Abstract In open marine aquaculture, rearing diverse fish species in adjacent net pens facilitates the waterborne spread of red sea bream iridovirus (RSIV). This study systematically evaluated the horizontal transmission dynamics of RSIV in black sea bream (BSB, Acanthopagrus schlegelii ) and Japanese sea bass (JS, Lateolabrax japonicus ) under immersion and cohabitation conditions. After viral exposure, BSB exhibited high survival rates despite significant viral loads and persistent shedding, effectively acting as an asymptomatic reservoir. Conversely, JS functioned as a highly susceptible host, experiencing 0% survival alongside rapid, high‐level viral replication and dissemination prior to mortality. These divergent epidemiological roles highlight the critical need for species‐specific biosecurity and disease management strategies in shared marine aquaculture zones.