In recent years, several studies have highlighted the high resilience of zoantharians to ocean warming. In particular, populations of Palythoa caribaeorum are proliferating and beginning to dominate the coastal ecosystems of the Canary Islands. This expansion has been associated with increasing sea surface temperatures (SSTs). Here, we provide new insights into the endosymbiont-P. caribaeorum associations during and after a heat stress experiment to understand the processes underlying their high resilience to elevated temperatures. For this purpose, 61 colonies collected in the Canary Islands were exposed to an increasing temperature gradient (from 24 to 32 °C) to assess their heat tolerance, and then transferred back to the control temperature (24 °C) to evaluate their resilience. Colonies performance was assessed by comparing host color changes (bleaching degree), analyzing their Symbiodiniaceae morphological condition, and determining the dominant lineage of Symbiodiniaceae using the psbAncr molecular marker. All colonies showed signs of bleaching during heat stress, evidenced by color loss and a decrease in healthy Symbiodiniaceae content. Nevertheless, P. caribaeorum showed high resistance to heat stress, as all colonies kept up to 30 °C were able to survive and significantly recover. Furthermore, as the experimental temperature increased, a new haplotype of the Cladocopium C1 lineage, not detected in the control and wild samples, emerged and dominated most of the colonies (59.09%). Our study demonstrates the resilience of P. caribaeorum to heat stress in the Canary Islands, raising important ecological concerns about the future of native macroalgae ecosystems in an ocean warming scenario.

This research was supported by the Agencia Canaria de Investigación, Innovación y Sociedad de la Información de la Consejería de Economía, Conocimiento y Empleo and by the European Social Fund (ESF) Programa Operativo Integrado de Canarias 2014-2020, Eje 3 Tema Prioritario 74 (85%). We thank Eulalia Peraza, María Elisa Lambre, and Melissa Denton for their invaluable assistance with the temperature stress experiment. Special appreciation goes to Rosa Fregel, whose insights into the benefits of IA distribution were instrumental. Additionally, we are indebted to Sabrina Clemente for her expertise in tracking the islander zoantharians across time and space and her support throughout the National Project ZoanSystem. We thank the reviewers for all their revisions, which improved this manuscript, and for supporting its publication.

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