Glutamatergic systems in Hydrozoa (Cnidaria)

The origins and early diversification of intercellular signaling molecules in animals remain poorly understood because comparative data across basal metazoan lineages are limited. Cnidarians form the sister group to bilaterian animals, and characterizing their transmitter systems is critical to understanding how complex adaptations within integrative systems shape evolutionary trajectories. Although glutamate is a well-established transmitter in bilaterian animals, its role in cnidarians remains unclear, and information on its neuronal function and signaling is limited. For most studied cnidarians, glutamate has been suggested to be a non-neuronal signaling molecule. Here, using glutamate immunoreactivity (Glu IR) in eight hydrozoan species with distinct ecologies (Aequorea victoria, Eutonina indicans, Clytia gregaria, Bougainvillia principis, Euphysa flammea, Polyorchis penicillatus, Aglantha digitalis, Nanomia septata), we identified and visualized distinct populations of glutamate-immunoreactive (Glu-ir) cells, including nematocytes, neurons, and muscle cells. A broad diversity of Glu-ir nematocytes was found in all studied species. Glu-ir neural cells were found only in three species (Aequorea, Nanomia, and Aglantha); their morphology and localization were species-specific. In addition, some striated and smooth myoepithelial cells were found to be either Glu-ir or GABA-ir. We propose that both glutamatergic and GABAergic systems were independently recruited more than 3 times as neurotransmitters across cnidarians, and that these recruitments are fundamentally rooted in bioenergetic demands.