The Respiratory and Gut Physiology of Fish: Responses to Environmental Change

The series of studies reported in this thesis begin (chapter two) with a collation of previously published data on the critical oxygen thresholds (Pcrit) of fish. Such data provide a quantitative insight into the variation of hypoxia tolerance across species and allows for the identification of a range of biotic / abiotic interactions. In addition, by critically reviewing the methodologies and principles behind investigations of respiratory physiology in fish, chapter two sets the scene for the following three empirical studies, within which such measurements are a re-occurring theme. The first of these empirical studies (chapter three) aims to experimentally determine the thermal and hypoxia tolerance of two closely related coral-reef snapper species - Lutjanus carponotatus and Lutjanus adetii. The contrasting biophysical environments occupied by these fish, the reef flat and reef slope, provide a useful model system in which to examine the extent to which variation in their spatial distribution is reflected by inter- and intraspecific variation in some common physiological tolerance trait measures (aerobic scope, critical temperatures and Pcrit). Such studies, linking the respiratory physiology of organisms to environmental conditions in their natural range, are of increasing interest for predicting possible shifts in the distribution of populations as the result of future climate change. In the fourth chapter, this thesis moves on to examine how the respiratory responses of the European flounder (Platichthys flesus) to hypoxia and hypercarbia influences their intestinal precipitation and excretion of carbonate. Measurements of Pcrit, ventilation volume, drinking rate and various blood parameters are made in order to elucidate the mechanistic link between carbonate excretion rate and environmental PO2 and PCO2. Finally, the study presented in chapter five builds upon the results of the proceeding chapter in order to explore how additional environmental factors such as temperature and seawater chemistry affect the rate of carbonate excretion by European flounder. Treatment conditions in chapter five are designed to reflect conditions thought to be prevalent in the warm, calcite seas of the Cretaceous period. Thus, this study provides the first experimental evidence for how piscine carbonate production may have varied in the geological past. In summary, the following thesis can be characterized as a series of fish ecophysiology studies linked by two major themes, respiratory physiology and gut carbonate production, both of which are set in the context of past, present and future environmental change.Many of the habitats occupied by fish are highly dynamic, naturally demonstrating substantial abiotic fluctuations over diurnal, tidal or seasonal cycles. It is also the case that throughout their 545 million year evolutionary history, fish have existed in aquatic environments very different to those of the present day. However, the past several decades have seen unprecedented rates of environmental change, at local and global scales, arising from human activities. The two major themes of the present thesis are: 1) Respiratory responses of fish to changes in environmental oxygen and temperature in the context of exploring intra- and inter-specific trait variation and its ecological implications 2) The effects of environmental factors (oxygen, carbon dioxide, temperature and seawater chemistry) on the intestinal precipitation and excretion of calcium carbonate by marine teleosts. In the first study (chapter two) a comprehensive database of fish critical PO2 (Pcrit) data compiled from the published literature is presented. The systematic review of this literature provided the opportunity to critically examine methodologies for determining Pcrit as well as its usefulness as an indicator of hypoxia tolerance in fish. The second study (chapter three) examines whether inter- and intra-specific variation in thermal and hypoxia tolerance in two reef snapper species (Lutjanus carponotatus and Lutjanus adetii) reflects their distributions across the contrasting biophysical environments of the reef flat and reef slope surrounding Heron Island on the Great Barrier Reef. L. carponotatus was clearly the most thermally and hypoxia tolerant of the two species, demonstrating a ~3.5 °C wider thermal tolerance zone (higher CTmax, lower CTmin) and ~26% lower Pcrit than L. adetii. These results suggest that the contrasting distribution of these species between flat and slope reef zones is reflected in their physiological tolerances. However, there was no evidence of intra-species variation in tolerance between flat and slope caught L. carponotatus individuals, indicating that this species does not form physiologically distinct subpopulations between these reef zones. The third study (chapter four) experimentally quantified the effect of hypercarbia (3000 μatm) and hypoxia (50% air saturation) on gut carbonate production by the European flounder (Platichthys flesus). Both hypercarbia and hypoxia resulted in a significant increase in carbonate excretion rate (1.5-fold and 2.4-fold, respectively) and acted synergistically when combined. In the final study (chapter five), gut carbonate production was measured in the European flounder undergoing conditions simulating the ‘calcite seas’ of the Cretaceous. The results of this study support the hypothesis that ocean conditions prevalent during the Cretaceous period resulted in piscine carbonate production rates substantially higher (~14-fold) than the present day. Ultimately, this thesis directly links the environmental physiology of fish at the individual level to wider scale implications (past, present and future), ranging from local ecological patterns all the way up to global carbon cycles.Natural Environment Research Council (NERC