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Arctic shelf zooplankton communities are dominated by the copepod Calanus glacialis. This species feeds in surface waters during spring and summer and accumulates large amounts of lipids. Autumn and winter are spent in dormancy in deeper waters. Lipids are believed to play a major role in regulating buoyancy, however, they cannot explain fine-tuning of the depth distribution. To investigate whether ion exchange processes and acid-base regulation support ontogenetic migration as suggested for Antarctic copepods, we sampled C. glacialis in monthly intervals for 1 yr in a high-Arctic fjord and determined cation concentrations and the extracellular pH (pHe) in its hemolymph. During the winter/spring transition, prior to the upward migration of the copepods, Li+ ions were exchanged with cations (Na+, Mg2+, and Ca2+) leading to Li+ concentrations of 197 mmol L−1. This likely decreased the density and promoted upward migration in C. glacialis. Our data thus suggest that Li+ has a biological function in this species. Ion and pHe regulation in the hemolymph were not directly correlated, but the pHe revealed a seasonal pattern and was low (5.5) in winter and high (7.9) in summer. Low pHe during overwintering might be related to metabolic depression and thus, support diapause.

The sensitivity of copepods to ocean acidification (OA) and warming may increase with time, however, studies >10 days and on synergistic effects are rare. We therefore incubated late copepodites and females of two dominant Arctic species, Calanus glacialis and Calanushyperboreus, at 0 °C at 390 and 3000 μatm pCO₂ for several months in fall/winter 2010. Respiration rates, body mass and mortality in both species and life stages did not change with pCO₂. To detect synergistic effects, in 2011 C. hyperboreus females were kept at different pCO₂ and temperatures (0, 5, 10 °C). Incubation at 10°C induced sublethal stress, which might have overruled effects of pCO₂. At 5 °C and 3000 μatm, body carbon was significantly lowest indicating a synergistic effect. The copepods, thus, can tolerate pCO₂ predicted for a future ocean, but in combination with increasing temperatures they could be sensitive to OA.

The combined effects of ocean warming and acidification were compared in larvae from two populations of the cold-eurythermal spider crab Hyas araneus, from one of its southernmost populations (around Helgoland, southern North Sea, 54A degrees N, habitat temperature 3-18A degrees C; collection: January 2008, hatch: January-February 2008) and from one of its northernmost populations (Svalbard, North Atlantic, 79A degrees N, habitat temperature 0-6A degrees C; collection: July 2008, hatch: February-April 2009). Larvae were exposed to temperatures of 3, 9 and 15A degrees C combined with present-day normocapnic (380 ppm CO(2)) and projected future CO(2) concentrations (710 and 3,000 ppm CO(2)). Calcium content of whole larvae was measured in freshly hatched Zoea I and after 3, 7 and 14 days during the Megalopa stage. Significant differences between Helgoland and Svalbard Megalopae were observed at all investigated temperatures and CO(2) conditions. Under 380 ppm CO(2), the calcium content increased with rising temperature and age of the larvae. At 3 and 9A degrees C, Helgoland Megalopae accumulated more calcium than Svalbard Megalopae. Elevated CO(2) levels, especially 3,000 ppm, caused a reduction in larval calcium contents at 3 and 9A degrees C in both populations. This effect set in early, at 710 ppm CO(2) only in Svalbard Megalopae at 9A degrees C. Furthermore, at 3 and 9A degrees C Megalopae from Helgoland replenished their calcium content to normocapnic levels and more rapidly than Svalbard Megalopae. However, Svalbard Megalopae displayed higher calcium contents under 3,000 ppm CO(2) at 15A degrees C. The findings of a lower capacity for calcium incorporation in crab larvae living at the cold end of their distribution range suggests that they might be more sensitive to ocean acidification than those in temperate regions.

