A Systems Approach to Homeostasis: What Euryhaline Fish Teach Us About Organismal Stress Responses.

Historically, organismal biologists have studied the organism's response to environmental variation from two complementary perspectives: one has focused on "stability" and the capacity of organisms to maintain a constant internal state (e.g., homeostasis) across environments, whereas the other has focused on "change" and how the expression of traits varies as a function of a continuous environmental factor (e.g., performance curves). While these approaches differ, they rely on the same fundamental principles dispersed across cell biology, physiology, endocrinology, ecology, and evolution and thus could be better integrated. Through the lens of systems biology, we offer a perspective that explores the idea that organisms maintain stability of critical physiological functions during environmental change through changes of lower-level traits within physiological regulatory networks. We assert that such network thinking and an emphasis on the cost of homeostatic systems are critical when relating the physiological responses of cells, tissues, hormones, etc to whole-organism performance and the ecological context in which the responses occur. We suggest that such an approach has the potential of transcending levels of biological organization by connecting approaches typically studied in isolation of each other and that this will help the organismal biologist relate physiological responses measured in the lab to performance and fitness in natural settings. To illustrate our perspective and aid in our presentation of practical tips for the experimental biologist, we use examples from our own research on osmoregulation in euryhaline fish.