Physiological disturbances at critically high temperatures: A comparison between stenothermal Antarctic and eurythermal temperate eelpouts (Zoarcidae)

The effect of gradually increased water temperature on the metabolism of te mperate eelpout from the North Sea (Zoarces viviparus) and Antarctic eelpou t (Pachycara brachycephalum) was investigated. Standard metabolic rate (SMR ) was similar in cold-adapted P, brachycephalum and cold-acclimated Z, vivi parus in the low temperature range. This indicates that Antarctic eelpout s how no metabolic cold adaptation las originally defined by Wohlschlag); how ever, they do show a compensatory increase of oxygen consumption compared t o warm-acclimated eelpout, SMR increased more strongly with rising temperat ure in P, brachycephalum than in Z, viviparus, which is reflected in a high er Arrhenius activation energy for oxygen consumption (99+/-5 kJ mol(-1), v ersus 55+/-3 kJ mol(-1) for cold-acclimated Z, viviparus; means +/- S,D,), The intracellular pH in the white musculature of Z, viviparus follows alpha stat regulation over the whole investigated temperature range and dropped a t a rate of -0.016 pH units per degrees C between 3 degrees C and 24 degree s C. In Antarctic eelpout white muscle pH declined at a rate of -0.015 pH u nits per degrees C between 0 degrees C and 3 degrees C, but deviated from a lphastat at higher temperatures, indicating that thermal stress leads to ac id-base disturbances in this species. The upper critical temperature limit (Tc-II; characterised by a transition to anaerobic metabolism) was found to be between 21 degrees C and 24 degree s C for Z, viviparus and around 9 degrees C for P, brachycephalum, In both species a rise of succinate concentration in the liver tissue turned out to be the most useful indicator of Tc-II, Obviously, liver is more sensitive to heat stress than is white muscle, Accordingly, the energy status of whit e muscle is not diminished at Tc-II, Heat-induced hyperglycaemia was observ ed in Antarctic eelpout (at 9 degrees C and 10 degrees C), but not in commo n eelpout, Based on our results and on literature data, impaired respiratio n in combination with circulatory failure is suggested as the final cause o f heat death. Our data suggest that the southern distribution limit of Zoar ces viviparus is correlated with the limit of thermal tolerance. Therefore, it can be anticipated that global warming would cause a shift in the distr ibution of this species.