Marine brachyuran and anomuran crustaceans are completely absent from the extremely cold (− 1.8 °C) Antarctic continental shelf, but caridean shrimps are abundant. This has at least partly been attributed to low capacities for magnesium excretion in brachyuran and anomuran lithodid crabs ([Mg2+]HL = 20–50 mmol L− 1) compared to caridean shrimp species ([Mg2+]HL = 5–12 mmol L− 1). Magnesium has an anaesthetizing effect and reduces cold tolerance and activity of adult brachyuran crabs. We investigated whether the capacity for magnesium regulation is a factor that influences temperature-dependent activity of early ontogenetic stages of the Sub-Antarctic lithodid crab Paralomis granulosa. Ion composition (Na+, Mg2+, Ca2+, Cl−, SO42−) was measured in haemolymph withdrawn from larval stages, the first and second juvenile instars (crabs I and II) and adult males and females. Magnesium excretion improved during ontogeny, but haemolymph sulphate concentration was lowest in the zoeal stages. Neither haemolymph magnesium concentrations nor Ca2+:Mg2+ ratios paralleled activity levels of the life stages. Long-term (3 week) cold exposure of crab I to 1 °C caused a significant rise of haemolymph sulphate concentration and a decrease in magnesium and calcium concentrations compared to control temperature (9 °C). Spontaneous swimming activity of the zoeal stages was determined at 1, 4 and 9 °C in natural sea water (NSW, [Mg2+] = 51 mmol L− 1) and in sea water enriched with magnesium (NSW + Mg2+, [Mg2+] = 97 mmol L− 1). It declined significantly with temperature but only insignificantly with increased magnesium concentration. Spontaneous velocities were low, reflecting the demersal life style of the zoeae. Heart rate, scaphognathite beat rate and forced swimming activity (maxilliped beat rate, zoea I) or antennule beat rate (crab I) were investigated in response to acute temperature change (9, 6, 3, 1, − 1 °C) in NSW or NSW + Mg2+. High magnesium concentration reduced heart rates in both stages. The temperature–frequency curve of the maxilliped beat (maximum: 9.6 beats s− 1 at 6.6 °C in NSW) of zoea I was depressed and shifted towards warmer temperatures by 2 °C in NSW + Mg2+, but antennule beat rate of crab I was not affected. Magnesium may therefore influence cold tolerance of highly active larvae, but it remains questionable whether the slow-moving lithodid crabs with demersal larvae would benefit from an enhanced magnesium excretion in nature.

The herbivorous Antarctic copepod Calanoides acutus overwinters inactively in a resting stage (diapause) at depths below 500 m. It is assumed that during diapause C. acutus is neutrally buoyant in order to retain energy reserves otherwise depleted by swimming activities. However, so far, no experimental observations on its buoyancy have been reported and our knowledge of buoyancy regulation mechanisms is incomplete. In the present study, species-specific differences in buoyancy were assessed visually. Observations were made of speci- mens from the diapausing cohort of C. acutus and com- pared to another herbivorous copepod Calanus propinquus, which overwinters actively feeding in the upper water layers. Freshly caught copepods were anaesthetized in a 3-amino-benzoic acid ethyl ester (MS222) in seawater solution in order to exclude the influence of swimming movements on buoyancy control. It was shown that C. propinquus was negatively buoyant, whereas diapausing C. acutus remained neutrally buoyant. This is the first record that neutral buoyancy in diapausing copepods is main- tained by the biochemical body composition without the additional need of swimming movements.

