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Alfred Wegener Institute for Polar and Marine Research, 27568 Bremerhaven, Germany
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Geographic distribution limits of ectothermal animals appear to be correlated with thermal tolerance thresholds previously identified from the onset of anaerobic metabolism. Transition to these critical temperatures was investigated in the spider crab (Maja squinado) with the goal of identifying the physiological processes limiting thermal tolerance. Heart and ventilation rates as well as PO2 in the hemolymph were recorded on-line during progressive temperature change between 12 and 0°C (1°C/h) and between 12 and 40°C (2°C/h). Lactate and succinate were measured in tissues and hemolymph after intermediate or final temperatures were reached. High levels of hemolymph oxygenation suggest that an optimum range of aerobic performance exists between 8 and 17°C. Thermal limitation may already set in at the transition from optimum to pejus (pejus = turning worse, progressively deleterious) range, characterized by the onset of a decrease in arterial PO2 due to reduced ventilatory and cardiac performance. Hemolymph PO2 values fell progressively toward both low and high temperature extremes until critical temperatures were reached at ~1 and 30°C, as indicated by low PO2 and the onset of anaerobic energy production by mitochondria. In conclusion, the limited capacity of ventilation and circulation at extreme temperatures causes insufficient O2 supply, thereby limiting aerobic scope and, finally, thermal tolerance.
aerobic capacity; anaerobic metabolism; optodes; partial pressure of oxygen
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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CRITICAL TEMPERATURES (Tc) have been defined for different marine invertebrate and fish species as being characterized by the onset of anaerobic metabolism, which is caused by a mismatch of O2 demand and O2 supply (for review, see Refs. 35 and 36). Extended exposure to temperatures above high Tc or below low Tc finally leads to death of the animal unless thermal acclimation, i.e., a shift of Tc values, occurs (40, 51). One hypothesis is that the adjustment of mitochondrial density and capacity is involved in setting thermal tolerance limits and is therefore related to geographic distribution (35, 36). As a consequence, the relationship between O2 availability to tissues and O2 demand appears to be crucial for survival of exposure to temperature extremes. Study of the processes of O2 uptake by ventilation and O2 distribution by circulation therefore appear important to further our understanding of the O2 limitation of thermal tolerance. Therefore, we chose to study these systemic aspects of thermal tolerance, selecting a crustacean, Maja squinado (Herbst), as a model organism. As yet, physiological studies of crustaceans have addressed temperature effects on O2 consumption, heart and ventilatory performance, or growth (12-14, 16, 23, 34, 44). In most of these studies, temperatures were chosen within the physiological range. Hence, there is little information on parameters that become limiting on both sides of the window of thermal tolerance in this group.
The present study was designed to investigate the effect of acute temperature change on PO2 in the hemolymph of Maja squinado combined with an analysis of ventilation and heart rates as the processes responsible for O2 uptake and distribution. Maja squinado was chosen under the assumption that the identification of a low Tc would be easier in this warm-adapted species than in cold-adapted species. Maja squinado is found between the warm waters of the north African coast and the west and south coasts of England and only occasionally occurs in the German Bight (9, 22).
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MATERIALS AND METHODS |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Animals.
Adult male and female Maja squinado were obtained from local
fishermen in Roscoff, France, in November 1998. Animals were kept in
large tanks with aerated natural seawater at 10°C, and most of them
were transported within 15 h from Roscoff to the Alfred Wegener
Institute in Bremerhaven, Germany. They were held in tanks with
aerated, recirculating natural seawater at 10 ± 0.5°C and 32
salinity. The animals were fed twice a week with pieces of cod
(Gadus morhua) and mussels (Mytilus edulis).
Experiments were carried out at the Station Biologique de Roscoff and
at the Alfred Wegener Institute.
Surgical procedure and on-line data recording. Animals were prepared for experimentation by drilling a small hole through the carapace directly behind the heart, avoiding injury to the hypodermis. The hole was covered with a latex dam to prevent hemolymph loss during the following surgery. A glass capillary was inserted through the latex dam 1 mm deep into the carapace and fixed with dental periphery wax. To identify the optimum position of the O2 sensor in the pericardial sinus, a cannula was fed through the capillary into the pericardial sinus until pulsation of the hemolymph level could be seen in the capillary. The cannula was removed, and a calibrated optode (a fiber-optic oxygen sensor; see description below and Refs. 25 and 26) was inserted through the glass capillary, brought to exactly the same position, and fixed with dental periphery wax.
