Introduction: One of the current threats to coral reefs is the loss of live coral cover. Massive predation associated with population outbreaks of the crown of thorns seastar, threatens the permanence of the reefs. Since 2017, there has been evidence of an increase in the density of the asteroid and an increase in coral mortality in the southern gulf of California. Objective: To describe the first event of massive predation on corals by Acanthaster planci in Espiritu Santo Island, gulf of California, Mexico. Methods: We visited El Corralito reef nine times between 2017 and 2019, using errant and band transect (25 x 3 m) visual censuses to determine Star density, behavior and damage. Over seven months, we monitored predation on colonies of Pavona gigantea. Results: The mean density of individuals in El Corralito in 2018-2019 was 607.40 ind/ha. Asteroid predation was evident in more than 60 % of diagnosed individuals (N = 827), with the greatest affectations in the P. gigantea colonies (80 %). 63 % of the 129 observed asteroid individuals were feeding mainly on Porites panamensis (68 % of cases). Acute injuries indicate that the predation event is active. In 81 days, 25 m2 of one of the monitored colonies were lost. Conclusions: There is an active outbreak event with significant negative consequences on the El Corralito reef, which could determine the loss of coral cover in a few months. Monitoring and management are required to establish the reasons that led to the outbreak and to control it.

Keywords: mortality; corallivory; reef degradation; outbreak; lesion.