To test the hypothesis that copepods are able to regulate their buoyancy via altering their ionic content, we analyzed both the cation concentration in the hemolymph of Antarctic pelagic copepod species in late winter and the vertical distribution pattern and population structure. High concentrations of ammonia/ammonium (NH3/ NH z ) were measured only in the hemolymph of Calanoides acutus, an Antarctic copepod definitely known to undergo vertical ontogenetic migrations and diapause at great depth, and in Rhincalanus gigas, in which a vertical ontogenetic migration pattern associated with diapause is still under debate. None of the other investigated species showed elevated ammonium concentrations in their hemolymph. We suggest that ion replacement by ammonium contributes to neutral buoyancy in diapausing calanoid copepods in the Southern Ocean. We hypothesize that ammonium buoyancy changes with season and is associated with shifts in extracellular pH and, therefore, most likely mediates metabolic depression during diapause. Copepods are dominant members of the zooplankton biomass worldwide (Longhurst 1985), and owing to their high production:biomass ratio of 4.4 (Voronina 1998) they contribute greatly to the total zooplankton production. In Antarctic waters the dominance of copepods is mainly due to large calanoid species such as Rhincalanus gigas, Calanus propinquus, Calanoides acutus, Metridia gerlachei ,a nd Paraeuchaeta spp., which can make up more than 50% of the total zooplankton biomass (Boysen-Ennen et al. 1991). The annual production of these large copepods significantly exceeds that of the Antarctic krill Euphausia superba (Voronina 1998). In polar seas herbivorous calanoid copepod species are strongly affected by the distinct seasonality in primary production and have developed specific behavioral and life history strategy adaptations to survive long periods of food scarcity. Species belonging to the Eucalanidae and Calanidae families are known to undergo an extensive ontogenetic vertical migration: They descend out of the surface waters as late copepodite stages to greater depths ($ 500 m), where they remain in a resting stage (diapause) for several months (Conover and Huntley 1991; Dahms 1995; Schnack-Schiel 2001). During this time they have severely reduced respiration and swimming activity (Schnack-Schiel et al. 1991; Dahms 1995), suggesting that they have little active control over their vertical distribution. This invokes a rather fundamental question: How can an animal with reduced metabolism and swimming activity remain at a particular depth layer for such long periods of time? For this to happen the animal must be neutrally buoyant at that depth.

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.

It is currently under debate whether organisms that regulate their acid–base status under environmental hypercapnia demand additional energy. This could impair animal fitness, but might be compensated for via increased ingestion rates when food is available. No data are yet available for dominant Calanus spp. from boreal and Arctic waters. To fill this gap, we incubated Calanus glacialis at 390, 1120, and 3000 µatm for 16 d with Thalassiosira weissflogii (diatom) as food source on-board RV Polarstern in Fram Strait in 2012. Every 4 d copepods were subsampled from all CO2 treatments and clearance and ingestion rates were determined. During the SOPRAN mesocosm experiment in Bergen, Norway, 2011, we weekly collected Calanus finmarchicus from mesocosms initially adjusted to 390 and 3000 µatm CO2 and measured grazing at low and high pCO2. In addition, copepods were deep frozen for body mass analyses. Elevated pCO2 did not directly affect grazing activities and body mass, suggesting that the copepods did not have additional energy demands for coping with acidification, neither during long-term exposure nor after immediate changes in pCO2. Shifts in seawater pH thus do not seem to challenge these copepod species.

Temperature-induced pH changes in white muscle tissue of three eelpout populations with different levels of eurythermy (the cold stenothermal Antarctic species Pachycara brachycephalum and the temperate eelpout Zoarces viviparus from the North Sea and the Baltic Sea) were monitored online by use of in vivo 31 P-NMR in unrestrained, unanaesthetized fish. An intracellular pH (pH i ) change of around −0.015 pH units/°C was observed in all eelpout populations in accordance with the α -stat hypothesis. The pH change was completed earlier (within 4 h) in the stenothermal Antarctic eelpout than in the Baltic population (within 8 h) and latest (not within 12 h) in the eurythermal North Sea population. These findings confirm the hypothesis that the kinetics of temperature-dependent pH i regulation is reflected by the relative contribution of active and passive processes to a temperature-induced pH change. The extent of passively induced pH i changes is in line with the general hypothesis that the temperature-dependent adjustment of pH i occurs mostly by active mechanisms in eurythermal animals, whereas in stenothermal animals pH changes are largely elicited by passive processes. Temperature changes had no influence on high-energy phosphates like phosphocreatine and ATP or on the Gibbs free energy change of ATP hydrolysis (Δ G /Δ ξ ).

