Reef ecosystems are composed of a variety of organisms, transient species of fish and invertebrates, microscopic bacteria and viruses, and structural organisms that build the living foundation, coral. Sessile cnidarians, corals and anemones, interpret dynamic environments of organisms and abiotic factors through a molecular interface. Recognition of foreign molecules occurs through innate immunity via receptors identifying conserved molecular patterns. Similarly, chemosensory receptors monitor the environment through specific ligands. Chemosensory receptors include ionotropic glutamate receptors (iGluRs), transmembrane ion channels involved in chemical sensing and neural signal transduction. Recently, an iGluR homolog was implicated in cnidarian immunological resistance to recurrent infections of bacterial pathogens. I postulate that iGluRs in cnidarians may act as danger-sensing and/or pathogen recognition receptors adjacent to immune defense and nervous system signaling. In Chapter One, I explain the exploration of diversity and divergence within cnidarian iGluRs, complimented with predicted functions in the context of correlated response to biological and environmental signals, setting the groundwork for functional characterization. In Chapter Two, I characterized the divergence of cnidarian iGluRs in comparison to other metazoans through maximum likelihood phylogenetic analyses, which revealed greater evolutionary expansion of cnidarian iGluR lineages, including a Cnidaria-specific class. Gene expression differentiation implies select iGluRs respond transcriptionally to bacterial challenge, supporting the hypothesis that cnidarian iGluRs respond to pathogen signals. In Chapter Three, I investigated a putative endogenous rhythm to iGluR expression, as chemosensory receptors may have the capacity to anticipate daily environmental fluctuations. While a circadian rhythm does not appear to be a primary contributor to biological rhythms in iGluR gene expression, symbiosis and diurnal fluctuations are implicated factors. In Chapter Four, I chromogenically localized Exaiptasia pallidaiGluR expression to the epidermis and concentrated within sensory tentacles, alongside cnidocytes. Expression of iGluRs in proximity of sensory cells is consistent with the putative function of iGluRs in cnidarian neural signaling. In the final chapter, I synthesized my research in its entirety; highlighting that cnidarian iGluRs expansions indicate cnidarian-specific neofunctionalization towards functions of chemosensory cnidarian-environmental signaling. New hypotheses and future research are presented to continue the study of iGluRs as chemosensory receptors within the cnidarian nervous system
