Temperature and the ecophysiology of marine ectotherms

Abstract

Temperature influences the physiology, behaviour, and distribution of organisms and the field of thermal ecology has been stimulated in recent decades by the need for a greater understanding of how climate change will affect Earth’s organisms. Although widely studied, a number of key knowledge gaps persist. I aim to contribute to these by investigating both fundamental and applied aspects of thermal ecology. The relationship between temperature and physiological rates is well studied and persistent patterns arise, such as that between temperature and metabolism. The thermal sensitivity of metabolism in ectotherms, consistently shows a Q10 value in the range of 2 – 3, indicating a doubling or tripling in metabolic rate with every 10°C increase in temperature. However, a small group of organisms have evolved specialised physiologies, allowing them to elevate their body temperature, thereby creating less of a connection between body temperature and ambient temperature. By studying the ecophysiology of these animals, we can explore how they break away from this metabolism-temperature constraint. But what are the ultimate drivers of these adaptations? In Chapter 2, I investigate two competing hypotheses generally proposed to explain the evolutionary drivers behind regional endothermy in fishes: thermal niche expansion and elevated cruising speeds. By compiling published biologging data and collecting precise speed measurements from free-swimming fishes in the wild, I directly tested whether endothermic fishes encounter broader temperature ranges, swim faster, or both. I found that regionally endothermic fishes do not encounter broader temperature ranges but that they swim at elevated cruising speeds, ≈ 1.6 times faster than their ectothermic counterparts, suggesting the significance of endothermy in fishes lies in the advantages it confers to swimming performance rather than facilitating occupation of broader thermal niches. Specialised physiology, such as this, is one way that animals have evolved to break the connection between body temperature and ambient temperature and break away from the constraints of the metabolic ‘Q10 effect’ (as expected under the Metabolic Theory of Ecology). However, it is increasingly realised that the aforementioned metabolic scaling ‘laws’ in fact exhibit significant variation and can be context dependent. Research to date has primarily focused on static temperature experiences (i.e., estimating metabolic rate at a range of constant temperatures). However, temperature in nature is rarely static, so our existing understanding from experiments may not truly reflect how temperature influences metabolism in natural systems. Using closed chamber respirometry, I investigate if rate of temperature change influences the oxygen consumption (a widely accepted proxy of aerobic metabolism) of an aquatic ectotherm under varying thermal conditions (Chapter 3). I show the rate of temperature change has a systematic effect on the oxygen consumption of ectotherms and as temperature increases more rapidly, the rate of oxygen consumption increases. It is currently uncertain if large-bodied fishes exhibit rapid temperature increases (such as those tested in Chapter 3), but we might expect this to occur during periods of physical exertion, such as during catch-and-release fishing interactions. When sharks are hooked, they often exhibit intensive swimming acceleration, as they attempt to escape. This can elevate the metabolic rate, resulting in the generation of heat. This excess heat may accumulate and manifest as an elevation of body temperature in these animals. In Chapter 4 I investigate this using state of the art biologging technology, in combination with blood biochemistry. In doing so, I record a previously undocumented thermal stress response in captured sharks (Chapter 4): I find ectothermic sharks experience acute elevations in body temperature during catch-and-release events, with subcutaneous temperature elevated by as much as 2.7°C. Furthermore, these elevations occur at rates as fast as 0.8°C min-1, significantly faster than any of the rates tested in Chapter 3, implying an even greater magnitude of metabolic increase, should this relationship extend to ectothermic sharks. Overall, I have addressed fundamental, unanswered questions in the field of thermal ecology and provided novel insights into the pathways by which temperature influences several key physiological parameters. I challenge long-held assumptions within the field, record previously undocumented thermal stress responses and identify key ecophysiological relationships which should be accounted for when predicting animals’ responses to thermal change. These novel findings contribute to an ever-expanding field and aid in our ability to predict ectotherm responses to climate change.