Differential thermal sensitivity among shallow-water octocorals and its association with holobiont composition

Octocorals are increasingly recognized as important structural components of reef ecosystems, yet the mechanisms underlying their responses to thermal stress remain poorly understood. In particular, it remains unclear whether differences in thermal sensitivity among octocorals correspond to variation in their symbiotic algae and associated bacterial communities. Here, we investigated the physiological and microbial responses of four shallow-water octocoral genera (Litophyton, Lobophytum, Sarcophyton, and Sclerophytum) collected from the same region and exposed to experimentally elevated temperatures. Physiological measurements revealed clear genus-specific differences in thermal sensitivity: Litophyton was the most sensitive, showing the most rapid decline in photosynthetic efficiency, the greatest loss of algal symbionts, and the highest mortality. Notably, this pattern did not track symbiont identity: Litophyton was dominated by the thermally tolerant symbiont Durusdinium, whereas the more resilient genera were associated primarily with Cladocopium. Microbiome analyses revealed host-specific bacterial assemblages, with Litophyton harboring a distinct community dominated by Endozoicomonas. Under heat stress, total Endozoicomonas abundance in Litophyton remained relatively stable, but its composition shifted at the ASV level, indicating fine-scale microbial restructuring. Together, these results suggest that thermal sensitivity was not explained by symbiont identity or bacterial community composition among the octocorals examined here. The factors underlying these genus-level differences remain to be identified, but host-level traits, such as morphology and evolutionary history, represent plausible candidates that warrant further investigation. More broadly, our findings caution against directly applying scleractinian-derived holobiont frameworks to octocorals, and highlight the need to better understand how octocoral-dominated reefs respond to continued warming.