). Discontinuities in the slopes of heart rate changes vs. temperature were calculated from intersections of fitted two-phase regressions according to the minimum sum of squares and were presented as breakpoints and critical temperatures (T c ). Linear regression lines were calculated with Prism 4 (GraphPad Software). Nonlinear regression curves were fitted using Boltzmann sigmoidal equation at Prism 4 (GraphPad Software). Q 10 values were calculated from the exponential phases of Fig. 1. Temperature dependent patterns of P e O 2 and heart rate of Hyas araneus exposed to different CO 2 concentrations during acute cooling from 10 to 0 • C (grey: normocapnia; red: 710 ppm; black: 3000 ppm). (a). P e O 2 , values under 3000 ppm were significantly different from those under normocapnia and 710 ppm (ANOVA, p<0.001). (b). Heart rate under 3000 ppm was significantly different from those under normocapnia and 710 ppm (ANOVA, p<0.001). Data are means ± SE, n=7 (normocapnia) and 8 (710 ppm, 3000 ppm), respectively. heart rate increments, after De Wachter and Wilkens (1996):

Results
Acute cooling from 10 • C to 0 • C resulted in a slight increase of arterial P e O 2 under all three conditions, i.e. normocapnia, 710 ppm, as well as 3000 ppm CO 2 , starting from values of P e O 2 which fell with rising CO 2 levels (Fig. 1a). Under normocapnia mean P e O 2 ranged from 9.49 kPa at 10 • C to 13.24 kPa at 0 • C, under 710 ppm values ranged from Temperature dependent patterns of P e O 2 and heart rate of Hyas araneus exposed to different CO 2 concentrations during acute warming from 10 to 25 • C (grey: normocapnia; red: 710 ppm; black: 3000 ppm). (a). P e O 2 values were significantly different between values under 3000 ppm and those under normocapnia and 710 ppm (ANOVA, p<0.01) and between values under normocapnia and 710 ppm (ANOVA, p<0.01). (b). Heart rate under normocapnia was significantly different from those under 710 ppm and 3000 ppm (ANOVA, p<0.001). Data are means ± SE, n=7 (normocapnia) and 8 (710 ppm, 3000 ppm). 8.72 kPa to 13.56 kPa and, under 3000 ppm, from 6.05 kPa to 10.65 kPa. The P e O 2 values of crabs incubated under 3000 ppm were significantly lower than in animals under both normocapnia and 710 ppm (ANOVA, p<0.001).
Heart rate decreased between 10 • C and 6 • C with different slopes depending on CO 2 concentration (Fig. 1b). The heart rate of crabs incubated under 3000 ppm CO 2 fell more strongly than in animals incubated under 710 ppm and normocapnia. The statistical comparison of the three data sets obtained between 10 • C and 0 • C revealed a significant difference between animals under normocapnia and 3000 ppm as well as between those under 710 ppm and 3000 ppm (ANOVA, p<0.001). Hyas araneus specimens incubated under 3000 ppm displayed a lower heart rate than those under 710 ppm or normocapnia. Heart rate remained more or less constant between 6 and 0 • C under all conditions, at a rate of 37.2±1.2 beats/min under 3000 ppm, 39.64±1.4 beats/min under 710 ppm and 40.27±1.4 beats/min under normocapnia.
Upon acute warming from 10 • C to 25 • C haemolymph P e O 2 values of Hyas araneus decreased (Fig. 2a), from a maximum of about 8.5 kPa at 10 • C under normocapnia to a minimum of about 2.5 kPa at 25 • C. Under the same warming protocol mean P e O 2 in crabs under 710 ppm fell from 6.3 kPa to 0.8 kPa, and in specimens under 3000 ppm from 6.9 kPa to 1.37 kPa. Differences were statistically significant between data obtained under normocapnia and 710 ppm (ANOVA, p<0.001) as well as between those under 3000 ppm and normocapnia or 710 ppm (ANOVA, p<0.01).