Animals were kept in a temperature-controlled 25-liter aquarium with air-saturated seawater. PO2 of the seawater changed slightly with temperature between 157.9 ± 1.1 mmHg at 0°C and 147.0 ± 1.2 mmHg at 40°C, depending on the temperature dependence of water vapor pressure. Arterial PO2 in postbranchial hemolymph was monitored on-line in the pericardial sinus by implanted optodes. Two different instruments, Microx I (PreSens, Neuburg/Donau, Germany) and Mops-4 (Comte, Hannover, Germany) were used. Both companies provide sensors with an oxygen-sensitive fluorophor [tris(2,2'-bipyride)-ruthenium(II)-chloride immobilized in silicone for Mops-4] fixed on the tip of an optic fiber. Sensor diameter is 50 µm in Microx I and 600 µm in Mops-4. Microx I quantifies the phase angle shift of emitted light based on oxygen-dependent dynamic quenching of luminescence (25, 26), whereas Mops-4 quantifies the intensity shift of the signal caused by different levels of PO2. Despite the differences in tip size and in the principle of measurement, the systems showed the same results with respect to time resolution and accuracy. For most of the measurements, Mops-4 was used because the tips are less fragile and easier to handle because of their larger diameter. Temperature shift of the signal was calculated by linear calibration curves recorded with each optode between 0 and 40°C. By calculating temperature-compensated calibration values at intervals of 0.1°C, it was possible to correct O2 recordings for temperature, which was monitored with a PT100 thermometer (isiTEC, Bremerhaven, Germany). Heart and ventilation activity were monitored by the noninvasive technique introduced by Depledge (11) and described in more detail elsewhere (18). Photoplethysmographs (isiTEC) were glued onto the carapace above the heart and on both sides below the scaphognathite. Data were recorded by a MacLab system (AD Instruments) at a rate of 0.1 Hz for PO2 and 20 Hz for heart and ventilation rates. Animals moved freely during the experiments; only the chelae were covered with two small pieces of tubing to prevent the crabs from destroying the ventilation sensors. They had a minimum of 12 h to recover at 12 ± 0.2°C before being cooled to 0°C within 12 h (1°C/h) or warmed to 40°C within 14 h (2°C/h). Time courses were set by the capacity of the thermostat (Lauda RC6CP, Lauda-Königshofen, Germany) used for temperature control. At the end of the experiments, optodes were removed and calibration was checked for drift due to damage caused by the penetrated tissue. Implanted optodes could be used for 48 h without drift by >2%. During longer implantations, an aggregation of proteins or other material appeared on the tip of the optode and response time was extended. All animals survived the surgical procedure.Lactate and succinate measurements.
Samples for the analysis of anaerobic end products were collected
during an experiment carried out with freshly caught animals in
Roscoff. Crabs (6 for each temperature) were maintained in 80-liter
aquaria. After a 12-h period at 12.5°C, temperature was increased or
decreased to 
0.3, 7.9, 21.6, and 33.3°C at a rate comparable to
that used during measurements of PO2 and heart
rate. After the final temperatures were reached, hemolymph samples were withdrawn via a cannula inserted through the articular membrane of the
coxa of the last walking leg. Animals were dissected by cutting off the
rostrum, including the cerebral ganglion, and opening the carapace.
Tissue samples were excised from heart, hepatopancreas, and musculature
of meropodites of different walking legs. The dissection was finished
within 60 s. Samples were freeze clamped immediately
(48) and stored in liquid nitrogen until analysis.
Statistics. PO2 and heart and ventilation rate data were analyzed for break points identifying a change in the relationship to temperature using a Q-BASIC program that determines intersections of fitted regressions at the minimized sum of residual sums of squares (49). Significance of differences between fitted regressions was tested using analysis of covariance (ANCOVA, SuperAnova, Abacus Concepts) and Student-Newman-Keuls test at the P < 0.05 level. Metabolite data were tested for significance of differences at the P < 0.05 level using ANOVA (SuperAnova). Data are given as means ± SD.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Animals exhibited great variability in hemolymph
PO2 and heart and ventilation rates at control
temperature and during temperature decrease or increase. No differences
could be found between male and female specimens (mean wt 595 ± 151 g). After 12 h at 12 ± 0.2°C, mean
PO2 was 92.6 ± 27.8 mmHg
(n = 13; for minimum and maximum values, see Table
1.). Mean heart rate was 52.9 ± 14.8 beats/min, and ventilation rate differed between left and right scaphognathites, as reported previously for different crustacean species (6, 31-33). Mean ventilation rate of both
scaphognathites in 13 different animals was 60.0 ± 28.3 beats/min
at 12°C. As an example, Fig. 1 shows
the typical variability in PO2 and heart and
ventilation rates for one specimen at 12°C over a period of 13 h. This animal exhibited large fluctuations in
PO2 with a low mean of 27.2 ± 12.5 mmHg
and no period of constant PO2 throughout. PO2 varied between 3.1 and 55.7 mmHg. Although
this animal exhibited a low mean PO2 value
compared with others, the pattern of PO2 fluctuations was similar in all investigated specimens. Periods of
apnea corresponded to very low hemolymph O2 levels.
