The stress related to prolonged aquatic submergence is the focus of this research proposal. This work attempts to quantify levels of circulating catecholamines in chronically submerged freshwater turtles and will provide useful background information for future studies on diving in turtles. Freshwater turtles typically employ one of two well-documented physiological homeostatic mechanisms for surviving submergence in water for long periods. First, 'thin-skinned' turtles (softshells; Apalone spinifera, musk turtles; Sternotherus odoratus, and painted turtles, Chrysemys picta) that overwinter in streams, ponds, or shallow lakes, rely on extrapulmonary mechanisms of gas exchange to provide sufficient oxygen for metabolism and carbon dioxide excretion. In essence, these turtles can remain aerobic throughout the winter by exchanging oxygen and carbon dioxide via extrapulmonary gas exchange sites that include the buccal cavity, the cloacal bursae, and the thin skin near the proximal aspect of the limbs. When metabolic demand for oxygen is low, (e.g. during winter submergence when water temperatures are near freezing) oxygen demand can be met by these comparatively 'low capacity' gas exchange sites. Another strategy for operating cellular machinery during long-term aquatic submergence is to burrow into the anoxic substrate (mud) and produce ATP anaerobically. This second strategy presumably minimizes access to turtles by predators and ectoparasites, but usually results in severe metabolic and ionic disturbances. Nevertheless, turtles that dive underwater for long periods, and employ either strategy, experience apnea. Thus they do not breathe using their lungs, and the success of either strategy depends upon the interactions among abiotic factors (i.e. water temperature, dissolved oxygen and carbon dioxide levels, and perhaps water pH) and biotic factors related mostly to anatomical correlates (i.e. thickness of skin, presence of cloacal bursae, cardiovascular and respiratory adjustments). Apnea, with or without hypoxia or anoxia is a stressful experience and catecholamines are released into the blood stream as a result. Release of stress hormones in response to 'stressful' situations can be adaptive, but chronic production and secretion of stress hormones (epinephrine and cortisol) usually leads to immunosuppression, morbidity and ultimately, mortality. This research will track the changes in stress hormone concentrations during normoxic and hypoxic/anoxic exposure. These data on stress hormone levels will be interpreted in the context of additional data concerning the effects of long-term submergence on cardiovascular function (including systemic, pulmonary, gastrointestinal, and renal blood flows) in freshwater turtles, which will be gathered in these studies as well. This project will produce qualitative and quantitative data that will be instrumental in helping biologists understand aspects of cardiorespiratory physiology in diving reptiles, particularly turtles.
