Molecular analyses of the Palythoa caribaeorum holobiont suggest its recent arrival in the Canary Islands

The widespread zoantharian Palythoa caribaeorum is known for its competitive behavior, characterized by rapid growth and high dispersal capacity. In the subtropical Canary Islands, populations of P. caribaeorum have expanded due to tropicalization, transforming macroalgae-dominated ecosystems into extensive areas of this species. This shift has reduced structural complexity and biodiversity, raising concerns for local ecosystems under global warming, especially given the limited historical data available on these populations. Here, we investigated the phylogeographic patterns of P. caribaeorum populations in the Canary Islands by genetically characterizing the holobiont from various sites and assessing phylogenetic relationships with global populations of P. caribaeorum and P. tuberculosa . We analyzed the ITS-rDNA of the host and Cyt b , CoxI, ITS2, and psbA ncr markers of the associated Symbiodiniaceae. In addition, 6 microsatellite loci were used to analyze Symbiodiniaceae population structure and connectivity across locations. Our results reveal that all P. caribaeorum specimens are phylogenetically nested within the same clade as the Indo-Pacific sibling species P. tuberculosa , with bPTP analysis supporting their inclusion within a shared species complex, thus prompting a taxonomic reassessment. Cladocopium proliferum , a generalist Symbiodiniaceae species found in Indo-Pacific coral reefs, was the only symbiont associated with P. caribaeorum in the Canary Islands. Symbiodiniaceae populations exhibited high levels of clonality and reduced allelic variation, even across a distance of ~400 km. This study highlights the non-native nature of P. caribaeorum in the archipelago, having likely arrived with C. proliferum , and reinforces the need to reconsider the current status and management of this colonizer species.