Bradycardia occurred together with apnea and large drops in
PO2, whereas smaller fluctuations in
PO2 and ventilation rate were not correlated
with bradycardia events. Short bursts of tachycardia seemed to
compensate for periods of bradycardia.
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When temperature decreased or increased, major changes occurred in cardiac and ventilatory performance and PO2. Initial trials confirmed that after temperature changed from 12 to 5 or 20°C, PO2 changed initially but remained at a constant mean level for 6 h after the new temperature was reached. This implies that no acute over- or undershoot reaction was involved. It also demonstrates that the rate of cooling or heating was sufficient to bring PO2 close to steady state for each temperature analyzed.
The drop in PO2 with rising or falling
temperature was curvilinear toward both ends of the temperature window,
with a range of maximum PO2 between 8 and
17°C (92.6 ± 6.4 mmHg). PO2 decreases at low and high temperatures were not symmetrical; a tailing was visible toward higher temperatures (Fig.
2). Ventilation rate increased linearly
between 6 and 17°C at an average rate of 8.2 ± 2.6 beats · min
1 · °C1. Below
8°C, the ventilation rate dropped sharply (P < 0.05, ANCOVA) and rose again to 18.8 ± 10.9 beats/min at 3.7°C
without a visible effect on PO2. During warming
above 17°C, no further increase in ventilation rate occurred but
ventilation leveled off significantly (P < 0.05, ANCOVA) and even decreased slightly, 2.1 ± 0.2 beats · min1 · °C1.
Beyond 30°C, a significant (P < 0.05, ANCOVA) steep
reduction (10.6 ± 0.3 beats · min1 · °C1)
occurred after a transitional rise at 30°C. Apnea occurred at 39.7°C.
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Heart rate also showed this triphasic behavior between 6 and 40°C,
with a slight but nonsignificant (P > 0.05, ANCOVA)
reduction in slope from 3.3 ± 0.1 to 1.8 ± 0.2 beats · min1 · °C1 at
14.4°C. A sudden, temporary increase at 30°C was followed by a
significant (P < 0.05, ANCOVA) decrease (9.3 ± 0.3 beats · min1 · °C1)
until acardia was reached at 40.5°C, which led to death when temperature was not reduced to control values immediately. At low
temperatures, a depression in heart rate below 8°C coincided with the
observed sharp decrease in ventilation rate and a slight reduction in
PO2. Below 4°C, heart rate decreased again,
followed by an increase to 12.9 ± 4.9 beats/min. This small
increase in heart rate around 1°C represents a final
"tachycardia" before acardia sets in, with no cardiac output being
detectable, as shown by Frederich et al. (17) using the
Doppler technique. Animals survived when temperature was increased
above 0°C again within 30 min.
Heart rate proved to be more resistant to temperature change than
ventilation rate and PO2. Therefore,
ventilatory performance seemed to be more crucial for surviving extreme
temperatures. As a consequence, Tc and preference and
tolerance ranges were identified especially from ventilation rate and
PO2 data. Calculated intersections of fitted
regressions (Table 2) confirmed visible break points at 8 and 17°C. Because temperatures of 8 and 17°C indicate the transition from optimum to an increasingly deleterious range (called the pejus range; see DISCUSSION), they are
called pejus temperatures (Tp I and Tp II).
The lower pejus temperature (Tp I) was characterized by a
sudden decrease in ventilation and heart rates accompanied by a slight
reduction in PO2 at 8°C. Apnea set in at
minimum PO2, and the temperature at this point
was defined as the low Tc (Tc I; 1°C)
because anaerobic metabolism was observed slightly below this value
(see Fig. 4). Similarly, the upper break point at 30°C was identified as the upper Tc (Tc II) because significant
anaerobic energy production occurred slightly above this value (see
Fig. 4 and DISCUSSION).
