Effects of thermal stress caused by the 2015–2016 El Niño on the biochemical composition, exoskeleton structure, and symbiont density of the fire coral Millepora alcicornis

Reef-forming cnidarians are essential for maintaining ecological balance. Unfortunately, coral reefs endangered due to bleaching, which interrupts mutualistic symbiosis between Symbiodiniaceae algae and their hosts. Bleaching events result in very high mortality the rapid deterioration of reef structures. Studies aimed at explaining causes, mechanisms, consequences bleaching have been mainly conducted with anthozoans, while impacts thermal stress responsible scarcely studied hydrozoans, such as Millepora species (phylum Cnidaria, class Hydrozoa), second most important reef-forming cnidarians. In present study, effects caused by 2015–2016 El Niño on symbiont abundance, exoskeleton structure, biochemical composition alcicornis were analyzed. Unbleached M. specimens exhibited a higher abundance Breviolum Durisdinium species, suggests that unbleached hydrocoral colonies might counteract hosting thermotolerant symbionts genus. Bleached hydrocorals lower levels calcification than well changes microstructure trabeculae zooid pores. contrast, did not affect total calcium carbonate carbohydrate content. tissues showed significantly protein refractory material, whereas lipid content decreased considerably. The study provides evidence bleached suffered decline structure exoskeletons after being exposed Niño. significant decrease primarily uses energy stores maintain vital cellular processes expense calcification.