H.O. Pörtner

Alfred-Wegener-Institute, Bremerhaven, Germany

Recent years have shown a rise in mean global temperatures and a shift in the geographical distribution of ectothermic animals. For a cause and effect analysis the present paper discusses those physiological processes that may limit thermal tolerance and thereby, geographical distribution in various phyla. These comparative studies, largely carried out on marine ectotherms, support a general view which links theprocesses and limits of thermal tolerance with the adjustment of aerobic scope and capacity as a crucial event in thermal adaptation. At temperatures above or below ambient aerobic scope of the animal starts to be reduced indicated by falling oxygen levels in the body fluids and the progressively insufficient capacity of circulatory and ventilatory mechanisms. At high temperatures, excessive oxygen demand causes decreasing oxygen levels in the body fluids, whereas at low temperatures the aerobic capacity of mitochondria may become limiting for ventilation and circu! lation. In accordance with the law of tolerance the onset of a drop in whole animal aerobic scope characterises low and high pejus thresholds. Further cooling or warming beyond these limits finally leads to low or high critical threshold temperatures (Tc) where aerobic scope is nil and transition to an anaerobic mode of mitochondrial metabolism is linked to progressive insufficiency of cellular energy levels. The adjustments of mitochondrial densities and their functional properties appear as a critical process in defining and shifting thermal tolerance windows on top of parallel adjustments at the molecular or membrane level. The finding of an oxygen limitation of thermal tolerance is in line with the concept of symmorphosis, i.e. the components and functional capacities of oxygen delivery are set to be optimal between the average highs and lows of environmental temperatures. Depending on the regular occurrence of more extreme temperatures only time limited survival is supp! orted by anaerobic metabolism and the delayed thermal denaturation of molecular functions owing to increased membrane or molecular stability, e.g. during protection by heat shock proteins and antioxidative defence. As a corollary, the first line of thermal sensitivity is due to capacity limitations, before individual, molecular or membrane functions become disturbed. These conclusions are in line with the general consideration that, as a result of the high level of complexity of metazoan organisms compared to prokaryotes and simple eukaryotes, thermal tolerance is reduced and set at a high complexity level, i.e. the overall capacity and integrated function of the oxygen delivery system.

Temperature preference in Daphnia – Hb or not Hb.

Paul Wiggins

Department of Zoology, La Trobe University, 3086, Victoria, AUSTRALIA
(wiggins@zoo.latrobe.edu.au)

Laboratory experiments have shown that haemoglobin (Hb)-rich Daphnia carinata (3 days exposure to hypoxia = 70 torr in the lab), when given a choice, prefer a higher temperature (19 °C) in hypoxia (70 torr) than their Hb-poor counterparts (16 °C) (Wiggins & Frappell 2000). This difference between cohorts is not found in normoxia (150 torr) where both have a preferred T_a of 23 °C. Furthermore, both Hb-rich and Hb-poor animals choose the same Ta in hyperoxia (PO₂ = 220 torr) as in normoxia. While these observations are interesting in themselves, they reveal little about the ecological significance of such differences in Hb-content. Long-term monitoring at three depths in a small, shallow, intermittently flowing freshwater body has revealed that very different PO₂ and dissolved oxygen (D.O.) profiles can occur, even across depth changes of less than one metre. Netting at fixed time intervals has revealed seasonal differences in diel vertical migration behaviour of Daphnia carinata. It has also been demonstrated that Hb-content differences of 150-200 % can occur in Daphnia cohorts taken from depths differing by less than half a metre. These observations have paved the way for further laboratory studies which reveal that temperature preference in Daphnia appears to depend on an animal's Hb-content and that Hb-content depends upon both the T_a and D.O. profile of the animal's environment. (partly funded by the ARC; PW is the recipient of an Australian Postgraduate Award).

Temperature preference, hypoxia, fat rats and a small marsupial capable of torpor.

Peter Frappell

Department of Zoology, La Trobe University, Melbourne, Victoria, 3086, AUSTRALIA
(p.frappell@latrobe.edu.au)

It is well established that mammalian species show a definite thermal preference when free to do so. In addition, it is well known that hypoxia attenuates thermogenesis and elicits hypothermia, particularly in smaller species. Further, many of these species when challenged with hypoxia will select a lower ambient temperature. The downward shift of the threshold temperature for thermogenesis, the selection of a lower ambient temperature and the lowering of body temperature that accompany hypoxia have been interpreted as a change in the thermal set-point rather than a deficit in thermoregulation. Thus behavioural thermoregulation, when it is applicable, may contribute to reducing the demand for oxygen at a time when oxygen is limited. In this address the behavioural response on exposure to hypoxia is examined in animals that naturally possess or have the capability to lower the thermal set-point: (1) obese Zucker rats that have a lower core temperature than their lean littermates and (2) Sminthopsis crassicaudata, a marsupial that enters daily torpor. (In part-collaboration with M. Maskrey and L. Withers. Funded by the ARC)

Temperature preference and behaviour of fish exposed to hypoxia

John Fleng Steffensen

Marine Biological Laboratory, University of Copenhagen, Denmark

Temperature preference was determined by using a computerized 2 compartments shuttle box. The position of the fish was determined with on-line video-analysis with a frame grabber. Based on the position of the fish the experimental temperature was either increased or decreased, with the aid of a computer controlling relays, solenoids and pumps. In addition the two compartments were constantly kept with a temperature difference of one °C. Based on the temperature, the time when leaving/entering one of the two compartments and the size of the fish, the temperature of the fish was calculated and stored in the PC.
The oxygen level in the shuttle box was measured with an oxygen electrode. The oxygen level could be controlled with a PC and a solenoid valve administering nitrogen.
The preferred temperature of Atlantic cod, Gadus morhua, decreased from 13.9 °C when exposed to normoxia to 8.8 °C when exposed to severe hypoxia at 15 % oxygen saturation. The 5.1 °C decrease in preferred temperature is equivalent with a decrease in oxygen consumption/requirement of the fish of about 40 %. By selecting a lower temperature at severe hypoxia the Atlantic cod can survive an otherwise lethal oxygen level.
Atlantic cod exposed to constant temperatures of 5, 10 and 15 °C decreased the swimming activity when exposed to progressive hypoxia.