The Dark Side of Mucus: Stress‐Induced Mucus Aggregate (<scp>SIMA</scp>) on Tubastraea Corals in a Heavily Impacted Bay

ABSTRACT This study investigates the absence of non‐indigenous sun coral Tubastraea in Guanabara Bay, Rio de Janeiro, despite its presence on nearby islands. Tubastraea corals are known for their high reproductive plasticity and resilience to varying environmental conditions. An ex‐situ experiment was conducted in which colonies of Tubastraea aurea were exposed to a continuous flow of seawater sourced from Guanabara Bay. During the experiment, colonies developed a Stress‐Induced Mucus Aggregate (SIMA) that progressively enveloped the polyps and was associated with marked tissue loss upon removal. This response is distinct from the coral's typical mucus secretion and is hypothesised to result from environmental stress‐induced alterations in the associated microbiota. The influx of organic matter and pollutants likely promotes a shift toward pathogenic and anaerobic microbial communities, which may further impair tissue respiration and contribute to coral mortality. 16S rRNA gene sequencing identified four ecologically significant bacterial genera— Vibrio , Pseudoalteromonas , Alcanivorax and Shewanella— commonly found in marine systems. These organisms demonstrate functional duality in coral microbiomes, simultaneously influencing both biofilm‐mediated community assembly and disease dynamics. Their context‐dependent roles markedly affect coral health outcomes under environmental stress regimes, highlighting their importance in reef ecosystem resilience. These findings suggest that microbial community dynamics in highly disturbed environments may play a key role in limiting the establishment and persistence of T. aurea in Guanabara Bay. Future research will focus on elucidating the role of physico‐chemical water parameters, sedimentation and microbial community shifts in the survival of Tubastraea corals in Guanabara Bay.