Depending on CO 2 treatment heart rate of Hyas araneus displayed different patterns upon acute warming between 10 • C and 25 • C (Fig. 2b). Lowest rates were seen under normocapnia with relatively stable mean values between 43.7 and 55.0 beats/min. In contrast, heart rate increased under 710 ppm from 48.9 beats/min at 10 • C to a maximum of 63.17 beats/min at 22.4 • C leveling off towards 61.67 beats/min at 23.5 • C and decreasing rapidly thereafter to 48.44 beats/min at 24.8 • C. Crabs under 3000 ppm displayed an increase in heart rate from 52.49 beats/min at 10 • C to 65.6 beats/min at 18.4 • C leveling off to 62.52 beats/min at 21.1 • C and decreasing thereafter to 50.05 beats/min at 24 • C. Data under 3000 ppm were significantly different from those obtained under normocapnia or 710 ppm (ANOVA, p<0.001). Figure 3 presents a comparison of P e O 2 values and heart rates within the whole temperature range of all three incubations (normocapnia (Fig. 3a), 710 ppm (Fig. 3b) and 3000 ppm (Fig. 3c)). Contrasting trends result for P e O 2 and heart rate. Clear changes in the development of heart rate upon warming under 710 ppm and 3000 ppm define the critical temperatures (T c ) from calculated breakpoints. The upper T c under 710 ppm was 23.5 • C and under 3000 ppm 21.1 • C. Under normocapnic conditions no breakpoint could be identified in the observed temperature range between 10 and 25 • C, confirming that the T c is found beyond 25 • C under normocapnia (T c >25 • C).

Thermal tolerance window of Hyas araneus
The aim of the present study was to illustrate the impact of anthropogenic CO 2 accumulation in the oceans on the thermal tolerance window of the spider crab (Hyas araneus) population from Helgoland. For an examination of the thermal tolerance window haemolymph oxygen partial pressure (P e O 2 ) and heart rate were measured during warming and cooling protocols. Hyas araneus exposed to the cooling protocol from 10 • C to 0 • C (Fig. 1a) displayed a moderate increase in arterial haemolymph oxygen partial pressure while heart rate decreased. Specimens exposed to warming from 10 • C to 25 • C (Fig. 2a) experienced a decrease in mean P e O 2 value while heart rate rose under normocapnic conditions. The comprehensive depiction in Fig. 3 shows that arterial haemolymph P e O 2 continued to rise down to 0 • C. These data contrast the first such data set elaborated in the spider crab Maja squinado  where the temperature dependent pattern of arterial P e O 2 characterized the thermal tolerance window. In Maja squinado P e O 2 fell upon cooling until a lower critical temperature was reached, subsequently anaerobic metabolism set in and indicated cold induced oxygen deficiency .
The optimum temperature range of Maja squinado was seen between low and high pejus temperatures of 8.9 and 17.8 • C, respectively , with a maximum arterial P e O 2 of 92.6 mmHg, equivalent to 12.3 kPa, which was more or less stable between those socalled pejus thresholds. The highest P e O 2 value recorded in Hyas araneus was 13.6 kPa at the coldest temperature of 0 • C. These data indicate that the optimum performance range of Hyas araneus is shifted to colder temperatures when compared to Maja squinado. As Hyas araneus reached their highest arterial P e O 2 values in the cold this may indicate that the species experiences no oxygen limitation at extremely cold temperatures, at least in its central organs close to the heart. However, recent data indicate that venous oxygen tensions may more closely reflect thermal limitation (F. Giomi, personal communication). Limitation in the perfusion of peripheral organs associated with low venous oxygen tensions or in general, in functional capacity upon further cooling, may thus occur in similar ways as shown in Maja squinado (Bock et al., 2001). Due to capacity limitation in neuromuscular systems all reptant decapod crustaceans were hypothesized to be excluded from permanently low temperatures of −1 • C in polar oceans .
The range of thermal tolerance of Hyas araneus is mirrored in its range of natural distribution from the North Sea around Helgoland, Germany, northward to the Arctic around Svalbard, Norway (Christiansen, 1969). Hyas araneus can thus be characterized as a cold-eurythermal species with a lower thermal optimum range than seen in the warmeurythermal Maja squinado . Mean ambient water temperature of the North Sea at Helgoland Roads is about 3 • C in winter and reaches 18 • C, maximally 20 • C, in summer (Wiltshire and Manly, 2004). Temperatures in Svalbard waters fluctuate between 0 and 6 • C (Svendsen et al., 2002). This wide range of habitat temperatures implies a wide thermal tolerance range of this spider crab.
