The gut-microbiota axis in brainless invertebrates: an evolutionary perspective

The gut-microbiota-brain axis has gained increasing attention due to its vital role in physiological functions, from metabolism to behavior. Although this axis has mainly been studied in mammals, exploring it from an evolutionary perspective provides an opportunity to expose fundamental mechanisms. This review examines how the gut microbiota supports essential processes that maintain homeostasis in marine invertebrates, including sponges, cnidarians, nematodes, echinoderms, and bivalve mollusks, even in the absence of centralized brain, although some possess nerve nets, ganglia, radial cords or complex digestive system. These organisms display primitive signaling mechanisms that enable functions such as digestion, detoxification, phagocytosis, and environmental adaptation. The presence of rudimentary nervous systems that allow effective tissue communication underscores functional similarities with more complex systems. These findings suggest that the interaction between the gut, microbiota, and nervous system may be an evolutionarily conserved principle. Studying the microbiota's role from an evolutionary and functional viewpoint helps identify universal principles in host-microbiota interactions, opening up new research possibilities in biomedicine with the potential to develop innovative therapeutic strategies.