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Arterial PO2 is clearly dependent on
ventilation rate because ventilation sets the amount of O2
available to the animal. Figure 3 shows
two different patterns of correlation depending on the temperature
range. Below 17°C, which was determined as the upper Tp
(Tp II) for ventilation, PO2
increased strongly with ventilation rate. Maximum oxygenation was
reached at ~40 beats/min. Further acceleration of scaphognathite beat
frequency did not result in elevated hemolymph
PO2. This situation changed in the temperature
range above 17°C. High levels of PO2 were
reached only at ventilation rates of 80-100 beats/min. A lower
scaphognathite beat frequency was not sufficient to maintain high
O2 levels in the hemolymph.
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Lactate was found to accumulate significantly in hemolymph,
musculature, hepatopancreas, and heart tissue at 33.3°C (Fig. 4). Lactate levels rose significantly in
the musculature already at 21.6°C but only to a small, nonsignificant
extent during cooling. A minor increase in the hepatopancreas and in
the hemolymph at 0.3°C was also nonsignificant. Succinate
accumulated significantly in all investigated tissues at 33.3°C and
in the hepatopancreas only at 0.3°C. As with lactate, the trend for
succinate levels to rise remained nonsignificant in the other tissues
at 0.3°C.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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The on-line recordings of arterial PO2 by fiber-optic oxygen sensors proved to be a suitable and reliable method for the quantification of dynamic temperature effects on hemolymph O2 levels in Maja squinado. This technique allows investigation of changes in PO2 with temperature over time and avoids disturbance by repeated sampling, as required by other experimental protocols (see, for example, Refs. 15, 30, 45). It also avoids immobilization of the animal, as required for the use of fragile microelectrodes (2). PO2 was investigated because it reflects both the efficiency of arterial O2 uptake and the pressure head for O2 diffusion from the hemolymph to tissue mitochondria.
Hemolymph PO2 is buffered by hemocyanin-bound O2. We refrained from studying temperature effects on O2 binding by hemocyanin, although different studies have shown that parameters such as half-saturation pressure (P50) and Bohr shift change with temperature in crustaceans (for review, see Refs. 7, 8, 27-29). Hemocyanin characteristics suggest that O2 binding at the gills may become insufficient at high temperature and O2 unloading in the tissues may be incomplete at low temperature (29). In our study, release of O2 from hemocyanin is likely to explain the tailing in the PO2 decrease between 23 and 32°C visible at a hemolymph PO2 of 24.5 ± 6.5 mmHg (Fig. 2). As expected, PO2 buffering occurred close to the P50 of Maja squinado hemocyanin, which amounts to 21.0 mmHg at 15°C and is expected to rise with temperature (28). In any case, hemocyanin oxygenation is a secondary parameter to quantify O2 availability to tissues, whereas arterial hemolymph PO2 is a direct indicator because it correlates with the level of physically dissolved O2 and represents the pressure head driving diffusion.
Fluctuations in PO2 and corresponding heart and ventilation rates under resting conditions were similar to those described for the crayfish Astacus leptodactylus (2). Mean PO2 values of 92.6 ± 6.4 mmHg between 8 and 17°C are in the upper range of PO2 data compiled by McMahon and Wilkens (33) and Mangum (28) for 28 decapod species and much higher than reported for Carcinus maenas and Necora puber (30). Forgue et al. (15) reported a mean PO2 of 45.8 mmHg at 15°C for Maja squinado. This value is still within the range of interindividual variability in PO2 seen in the present study.
To investigate the response to fluctuating temperature, we changed temperature quite rapidly over a wide range. The fast, progressive temperature change minimized acclimation phenomena that may involve shifts in thermal thresholds (e.g., associated with changes in mitochondrial density; Ref. 36). Reactions to such a progressive temperature change will take longer at low than at high temperatures because of the Q10 effect. Stable PO2 readings during extended incubations at selected temperatures confirmed that this was accounted for by the slower rate of temperature change in the range between 12 and 0°C.
