On the collapse of an endemic reef-building coral species

Coral species are increasingly threatened worldwide, yet population-level data remain scarce for informing effective conservation strategies. Here, we used the endangered reef-building coral Mussismilia harttii, endemic to the tropical Southwestern Atlantic, to identify drivers of population decline across 15 sites spanning about 1500 km of its distribution. Using data collected in 2016, we quantified colony density, colony size, corallite size, asymmetry and percentile tail ratio of colony size distributions, and examined their relationships with sea surface temperature, salinity, turbidity, and distance from the coast. A Bayesian model selection framework was applied to identify the best predictors of population metrics. Living colonies were absent from four sites and occurred at densities as low as 0.01-0.04 colonies m^-2 at eight others, compared with 0.10-0.15 colonies m^-2 at three sites that still resembled the baseline populations described by Laborel in the 1960s. Only 12.2% of colonies exceeded the historically common size of 400 corallites, indicating a marked loss of large colonies and resulting in strongly right-skewed size distributions across all sites, characterized by positive asymmetry and low percentile tail ratios. Proximity to the coast consistently emerged as the strongest predictor of population decline, likely reflecting cumulative impacts of tourism and coastal development. Our results indicate that M. harttii qualified for uplisting to Critically Endangered under the IUCN Red List as early as 2016, preceding the recurrent marine heatwaves of the last decade. Conservation actions targeting nearshore stressors, together with climate change mitigation, are essential to prevent further population collapse and extinction.