Effects of global warming on animal distribution and performance become visible in many marine ecosystems. The present study was designed to develop a concept for a cause and effect understanding with respect to temperature changes and to explain ecological findings based on physiological processes. The concept is based on a wide comparison of invertebrate and fish species with a special focus on recent data obtained in two model species of fish. These fish species are both characterized by northern and southern distribution limits in the North Atlantic: eelpout (Zoarces viviparus), as a typical non-migrating inhabitant of the coastal zone and the cod (Gadus morhua), as a typical inhabitant of the continental shelf with a high importance for fisheries. Mathematical modelling demonstrates a clear significant correlation between climate induced temperature fluctuations and the recruitment of cod stocks. Growth performance in cod is optimal at temperatures close to 10°C, regardless of the population investigated in a latitudinal cline. However, temperature specific growth rates decrease at higher latitudes. Also, fecundity is less in White Sea than in North and Baltic Sea cod or eelpout populations. These findings suggest that a cold-induced shift in energy budget occurs which is unfavorable for growth performance and fecundity. Thermal tolerance limits shift depending on latitude and are characterized by oxygen limitation at both low or high temperatures. Oxygen supply to tissues is optimized at low temperature by a shift in hemoglobin isoforms and oxygen binding properties to lower affinities and higher unloading potential. Protective stimulation of heat shock protein synthesis was not observed. According to a recent model of thermal tolerance the downward shift of tolerance limits during cold adaptation is associated with rising mitochondrial densities and, thus, aerobic capacity and performance in the cold, especially in eurythermal species. At the same time the costs of mitochondrial maintenance reflected by mitochondrial proton leakage should rise leaving a lower energy fraction for growth and reproduction. The preliminary conclusion can be drawn that warming will cause a northern shift of distribution limits for both species with a rise in growth performance and fecundity larger than expected from the Q10 effect in the north and lower growth or even extinction of the species in the south. Such a shift may heavily affect fishing activities in the North Sea.

A low capacity for regulation of extracellular Mg(2+) has been proposed to exclude reptant marine decapod crustaceans from temperatures below 0°C and thus to exclude them from the high Antarctic. To test this hypothesis and to elaborate the underlying mechanisms in the most cold-tolerant reptant decapod family of the sub-Antarctic, the Lithodidae, thermal tolerance was determined in the crab Paralomis granulosa (Decapoda, Anomura, Lithodidae) using an acute stepwise temperature protocol (-1°, 1°, 4°, 7°, 10°, and 13°C). Arterial and venous oxygen partial pressures (Po(2)) in hemolymph, heartbeat and ventilation beat frequencies, and hemolymph cation composition were measured at rest and after a forced activity (righting) trial. Scopes for heartbeat and ventilation beat frequencies and intermittent heartbeat and scaphognathite beat rates at rest were evaluated. Hemolymph [Mg(2+)] was experimentally reduced from 30 mmol L(-1) to a level naturally observed in Antarctic caridean shrimps (12 mmol L(-1)) to investigate whether the animals remain more active and tolerant to cold (-1°, 1°, and 4°C). In natural seawater, righting speed was significantly slower at -1° and 13°C, compared with acclimation temperature (4°C). Arterial and venous hemolymph Po(2) increased in response to cooling even though heartbeat and ventilation beat frequencies as well as scopes decreased. At rest, ionic composition of the hemolymph was not affected by temperature. Activity induced a significant increase in hemolymph [K(+)] at -1° and 1°C. Reduction of hemolymph [Mg(2+)] did not result in an increase in activity, an increase in heartbeat and ventilation beat frequencies, or a shift in thermal tolerance to lower temperatures. In conclusion, oxygen delivery in this cold-water crustacean was not acutely limiting cold tolerance, and animals may have been constrained more by their functional capacity and motility. In contrast to earlier findings in temperate and subpolar brachyuran crabs, these constraints remained insensitive to changing Mg(2+) levels.