Measurements of PO2 as well as heart and ventilation rates allowed determination of threshold temperatures at the organismic level in the context of changes in lactate and succinate concentrations. Previously, critical temperatures (Tc I and Tc II) have been identified by the onset of succinate (and/or acetate) formation, indicating tissue hypoxia, the transition to an anaerobic mitochondrial metabolism, and an elimination of aerobic scope for activity at both critical temperatures (for review, see Refs. 35, 36). The present data confirm that critical temperatures exist in Maja squinado, because the accumulation of lactate and succinate coincided with very low arterial PO2 values and indicates the onset of anaerobiosis at extreme temperatures (Fig. 4). However, major changes in circulatory performance and hemolymph PO2 occurred before critical temperatures were reached (Fig. 2). Threshold temperatures (Tp I and Tp II) became visible, which indicated the transition from a temperature range with maximum arterial PO2 to low or high ranges of temperature characterized by falling PO2 values. The progressive drop in arterial PO2 reflects a reduction of O2 availability to mitochondria. This is equivalent to an apparent reduction of mitochondrial aerobic capacity and thereby a reduced scope for aerobic activity of the whole animal despite maintenance of standard metabolism. The concept of "aerobic scope for activity" as defined on the basis of measurements of O2 consumption in fish (4) reflects the range between minimum and maximum O2 consumption by mitochondria and depends on sufficient O2 delivery by ventilation and circulation. Therefore, this concept extends from the whole animal to mitochondrial levels.
The high PO2 between Tp I and Tp II was maintained by a progressive increase in ventilation rate with temperature as required to compensate for the rise in O2 demand (Fig. 3). Tp II was characterized by a change to more or less constant ventilation rates and onset of a decrease in PO2, reflecting a rising O2 demand in metabolism that was no longer compensated for by ventilation. Tp I was characterized by a sudden decrease in ventilation and heart rates, followed by a decrease in PO2. This threshold is furthermore characterized by a disproportionate reduction in cardiac output and a redistribution of hemolymph from lateral into sternal and hepatic arteries, as shown elsewhere (17). Critical temperatures characterized by the onset of anaerobic metabolism were reached close to 1 or 30°C, respectively (see Figs. 2 and 4). Lactate accumulated as the main anaerobic end product in crustaceans (1, 19, 20, 43). In addition, an increase in succinate concentrations indicated mitochondrial anaerobiosis (35). Because of the large interval between sampling temperatures and the fast rate of temperature change, a more precise quantification of Tc I and Tc II is not possible. For survival in the natural environment, however, the thresholds Tp I and Tp II may have greater importance (see below).
The processes causing tissue hypoxia appeared to be similar at low and high temperatures. Between 8 and 0°C, a reduction in heart rate, ventilation rate, and PO2 occurred with similar slopes (10 ± 1.7%/°C). Cooling caused whole animal metabolism and all O2-consuming and O2-delivering processes (ventilation rate and hemolymph circulation) to slow down. Finally, insufficient performance of ventilation and circulation caused tissue hypoxia and transition to anaerobiosis at the critical temperature before torpor and death. Accumulated amounts of anaerobic end products were small (Fig. 4), reflecting the low metabolic rate and, possibly, a considerable fraction of aerobic metabolism just below Tc I.
Reduced O2 availability also became visible during warming above 17°C. Heart rate increased but no longer followed the Q10 relationship. Ventilation rate and, therefore, O2 supply no longer continued to rise above 17°C. Again, limited performance of ventilation and circulation contributed to the decrease in PO2 observed between 17 and 30°C. Above 30°C, tissue hypoxia caused lactate and succinate to accumulate in different tissues and heart beat and ventilation started to collapse. The concentrations of lactate found at 33.3°C after progressive warming were of the same order of magnitude as reported for different decapod species after 6-12 h of exposure to environmental hypoxia at intermediate temperatures (20, 21, 50). Survival under these anaerobic conditions will depend on the combination of both the duration of exposure and temperature.
