A catastrophic warming event occurred during 2005 throughout the northeastern Caribbean Sea that caused a mass coral bleaching event in Puerto Rico the was followed by an unprecedented mass mortality of the Montastraea annularis species complex, the most significant reef-building coral in the Atlantic. It resulted in a severe net physiological fragmentation of large coral colonies. Permanent photo-stations were established in 4-6m deep reef terraces dominated by M. annularis. at four sites in Culebra Island, PuertoRico. Digital photography was used to document changes in benthic community structure before(2005) and after(2007-2009) this event. Mass coral mortality caused a 70 to 99% decline in% living tissue cover. There was a significant difference in% living tissue cover loss through time, among sites, particularly atacontrolsiteoutsideano-take MPA. Abundant physiological tissue fragments were formed in each colony, typically ranging from just below 1 to 105 cm2, but mostly in mean sizes below 10 cm2, which showed higher mortality trends. There was no significant difference in mean fragment size distribution among sites. Fragment density was significantly higher(p<0.0001) at Cayo LuisPeña(97/m2) in comparison toother sites(24-67/m2). Fragment density declined from 15 to 31% between 2007 and 2009. There are not known precedents through the Caribbean of catastrophic events of this magnitude. The synergistic consequences of climate change and variable local anthropogenic impacts in major reef engineer taxa still remain largely unknown. However, a single warming event was unequivocally capable of causing an acute coral mortality event that resulted in a major ecological collapse.National Science Foundation, HRD #0734826 University of Puerto Rico, Central Administration and Río Piedras Campus