Characteristics of temperature-dependent metabolic adaptation, as well as their impli- cations for climate-dependent energy budgets, biogeography and fitness are reviewed and analysed for populations of Atlantic cod Gadus morhua in relation to findings in other fish species from north- ern and southern hemispheres, especially various species of eelpout (Zoarcidae). The analysis builds on the recently posited concept of oxygen- and capacity-dependent thermal tolerance in aquatic ectotherms. Mechanistic physiological studies are used to explain both functional differences between populations and current observations (such as the northward movement of cod, or the changes in seasonal abundance of eelpout due to global warming). Available data support the hypothesis that natural selection favours individuals that maximize growth and energy efficiency at the expense of ranges of thermal tolerance. The levels of energy turnover are subject to the con- straints of resource availability and temperature variability. Temperature variability in the cold, such as in the sub-Arctic, causes a rise in maintenance costs at the expense of growth, but possibly in favour of motility, and thus of foraging capacity. These different trends are mirrored in higher capac- ities for gene expression of key enzymes involved in aerobic metabolism (during cold acclimation) in northern as compared to southern cod populations of the East Atlantic. However, detailed patterns, as well as many of the underlying mechanisms, remain underexplored, especially with respect to the suggested hierarchy of energy allocation to energy budget components.

The hypothesis of an oxygen-limited thermal tolerance due to restrictions in cardiovascular performance at extreme temperatures was tested in Atlantic cod, Gadus morhua (North Sea). Heart rate, changes in arterial and venous blood flow, and venous oxygen tensions were determined during an acute temperature change to define pejus (“getting worse”) temperatures that border the thermal optimum range. An exponential increase in heart rate occurred between 2 and 16°C (Q10= 2.38 ± 0.35). Thermal sensitivity was reduced beyond 16°C when cardiac arrhythmia became visible. Flow-weighted magnetic resonance imaging (MRI) measurements of temperature-dependent blood flow revealed no exponential but a hyperbolic increase of blood flow with a moderate linear increase at temperatures >4°C. Therefore, temperature-dependent heart rate increments are not mirrored by similar increments in blood flow. Venous Po2(PvO2), which reflects the quality of oxygen supply to the heart of cod (no coronary circulation present), followed an inverse U-shaped curve with highest PvO2levels at 5.0 ± 0.2°C. Thermal limitation of circulatory performance in cod set in below 2°C and beyond 7°C, respectively, characterized by decreased PvO2. Further warming led to a sharp drop in PvO2beyond 16.1 ± 1.2°C in accordance with the onset of cardiac arrhythmia and, likely, the critical temperature. In conclusion, progressive cooling or warming brings cod from a temperature range of optimum cardiac performance into a pejus range, when aerobic scope falls before critical temperatures are reached. These patterns might cause a shift in the geographical distribution of cod with global warming.

Temporal pattern of hatching was studied in the subantarctic lithodid crabs Lithodes santolla (Molina) and Paralomis granulosa (Jaquinot) from the Argentine Beagle Channel. In both species, larval hatching occurred in low daily numbers over an extended period of up to several weeks, depending on hatch size. Low daily hatching activity and low oxygen-consumption rates in freshly hatched P. granulosa larvae are discussed as life history adaptations to, and/or physiological constraints by, the environmental conditions of high latitudes. Fil:Calcagno, J.A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina. Fil:Lovrich, G.A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.

We investigated the temperature dependence of some physiological parameters of common eelpout (Zoarces viviparus) from different locations (North Sea, Baltic Sea and Norwegian Sea) on acclimation temperature (3 degrees C and 12 degrees C) and acute temperature variation. The lethal limit of 12 degrees C-acclimated eelpout was determined as the critical thermal maximum [loss of equilibrium (LE) and onset of muscular spasms (OS)] and it was found to be 26.6 degrees C for LE and 28.8 degrees C for OS for all populations. However, these parameters do not have any relevant ecological interpretation. We therefore investigated the effect of gradually increased water temperature on standard metabolic rate (measured as resting oxygen consumption Mo2) and critical oxygen concentration ([O2]c) of eelpouts. Acclimation to low temperature (3 degrees C) resulted in partial compensation of Mo2, paralleled by a decrease of activation energy for Mo2 (from 82 kJ mol(-1) at 12 degrees C to about 50 kJ mol(-1) at 3 degrees C) in North Sea and Baltic Sea eelpouts. At the same time, Norwegian eelpout showed no acclimation of oxygen demand to warm temperature (12 degrees C) at all. The scope for eelpout aerobic metabolism shrank considerably with increased acclimation temperature, as [O2]c approached water oxygen concentrations. At 22.5+/-1 degrees C the [O2]c reached air saturation, which is equivalent to the upper critical temperature (TcII) and at this temperature the aerobic scope for the metabolism completely disappeared. In line with previous insight, the comparative analysis of the temperature dependence of Mo2 of Z. viviparus from different populations suggests that a pejus (sub-critical) temperature for this species is about 13-15 degrees C. In conclusion, the capacity to adjust aerobic metabolism relates to thermal tolerance and the bio-geographical distribution of the species. Global warming would thus be likely to cause a shift in the distribution of this species to the North.

