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This work investigates the ecophysiology of marine amphipods. Amphipods are an important part of the zooplankton community in the pelagic environment. Amphipods are a food source for a variety of fishes and also have a role in carbon cycling. Little is known about their physiology and how they have adapted to environmental variation. The Intergovernmental Panel on Climate Change (IPCC) reports that global warming is causing temperatures to rise throughout the world’s oceans, a trend that will continue with rising human carbon emissions. As temperature and CO2 levels increase, oceanic oxygen levels are predicted to decrease, and as a result, oxygen minimum zones will expand. Ocean general circulation models have shown that the detectable decrease in dissolved oxygen concentrations is driven by increasing ocean surface temperatures and enhanced stratification. Low oxygen concentrations and high temperatures affect physiological performance and, consequently, vertical distribution and ecology of marine organisms. Vertically migrating amphipods living in the Eastern Tropical North Pacific currently experience temperature changes of 15 degrees Celsius or more and changes in oxygen concentration from saturation to near anoxia. Metabolic depression is the reduction in total metabolic rate, including aerobic and anaerobic ATP consumption, to below the basal metabolic rate. This happens in response to environmental stress such as extreme temperatures, desiccation, anoxia and food deprivation. The tolerance of an organism to low oxygen is inversely related to the extent of their metabolic. Organisms subjected to physiological stress, such as stresses that cause proteins to denature, will respond by producing heat shock proteins (hsps). Hsps act as molecular chaperones and are able to prevent/reduce denaturing of proteins and target proteins that are irreversibly denatured for removal from the cell via the ubiquitin-proteosome pathway. No previous studies have been done on midwater amphipods to see if the temperature gradient they experience during diel vertical migration induces a stress response. Chapter 1 examines how temperature and oxygen gradients affect the physiology of the amphipod Phronima sedentaria by quantifying the aerobic and anaerobic metabolic rates at oxygen levels consistent with those experienced across Phronima’s vertical range in tropical regions. Total ATP production (metabolic rate) was compared in specimens subjected to night time surface conditions (oxygenated) and day time conditions (hypoxia). In Chapter 2 protein concentrations of hsp 70 were measured in specimens subjected to a range of temperatures within and above what they typically experience. Understanding the adaptations of pelagic amphipods to their current environment will help predict the physiological impacts of global warming for amphipods and their predators. One adaptation for living in hypoxia is metabolic depression. Metabolic rates of organisms are affected by a number of variables, particularly by temperature, body mass and ecology. Metabolic rate typically doubles or triples for every 10°C change in body temperature. Routine oxygen consumption rates of most vertically migrating, visually oriented, midwater crustaceans decline with depth primarily due to temperature, but also due to the low light and consequential lack of visual cues which reduces locomotion needs. Transparent organisms in epipelagic regions would be relieved of this selective pressure because they are hidden from their visual predators. Hyperiid amphipods are the only group of crustaceans that are truly dominated by transparency. The influence of transparency on metabolic rate has not been examined in amphipods. Chapter 3 sought to determine what environmental and ecological factors influence the rate of metabolism in marine amphipods by examining a broad data set from polar to tropical environments, and including transparent specimens. The data set for this study was obtained from the literature and original data. Recent molecular work allowed us to look at hyperiid metabolism in a phylogenetic context. Understanding patterns of pelagic and deep sea metabolism is important for further understanding of global carbon flux and the consequences of climate change on migration strategies.

METABOLISM, HYPOXIA TOLERANCE AND HEAT SHOCK RESPONSE OF AMPHIPODS, EMPHASIZING THE HYPERIID AMPHIPOD PHRONIMA SEDENTARIA