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Animals have varying reactions to temperature changes within their zone of tolerance. Reaction to temperature within an animal’s environmental range is usually assessed using a rate function, heartbeat for example, or a filtration rate for species such as clams that pump water through their feeding apparatus. Most commonly, metabolic rate is used; metabolism is an excellent index of an animal’s rate of energy consumption and is readily measured by monitoring an individual’s rate of oxygen consumption. The rate of increase or decrease in reaction rate over a T°C change is standardized by the Q₁₀ approximation, which is the factor by which a reaction velocity (e.g. rate of oxygen consumption) is increased for a rise of 10 °C.

(2.1)

where K₁ and K₂ are velocity constants corresponding to temperatures T₁ and T₂. Reaction velocity is generally used instead.

For virtually all rate functions in which we are interested, the biological rate increases by a factor of approximately 2 for each 10 °C rise in temperature: that is, Q₁₀ ≈ 2. However, the Q₁₀ of an animal’s metabolic rate varies slightly over a range of temperatures, being higher in the lower ranges. Therefore, when providing Q₁₀ for metabolic rates, it is imperative to also provide the temperature range over which the measurements were taken.

The fact that metabolic rate doubles with a temperature change of 10 °C (or halves with a 10 °C drop) has a profound effect on ectothermic species. A vertical migrator in the tropical ocean swimming from a depth of 500 m to near surface waters at sunset (a common occurrence, as we shall see) will encounter changes of >10 °C on its way up and again on its way back to depth. As a consequence, it will endure profound changes in its metabolic rate during each leg of its excursion.

In 1914, August Krogh, the father of comparative physiology, first attempted to define a pattern for the change in Q₁₀ with temperature by subjecting a narcotized goldfish to temperatures ranging between 0 and 25 °C and measuring its oxygen consumption rate. The curve he derived is called the “Normal Curve.” It was popularized considerably in later years when it was found that a similar relationship between metabolism and temperature existed for many species, with the exception of the large Q₁₀ in the 0–5 °C temperature interval (e.g. Winberg 1956). Even Krogh stated that his Q₁₀ value of 10.9 between 0 and 5 was “obviously erroneous.” The general trend was remarkably accurate though, as were the numbers generated above 5 °C. The Q₁₀’s this curve represents is shown below.

The Q₁₀ approximation is a fundamental molecular response to temperature: it applies to chemical reactions taking place in a beaker as well as to rate processes in ectothermic species. However, it is not intuitively obvious why reaction rates should double for every increase in a temperature of 10 °C. The answer is in a concept termed “activation energy,” which was pioneered by the Swedish physical chemist Svante Arrhenius, and which earned him a Nobel Prize in 1903.

In the realm of physical chemistry, temperatures are expressed in the absolute temperature scale, in degrees Kelvin. A Kelvin degree is equal to a degree Celsius, but the scale begins at absolute zero, the temperature where all molecular motion ceases: −273 °C. Thus, a temperature of 0 °C is equal to 273 K, and 20 °C is equal to 293 K; by convention, the degree symbol is not used for degrees Kelvin. The temperature range most relevant to the pelagic fauna, −2 to 40 °C (271–313 K), only covers about a 10% change of temperature on the absolute scale. In our range of concern, a change of 10 °C is roughly 3% of the absolute temperature. Why, then, do reaction rates double?

The breakthrough of Arrhenius was his idea that within a population of molecules, only a fraction have sufficient energy to be reactive: those that exceed the activation energy threshold (Figure 2.3). The average thermal energy of the molecules gives us the temperature, but it is not the average that is most important, it is the proportion of molecules that have enough energy to exceed the activation energy threshold and be competent to react. When heat is added to a system, the proportion of molecules that exceeds the activation energy increases more quickly than the average temperature. In fact, an increase in temperature of 10 K results in a doubling of the fraction of molecules exceeding the activation energy. The activation energy concept thus explains the Q₁₀s we observe with biological rates.

Experimentation in the 1940s, 1950s, and 1960s further defined temperature responses as a function of time and acclimation period. Three general time courses were identified.

1 Direct responses of rate functions to changes in temperature persisting for hours: acute measurements

Figure 2.3 Energy distribution curves for a population of molecules at two different temperatures. Only those molecules having energies equal to or greater than the activation energy are reactive.

Source: Hochachka and Somero (2002), figure 7.1 (p. 296). Reproduced with the permission of Oxford University Press.

Rates measured in this way, with no acclimation period, reflect the short‐term flexibility of biological systems. In some cases, acutely measured responses to temperature can show Q₁₀ values greatly different from 2. When metabolism is the rate being measured, such a response is termed “metabolic overshoot.” It would correspond to a “type 5” response described below. It is the result of a system that is still in the process of adjusting to a new temperature and it can happen in a transition to a warmer or colder temperature.

1 Compensatory acclimation to days or weeks of exposure: the acclimated response

An animal is acclimated only when its rate processes have stabilized to the new temperature. Acclimated animals were utilized in constructing the temperature tolerance polygon shown in Figure 2.2a.

1 Evolutionary adaptation through natural selection: climatic adaptation