In vivo Magnetic Resonance Imaging (MRI) and Spectroscopy (MRS) experiments were carried out in non-anaesthetized marine fish with a maximum body length of 40 cm using a 4.7 T horizontal magnet. In flow-through animal chambers long-term investigations could be carried out for several days in a sessile benthic zoarcid species, the eelpout Zoarces viviparus, as well as in a pelagic gadid species, the cod Gadus morhua. Sea water adapted probes and optimised excitation and refocusing pulse shapes reduced the negative effects of the highly conductive medium. In both species, in vivo 31P-NMR spectra were collected within 5 minutes with a reasonable signal to noise ratio (PCr/Pi ratio > 40 in cod) and typical line widths in the range of 20 to max. 40 Hz. Magnetic resonance images were recorded in the sessile species without movement artefacts. In these fish it was even possible to measure the blood flow of different vessels with a flow compensated gradient echo sequence and to carry out localized in vivo 1H-NMR spectroscopy in different organs.

Temperature-dependent adjustments of intracellular pH are thought to play a major role in the maintenance of protein function. Comparative studies were carried out in two species from the same fish family (Zoarcidae), the stenothermal Antarctic eelpout (Pachycara brachycephalum) and the eurythermal eelpout (Zoarces viviparus), to find out whether pH regulation is modified by temperature in the closely related species and to what extent the respective pattern differs between eurytherms and stenotherms. Previous invasive studies had compared individual animals sampled at various temperatures and suggested that a decrease in intracellular pH (pHi) values occurs at rising temperatures, as predicted by the alpha-stat hypothesis of acid-base regulation. The present study used non-invasive in vivo 31P-NMR spectroscopy in non-anaesthetized, unrestrained fish for long-term online recordings in individual specimens. Control spectra obtained at T= 0°C for P. brachycephalum and at 12°C for Z. viviparus indicated low stress conditions, as well as a high stability of energy and acid-base status over time periods longer than 1 week. Temperature changes had no influence on the concentration of high-energy phosphates like phos- phocreatine or ATP. Temperature-induced pH changes were monitored continuously in a range between 0 and 6°C for polar, and 12 and 18°C for temperate eelpout. A pHi change of around -0.015 pH units/°C was observed within both species, in accordance with the alpha-stat hypothesis; however, extrapolation to the same temperature revealed different set points of pH regulation in the two species. These findings confirm that an alpha-stat pattern of pH regulation can be found in stenothermal Antarctic animals, at set points deviating from an alpha-stat pattern, however, in a between-species comparison.

