Beneath the Surface: Aquatic Models Illuminate Host–Microbe Interactions

Microbial outcomes emerge from dynamic interactions among host defenses, microbial traits, and environmental context. This narrative review examines aquatic host–microbe systems, with emphasis on zebrafish and Vibrio species, as experimentally tractable aquatic models for understanding how interactions shift across a spectrum from mutualism to pathogenesis. We first define major aquatic invertebrate and vertebrate models, highlighting how each balances scalability, genetic accessibility, physiological complexity, and translational relevance. We then use Vibrio species as a framework for moving beyond the pathogen versus non-pathogen classification, emphasizing conditional pathogenicity, host-dependent outcomes, and shared molecular toolkits between symbionts and pathogens. Finally, we highlight host responses including barriers and immunity. Aquatic models reveal that disease results from context-dependent regulation of both host and microbial programs. By integrating the diversity of aquatic models, aquatic host–microbe research presents a unique opportunity to define the continuum of outcomes from mutualism to pathogenesis.