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It is important to note that the change in species composition at oceanic boundaries such as the Antarctic Polar Front is not due to the short‐term lethal effects of temperature change. Instead, a suite of factors is involved, including inefficiencies in reproductive strategies and timing, metabolic inefficiency, absence of preferred prey, and competition from similar species for resources that result in the gradual demise of the replaced species. However, characterizing a species’ tolerance to temperature is highly instructive because it introduces two basic rules of physiological response to temperature and to other environmental challenges like salinity. The first rule is that the short‐term range of temperature tolerance within a species, population, or individual is not rigid or immutable; animals can adjust their range of tolerance over a period of time in response to changes in external temperature. The second rule is that upper and lower limits exist for all species that cannot be exceeded, even after allowing for biological adjustment.

The internal adjustment process that raises or lowers lethal limits takes time to accomplish and is described by two terms. When the adjustment phenomena take place in the natural habitat (e.g. seasonal temperature change), the process is called acclimatization. When adjustment is induced in the laboratory, the phenomenon is called acclimation.

The best way to define the level of eurythermicity in a species is using an approach that incorporates a species’ ability to biologically adjust its temperature range: the thermal tolerance polygon (Figure 2.2a). First introduced in 1952 by the Canadian fish physiologist John R. Brett, the tolerance polygon uses a rigorous experimental protocol to define the upper and lower lethal limits of a species. The lethal T°C was theoretically defined as that temperature at which 50% of a population could withstand for an infinite time. To determine this, a sample of fishes acclimated to a given temperature was subjected to a series of temperatures higher (or lower) levels of which resulted in complete mortality of the sample. The period of tolerance prior to death was termed the resistance time. In each instance, the logarithms of the median resistance time were plotted against temperature and the results formed a straight line (Figure 2.2b). The slope of this line is relatively consistent for most species. In every case, an abrupt change in slope occurred, indicating that mortality due to temperature had effectively ceased, marking the change from resistance to tolerance. That point was termed the incipient lethal temperature. Low and high incipient lethal temperatures were determined for each acclimation temperature to form the polygon shown in Figure 2.2a.

Figure 2.2 Thermal tolerance and lethal limits. (a) Thermal tolerance polygon. Upper and lower lethal limits of the sockeye salmon Oncorhynhcus nerka in relation to acclimation temperature. (b) Median resistance times of young chum salmon Oncorhynchus keta acclimated to the temperatures indicated.

Sources: (a) Adapted from Fry and Hochachka (1970), figure 2 (p. 81); (b) Brett (1952), figure 7 (p. 282).

The polygon for Oncorhynchus keta indicates that it is a fairly eurythermal species. Polygons for Antarctic species would encompass only a small fraction of the lower range, whereas highly eurythermal species such as the brown bullhead catfish (Ameriurus nebulosus) would be very much larger.

Studies of temperature tolerance in a variety of different organisms suggest the following.

1 Generally, upper and lower lethal limits can be modified considerably by different acclimation temperatures, e.g. the warmer the temperature of acclimation, the higher the upper lethal limit.

2 There are absolute upper and lower lethal limits beyond which an organism cannot adapt, and these limits can be determined with precision.

3 It takes longer to acclimate to cold temperatures than to warm ones.

4 The tolerance polygon of an organism relates well to habitat and geographic area, as shown in the example below.

In addition to knowing the zones of tolerance or the limits to survival of a species, it is important to understand the physiological responses of an organism to temperatures within its environmental range.