Reptant decapod crustaceans are almost absent from the Southern Ocean south of the Antarctic Convergence. We tested the hypothesis that this may be due to the reduced ability of this group to regulate Mg(2+) levels in the haemolymph ([Mg(2+)](HL)). Mg(2+) acts as an anaesthetic in marine invertebrates and its level is higher in Reptantia (crabs such as Cancer spp., Chionoecetes spp., Maja spp., 30–50 mmol l(−)(1)) than in Natantia (prawns such as Pandalus spp., Palaemon spp., Crangon spp., 5–12 mmol l(−)(1)). We varied [Mg(2+)](HL) in three species of reptant decapod crustaceans, Carcinus maenas, Hyas araneus and Eurypodius latreillei, and investigated heart rate, the rate of oxygen consumption and levels of spontaneous and forced activity at different temperatures. The rate of oxygen consumption and heart rate increased significantly with reduction in [Mg(2+)](HL) over the entire temperature range investigated in E. latreillei. In H. araneus, an increase in metabolic and heart rates compared with control values was found only at temperatures below 2 degrees C. Forced and spontaneous activity levels increased significantly in the group of [Mg(2+)](HL)-reduced animals below 0 degrees C, at which control animals were mostly inactive. At a reduced [Mg(2+)](HL) of 5–12 mmol l(−)(1), which is the [Mg(2+)](HL) of caridean shrimps in the Southern Ocean, Q(10) and activation energy were reduced for all these variables and extended the temperature range over which physiological functions were maintained. We suggest that the high [Mg(2+)](HL) in Reptantia causes relaxation of the animals and reduces their scope for activity, especially at temperatures below 0 degrees C. The hypothesis that the synergistic effects of high [Mg(2+)](HL) and low temperature probably prevented the Reptantia from recolonizing the permanently cold water of polar areas is discussed.

Temperature is considered to be one of the most important abiotic factors shaping marine ecosystems due to its major impact on all biological processes. Therefore, low or high temperature extremes characterise the limits of geographical distribution of many species, and global change has already caused a change in the distribution of species (Southward, Hawkins & Burrows, 1995). An investigation of marine ectotherms surviving in seasonally or permanently cold ocean environments and their comparison with ectotherms from temperate and warm waters should help to reveal those biochemical or physiological mechanisms which determine geographical distribution limits. These studies should also reveal which molecular, cellular and systemic functions have been shifted to levels compatible with the steadystate maintenance of all life-sustaining processes in the cold. In permanent cold, the latter must also include growth and reproduction, whereas during seasonal cold exposure these processes may be suspended. Adaptation to cold started from life forms that evolved in warm waters (e.g. Arntz, Brey & Gallardo, 1994; Thiel, Portner & Arntz, 1996). Therefore, the characteristics of the adaptational process must be seen in the light of this evolutionary trend. Cold adaptation then becomes a special physiological feature rather than a basic ability of all life forms. If life conquered the cold after having evolved in warm waters, the question arises about what were the limiting factors in this adaptational process and how would these limiting factors affect the whole organism, thereby preventing an easy access to cold ocean environments.

The functional relevance of oxygen transport by hemocyanin of the Antarctic octopod Megaleledone senoi and of the eurythermal cuttlefish Sepia officinalis was analyzed by continuous and simultaneous recordings of changes in pH and hemocyanin oxygen saturation in whole blood at various temperatures. These data were compared to literature data on other temperate and cold-water cephalopods (octopods and giant squid). In S. officinalis, the oxygen affinity of hemocyanin changed at deltaP50/degrees C = 0.12 kPa (pH 7.4) with increasing temperatures; this is similar to observations in temperate octopods. In M. senoi, thermal sensitivity was much smaller (0.01 kPa, pH 7.2). Furthermore, M. senoi hemocyanin displayed one of the highest levels of oxygen affinity (P501 kPa, pH 7.6, 0 degrees C) found so far in cephalopods and a rather low cooperativity (n50 = 1.4 at 0 degrees C). The pH sensitivity of oxygen binding (delta log P50/delta pH) increased with increasing temperature in both the cuttlefish and the Antarctic octopod. At low PO2 (1.0 kPa) and pH (7.2), the presence of a large venous oxygen reserve (43% saturation) insensitive to pH reflects reduced pH sensitivity and high oxygen affinity in M. senoi hemocyanin at 0 degrees C. In S. officinalis, this reserve was 19% at pH 7.4, 20 degrees C, and 1.7 kPa O2, a level still higher than in squid. These findings suggest that the lower metabolic rate of octopods and cuttlefish compared to squid is reflected in less pH-dependent oxygen transport. Results of the hemocyanin analysis for the Antarctic octopod were similar to those reported for Vampyroteuthis--an extremely high oxygen affinity supporting a very low metabolic rate. In contrast to findings in cold-adapted giant squid, the minimized thermal sensitivity of oxygen transport in Antarctic octopods will reduce metabolic scope and thereby contribute to their stenothermality.