The heart rate of Hyas araneus decreased exponentially between 10 • C and 0 • C under normocapnic conditions (Fig. 1b). Heart rate reached 37.2 beats/min at 0 • C. This value appears high compared to the 20 beats/min reported for Hyas araneus at 0 • C . In Maja squinado, heart rate at 0 • C, below the lower critical temperature and within the pessimum range was 10 beats/min. A lower heart beat rate might not only reflect thermal limitation but also relate to the somewhat larger body size of M. squinado (carapace length: 142.5±30.5 mm (Bernárdez et al., 2005) compared to carapace length: 68.8±2.8 mm in H. araneus) (Ahsanullah and Newell, 1971;DeFur and Mangum, 1979). Maja squinado's heart rates in the optimum range (9.3 • C to 17.3 • C) were about 40 to 60 beats/min , at the low end of rates seen in H. araneus at 10 • C and beyond. Close to 0 • C, high P e O 2 values combined with relatively high heart rates of Hyas araneus reflect maintenance of performance at cold temperatures. The rise in arterial P e O 2 in the cold reflects the facilitation in oxygen supply in cold waters, once tissue functional capacities are cold adapted. This facilitation is due to rising oxygen solubility in the cold in water and body fluids combined with a putative cold induced reduction of metabolic rates (cf. Pörtner, 2002). This conclusion is supported by the relaxed oxygen supply situation and the respective molecular to systemic adaptations of polar stenotherms (cf. . Under normocapnia and both elevated CO 2 tensions tested Hyas araneus heart rate decreased from 10 to 6 • C and was nearly constant between 6 and 0 • C, reflecting the lower end of an exponential decline phase which characterizes the lower end of the thermal window. This pattern is similar to the pattern of oxygen consumption within thermal tolerance windows as seen in the squat lobster Munida rugosa (Zainal et al., 1992) and in other marine invertebrates, e.g. the lugworm, Arenicola marina (Wittmann et al., 2008). Upon warming from 10 to 25 • C the heart rate of Hyas araneus remained more or less constant under normocapnic conditions (Fig. 2b). The exponential phase of the heart rate was seen between 6 and 12 • C under all CO 2 conditions. The upper point of change between exponential and linear phase is defined as the upper pejus temperature, where the circulatory performance of H. araneus from the Helgoland population reaches its upper capacity limit. At higher temperatures than T p the analysis revealed no further discontinuities in the normocapnic data (Fig. 3). For comparison, heart rate data of Maja squinado displayed a break at 31.5 • C, close to their critical temperature identified by the onset of anaerobic metabolism . We conclude that the upper critical temperature of the Hyas araneus from the population at Helgoland under normocapnia is likely reached above 25 • C.
The observations that the maximal P e O 2 of Hyas araneus is found close to 0 • C, that the upper pejus temperature is likely seen around 10 • C to 12 • C and that the critical temperature of Hyas araneus is found above 25 • C indicate that the width of the pejus range starting beyond 10 • C to 12 • C is similar or somewhat broader than that of Maja squinado which displays a pejus range between 17.3 and 31.1 • C . This and the progressive rise in P e O 2 within the optimum range towards the more extreme cold distinguishes Hyas araneus as a cold-eurythermal species from the warm-eurythermal Maja squinado.

CO 2 effects on thermal tolerance
During exposure to increased CO 2 concentrations (710 ppm and 3000 ppm) the P e O 2 of Hyas araneus displayed various decline phases between 0 • C and 25 • C resembling those under normocapnic conditions (Fig. 3). The same was true for the pattern of heart rate. However, levels of heart rate and P e O 2 differed between CO 2 levels. Heart rate resulted lower between 10 and 0 • C under 3000 ppm than under 710 ppm or normocapnia. This observation together with a trend for P e O 2 to be lower under high CO 2 levels indicate a reduction in functional capacity of oxygen supply in CO 2 exposed specimens in the cold.
Above 10 • C, CO 2 concentration influenced the temperature dependent rise in heart rate, which resulted steeper with higher CO 2 levels. The increase in heart rate under 3000 ppm CO 2 was larger than under 710 ppm or under normocapnia (Fig. 2). The stronger thermal stimulation of heart rate under increasing CO 2 levels may reflect a response to lower oxygen tensions and/or a chemosensory response. It may also reflect a stronger increase in metabolic rate. Exposure to acidification likely results in enhanced ventilation and a parallel rise in heart rate, which supports CO 2 release and thus the alleviation of CO 2 induced pH disturbances (cf. Pörtner et al., 2005).
Starting from a putative T c above 25 • C for Hyas araneus under normocapnic conditions CO 2 clearly induces a reduction in upper critical temperature to 23.5 • C under 710 ppm and to 21.1 • C at 3000 ppm (Fig. 3). These results confirm those obtained earlier in Cancer pagurus by Metzger et al. (2007), where measurements of PaO 2 demonstrated a downward shift of critical temperature from 20.5 • C (normocapnia) to 15.5 • C under 1% CO 2 (=10 000 ppm CO 2 ). All of these findings support the hypothesis that with higher CO 2 concentrations and a stronger heat induced stimulation of heart rate the upper critical temperature falls, as a result of synergistic effects of temperature and CO 2 . In contrast to observations in the cold it may also involve a CO 2 induced stimulation of metabolic costs in the warmth.
