The goal of this research is to understand the control of bioluminescence in the hydrozoan coelenterate, Obelia, as part of a more general understanding of how epithelial action potentials control effector responses in lower invertebrate species. In this animal all of the behavioral responses are under the control of action potentials in electrically coupled epithelial cells, rather than under neural control as in higher animals. Luminescence in these organisms results from the activation of a calcium-dependent photoprotein which provides an endogenous intracellular calcium indicator. Previous experiments using a combination of patch clamp analysis and video image analysis have shown that calcium entering nonluminescent epithelial cells during an action potential results in the passage of a chemical signal into the luminescent cells via gap junctions. This signal (hypothesized to be calcium, itself) then results in activation of the intracellular photoprotein. Experiments are proposed which will establish the identity of the chemical signal. In addition to the demonstration that luminescence is triggered as the result of a chemical signal passing across gap junctions, other experiments have disclosed the existance of a novel type of voltage independent calcium channel within the luminescent cell itself. Like the initiation of luminescence, the activation of this current is also dependent on chemical signalling through the gap junction. In further studies, whole cell and single channel recording techniques will be used to characterize this channel which represents a novel mechanism of calcium entry across the plasma membrane. This research on the physiological mechanism controlling light emission by a luminescent marine organism has already produced important insights into the function of junctions that mediate communication between cells. "Gap junctions" are structurally specialized appositions between cells and are found in many tissues of animals at all levels of complexity. Much indirect evidence has led to the belief that these junctions serve in transmission of chemical signals between cells. The results of this research have provided the first clear demonstration that chemical signal transmission across gap junctions is involved in an important physiological function. In addition, a novel type of channel for calcium entry into cells has been discovered. A continuation of the study of this interesting system will surely expand our understanding of the control of light emission by these small marine organisms, but this study is also likely to provide further fundamental insights into the general mechanisms of cellular communication.