We believe that these findings may have general relevance for
understanding thermal tolerance and, possibly, for explaining the
geographic distribution of marine invertebrates (see below). The
ecological concept of the "law of tolerance" of Shelford (38, 39) was defined according to the range of tolerance to abiotic factors like temperature, humidity, or light. The optimum in the middle
of the tolerance range is encompassed by both a low and a high pejus
range (Ref. 37; pejus = turning worse, becoming deleterious)
during transition to upper and lower limits. Within pejus ranges
survival is still possible, but performance is restricted. Our data
suggest that it is possible to use physiological break points within
this concept. In Maja squinado, an optimum range was found
between 8 and 17°C (Fig. 5). This range
is characterized by maximum PO2 and represents
the temperature window of maximum scope for aerobic activity. It
therefore indicates the range of optimum performance supporting
successful survival in the natural environment. We suggest that those
thresholds limiting the optimum range be called "pejus
temperatures," Tp I (8°C) and
Tp II (17°C). Beyond Tp I and
Tp II, falling PO2 values indicate a reduced scope of aerobic energy production until critical temperatures Tc I and Tc II are reached,
characterized by low PO2 values and the
transition to anaerobic metabolism (Fig. 5). Finally, pessimum ranges
(pessimum = worst, lethal range) extend beyond critical
temperatures and the limits of tolerance. Animals are still able to
survive within the physiological tolerance range between
Tc I and Tc II, however, with largely reduced
scope for activity toward both extremes (pejus ranges).
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As a corollary, limited ventilation and circulation performance leads to an O2 limitation of thermal tolerance at both low and high temperature extremes. The development of hemolymph PO2 with changing temperature characterizes the range of optimum O2 availability as well as these O2 limitations. Pejus temperatures define the transition to low and high ranges of progressively decreased O2 availability (pejus ranges) until critical temperatures finally indicate tissue hypoxia and onset of anaerobic metabolism. Future study must investigate these ranges and thresholds in a wide range of animal species to test the general applicability of concepts and terms used in the present paper.
Perspectives
Two key questions characterize the perspectives originating from this study.What is the ecological relevance of an oxygen-limited thermal tolerance and associated threshold (i.e., pejus and critical) temperatures? As a preliminary answer, the pejus temperatures at 8 and 17°C agree well with the temperature fluctuations of the English Channel, which is the natural environment of the investigated population. Bottom (60 m) mean temperatures vary between 9.1°C in winter and 16.0°C in summer (10, 42). It thus appears possible that pejus temperatures, i.e., those temperatures limiting the range of maximum performance and scope for activity, are equivalent to those defining the limits of geographic distribution. However, the animals used in the present study were acclimated to only one (mean) temperature, and the experimental protocol led to the determination of the acute tolerance window. This window may shift between seasons depending on thermal acclimation. This immediately leads to the second question.
Which mechanisms may be responsible for setting pejus and critical temperatures depending on acclimation, season, and latitude? In the lugworm Arenicola marina, it has been demonstrated that both critical temperatures shift in parallel, as shown for two different populations in a latitudinal cline (40, 41). Higher mitochondrial densities were found in cold-adapted populations of Arenicola marina (41) and in cold-adapted perciform fishes (24). Our current hypothesis is that a downward shift in critical (and possibly pejus) temperatures involves a rise in mitochondrial density and tissue aerobic capacity, whereas an upward shift is linked to a decrease in these parameters (35, 36). Mitochondrial proliferation in the cold supports maintenance of sufficient aerobic capacity and function in all tissues, including those responsible for ventilation and circulation. In the warm, however, O2 demand by mitochondria may become limiting, especially at high densities. Proton leakage at the inner mitochondrial membrane is suggested to represent the cost of mitochondrial maintenance, leading to a higher O2 demand with a higher number of mitochondria present and, accordingly, a downward shift of upper critical (and possibly pejus) temperatures in cold-adapted animals (35, 36). Data on O2 consumption in fiddler crabs, Uca sp., from different latitudes (46, 47) suggest that the process of cold adaptation and an associated rise in metabolic rate take place in decapod crustaceans as well. Accordingly, thermal limits of in vivo heart rates varied between species of porcelain crabs depending on the temperatures experienced at different vertical levels in the intertidal zone (44). However, it remains to be investigated to what extent Maja squinado would be able to shift its range of thermal tolerance in accordance with its wide range of geographic distribution.
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ACKNOWLEDGEMENTS |
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The technical support of D. Storch and I. Pinz during tissue sampling and metabolite measurements is gratefully acknowledged.
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FOOTNOTES |
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Present address of M. Frederich: Brigham and Women's Hospital, Harvard Medical School, Boston, MA 02115.
Address for reprint requests and other correspondence: H. O. Pörtner, Alfred-Wegener-Institute for Polar and Marine Research, Columbusstrasse, 27568 Bremerhaven, Germany (E-mail: hpoertner{at}AWI-Bremerhaven.de).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 6 December 1999; accepted in final form 25 May 2000.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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) and 17-40°C (
).
