Climate-driven introduction of an opportunistic symbiont in the most extensive reef system of the Southwestern Atlantic Ocean

Coral symbionts of the Symbiodiniaceae family range from strictly mutualistic to highly opportunistic species. Symbiodinium necroappetens is an opportunistic species and a putative necrophile of dead coral tissue, hitherto reported as endemic to the Caribbean. Symbiodiniaceae were characterized by ITS2 barcoding along the most severe thermal anomaly registered in the Abrolhos Bank, southwestern Atlantic Ocean (SWAO). S. necroappetens relative abundance and prevalence went from undetectable before the anomaly to as much as 73% and 100%, respectively, during and after the heatwave. Sequences were still detectable four years after being initially observed. We are likely observing a climate-driven introduction of S. necroappetens in the SWAO. Given the increasing frequency and intensity of thermal anomalies hitting the global ocean, we can only speculate on the ecological consequences of such invasion for the Abrolhos reefs.