**5. Conclusions**

This study represents a broader integrative approach to investigating the presence and density of Symbiodiniaceae in antipatharians than has been used in previous studies. We combined microalgal cell extraction, histological examination and transmission and scanning microscopy with attempted sequencing of DNA samples. We found that Symbiodiniaceae-like cells were present within some of the antipatharian samples of *S. maldivensis* and *C. abies* from SW Madagascar, although the overall density of macroalgae cells in both antipatharian species from both shallow and mesophotic reefs was very low. This low density aligns with the majority of previous findings, indicating that high Symbiodiniaceae densities are not characteristic of antipatharians. These findings are significant in the context of extensive 'coral bleaching' events threatening the integrity of coral reefs, suggesting that most antipatharians are likely to be less prone to this phenomenon. Nonetheless, considering that high densities of dinoflagellates have been documented in three colonies of one antipatharian species, more studies are desirable to understand the mechanism and implications of the coral–algae relationship within this coral taxon. Studies on other effects that climate change might have on antipatharians are also desirable.

**Author Contributions:** Conceptualization, E.G. and I.E.; Data curation, E.G., M.G. and J.R.; Formal analysis, E.G.; Funding acquisition, I.E., P.D. and L.T.; Methodology, E.G., I.E. and L.T.; Writing original draft, E.G.; Writing—review and editing, I.E. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Fonds National de la Recherche Scientifique, Belgium (no. PDR T0083.18), under the 'Conservation Biology of Black Corals' research project co-directed by the University of Mons, the University of Liège, and the Free University of Brussels, in Belgium.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The dataset generated for this study are available on request to the corresponding author. No sequence data was generated on this study.

**Acknowledgments:** We thank: All the members at the Marine Organisms Biology and Biomimetics (BOMB) Laboratory in Belgium for their assistance during sample analyses. Nicolas Sturaro, University of Liege, for help with sample collection. Members at the University of Toliara, Madagascar, for support during field work. Liz Tynan, James Cook University, for editorial comments and Rupert Ormond, Heriot-Watt University, for editorial review. Figure 1b created on R (R Team 2019) with an adapted script provided by J.C. Fischer, University of Bayreuth, and A. Wiefels, University of Reunion Island. We are very grateful to Dennis Opresko, Smithsonian Institute, for continuous support.

**Conflicts of Interest:** We declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
