Background
The critical thermal maximum (CTM) of an organism is the upper (warm) temperature at which the onset of behavioral incapacitation (usually loss of equilibrium) occurs and - in the words of the authors quoting Hutchison (1961) - "the animal loses its ability to escape from conditions that will promptly lead to its death." And although it sounds like a fixed immutable value for a given species, the CTM has been proven to be a function of temperature itself.

What was done
In a study of three species of freshwater mussels (Alasmidonta varicosa, Elliptio complanata and Strophitus undulatus) that had been acclimatized to a water temperature of either 15 or 25°C, Gailbraith et al. gradually raised the temperature of the water in which the mussels were immersed at a rate of 0.35°C per minute, until they observed the onset of mussel extreme gaping (a periodic valve movement of the mussels resulting in the rapid opening and closing their shells) that is indicative of their impending demise, unless relief is immediately applied to the benthic filter-feeders.

What was learned
The three U.S. researchers report that "responses varied by species, but mussels acclimated to 25°C generally had a higher CTM than mussels acclimated to 15°C." And for one of the three species (E. complanata), they observed that the effects of acclimation and another variable they threw into the mix (aeration) were interactive, leading them to conclude that "combinations of environmental stressors may influence thermal tolerance," and that "such responses vary among species." In addition, they state that results similar to theirs "are well documented for other freshwater organisms, particularly for fish (Becker and Genoway, 1979; Elliott, 1981) but also for the zebra mussel, Dreissena polymorpha (McMahon and Ussery, 1995; Lutterschmidt and Hutchison, 1997)."

What it means
Although multiple factors may thus come into play in determining an animal's current CTM, it is evident from the results of Galbraith et al. that some degree of warming above that species-specific value can at times lead to a longer-term increase in the species' CTM. And in the case of global warming, it suggests that species may well adjust their individual CTMs upwards in response to periodic heat waves or significantly warmer years that are superimposed upon the ploddingly-slow decadal to century-scale warming that is characteristic of the CO2-induced phenomenon envisioned by the world's climate alarmists.

References
Becker, C.D. and Genoway, R.G. 1979. Evaluation of the critical thermal maximum for determining thermal tolerance of freshwater fish. Environmental Biology of Fishes 4: 245-256.

Elliott, J.M. 1981. Thermal stress on freshwater teleosts. In: Pickering, A.D. (Ed.), Stress and Fish. Academic Press, New York, New York, USA, pp. 207-245.

Hutchison, V.H. 1961. Critical thermal maxima in salamanders. Physiological Zoology 34: 92-125.

Lutterschmidt, W.I. and Hutchison, V.H. 1997. The critical thermal maximum: history and critique. Canadian Journal of Zoology 75: 1561-1574.

McMahon, R.F. and Ussery, T.A. 1995. Thermal Tolerance of Zebra Mussels (Dreissena polymorpha) Relative to Rate of Temperature Increase and Acclimation Temperature. U.S. Army Corps of Engineers, Washington, D.C.