The endocrine regulation of salt and water balance in aquatic organisms

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My research program examines how endocrine factors (e.g. hormones) allow aquatic animals to maintain salt and water balance. To put this in context, all freshwater (FW) animals and a great many marine species live under conditions where passive salt movement either into our out of the body will create life-threatening problems if not held in check by tissues and organs that specialize in salt transport. For example, the salt content (e.g. ions such as sodium, chloride and calcium) of freshwater (FW) is considerably lower than that of animal blood and ions are lost from the body to the surroundings by passive diffusion. In contrast, many seawater (SW) animals suffer the opposite problem because in these species SW is saltier than blood, and ions diffuse into the animal. Furthermore, aquatic animals may live where natural alterations in environmental salt content occur (e.g. changing tides in an estuary) or where human disturbance (e.g. road salting) is changing water salt content. In all these circumstances, the action of hormones on specialized ion transporting tissues adjusts the form and function of the tissues so as to allow the maintenance of salt and water balance. In aquatic animals, these specialized tissues include epithelia of the gill, kidney, gastrointestinal tract and skin. Epithelial tissues establish barriers that separate and regulate the movement of biological material between fluid compartments (e.g. the gill epithelium separates blood and water and regulates ion movement between them). Transepithelial ion transport, from one fluid compartment to another, occurs by two routes. The first is through cells (the transcellular pathway) and the second is between cells (the paracellular pathway). Transcellular ion transport is governed by a complex suite of transport proteins and ion channels that are well described in aquatic animals, as is the effect of hormones on their function. The paracellular route is governed by proteins of the tight junction (TJ) complex in vertebrates or septate junction (SJ) complex in invertebrates. In contrast to transcellular transport proteins, very little is known about TJ and SJ proteins in aquatic animals, and almost nothing is known about how (or what) endocrine factors influence them. This is alarming because the permeability of the paracellular pathway in aquatic animals is broadly acknowledged to play a vital role in the regulation of salt and water balance. As such we are presented with a serious gap in our fundamental knowledge of how aquatic animals function in their natural environment and how these organisms are equipped to cope with environmental change. My work addresses this lack of knowledge by identifying important TJ and SJ proteins in aquatic animals and looking closely at the endocrine regulation of TJ and SJ permeability using established and new state-of-the-art techniques. In this regard, our research is important to anyone who has an interest in how aquatic organisms maintain "homeostasis" (i.e. internal stability) or who are interested in how biological material moves across the epithelial barriers of aquatic animals. This is a broad populace that will range from those with basic research interests, to those who investigate the effects of pollutants and the impact of human activity on aquatic organisms, as well as those that seek to optimize the delivery of biological material (e.g. drugs) to aquatic organisms via the paracellular pathway.