The additional decrement in heart rate at low temperatures may possibly involve an accumulation and effect of adenosine under CO 2 exposure. Adenosine was found to accumulate under elevated CO 2 levels and depress ventilation rate in Sipunculus nudus (Reipschläger et al., 1997). The role of pH in this effect is not clear (Pörtner, 2008). In crustaceans, adenosine also depresses spontaneous activity and the responsiveness of interneurons to electrical and chemical stimuli in the brain (Derby et al., 1987) and elicits bradycardia (Brevard et al., 2003). In contrast, adenosine can display a stimulatory effect on heart rate, haemolymph flow and scaphognathite frequency (Maurer et al., 2008;Stegen and Grieshaber, 2001). This apparent discrepancy resembles the contrasting CO 2 effects at low and at high temperatures. A stimulatory effect might in fact be involved in the increase in heart rate with rising CO 2 concentrations in the warmth (Fig. 4). Further experiments are required to test these hypotheses. Stillmann and Somero (1996) identified upper thermal limits in heart rates of the high intertidal crab Petrolisthes cinctipes and the low intertidal crab Petrolisthes eriomerus, which correlated with the natural habitat temperature. Heart rate measurements of Petrolisthes species identified a critical temperature which in the light of our present data and those by  may indicate a shift from aerobic to anaerobic metabolism. The low intertidal crab Petrolisthes eriomerus displayed a narrower thermal tolerance window and a lower T c of 26.6 • C with some accumulation of L-lactate at 25 • C in comparison to the high intertidal crab Petrolisthes cinctipes (T c =31.5 • C) (Stillmann and Somero, 1996). The example of low and high intertidal Petrolisthes species illustrates the specialization of marine species on limited thermal ranges and the ability to live near their thermal limits. For eurythermal Hyas araneus we suggest, that the thermal tolerance window enables the large scale biogeographical distribution of the species. Under normocapnia, the response of Hyas araneus from Helgoland resembles the one of the high intertidal species P. cinctipes. In contrast, Hyas araneus under elevated CO 2 showed a response similar to P. eriomerus, which is a low intertidal species and does not possess such a wide thermal tolerance range. A CO 2 induced narrowing of the thermal tolerance will therefore most likely restrict the geographical distribution of a species. Hyas araneus from Helgoland would already exploit its upper pejus range during present summers (temperature maxima about 20 • C; Wiltshire and Manly, 2004), however without reaching the upper T c . The situation may change under elevated CO 2 concentrations of 3000 ppm (scenario 2300), where the critical temperature of Hyas araneus was determined at 21.1 • C, which would imply an increase in heat stress during extreme summers and elevated mortality rates.
A schematic model of heart rate changes in Hyas araneus illustrates how the thermal tolerance window is narrowed under the influence of CO 2 (Fig. 4). The CO 2 induced rise in Q 10 values in the exponential phases of heart rate may be involved in eliciting the narrowing of thermal windows, in similar was as the Q 10 enhancement of metabolic rate as recently seen in the lugworm Arenicola marina (Wittmann et al., 2008). In the lugworm model the thermal tolerance window is influenced by seasonal acclimatization to temperature resulting in a narrower window during winter, associated with lower metabolic rates and higher Q 10 values than in summer (Wittmann et al., 2008).
We can conclude that CO 2 induced ocean acidification has the potential to cause a narrowing of thermal windows. The present mechanism based projections indicate that specimens from the southernmost population of a species, when permanently exposed to acidification may lose their capability to acclimate to extreme temperatures. In the future, long term exposures which mimic the long term nature of ocean acidification scenarios more closely will have to complement the present experiments. The North Sea around Helgoland already showed a warming trend during the last 40 years of 1.1 • C to a mean temperature of 18 • C and with maximum temperatures of about 20 • C (Wilthire and Manly, 2004). Personal observations indicate a drastic decrease in the abundance of H. araneus around Helgoland, which might be linked with summer warming. The present study emphasizes that a further increase in ambient temperature as predicted by the IPCC (2001IPCC ( , 2007 combined with increased ocean acidification (Caldeira and Wickett, 2005) may cause animals to reach their physiological limits even sooner. As a consequence Hyas araneus may lose its southern habitats and experience a stronger northward shift of biogeographical boundaries.