Stress-Tolerant Coral Faces Bioerosion: Sponge Colonization Under Mass Bleaching

Global climate change has accelerated ocean warming, increasing thermal stress events that cause mass coral bleaching. In contrast, some tolerant organisms, such as certain sponge species, thrive under these novel ocean conditions. Despite their ecological importance, bioeroding sponges remain understudied in tropical reef systems. Here, we report the presence and ecological patterns of the coral-excavating sponge Cliothosa delitrix on Southwestern Atlantic reefs during the Fourth Global Coral Bleaching Event (2024). The sponge was found on six different low-latitude reef sites between 15 to 30 meters depth, colonizing both healthy and bleached colonies of the dominant, stress-tolerant coral Siderastrea stellata . Of the 347 coral colonies, ~ 52% showed bleaching, while 48% showed no signs of this phenomenon. Bioerosion by C. delitrix was dramatically higher in bleached coral colonies, with an incidence approximately 13-fold greater compared to unbleached colonies (~ 38% vs. ~3%, respectively). This suggests the increased vulnerability of thermally stressed corals to sponge colonization. Sponge coverage was positively correlated with S. stellata available substrate (r = 0.93), and the abundance of small sponge individuals (< 5 cm²) suggests high post-bleaching recruitment. Furthermore, there was no significant correlation (r = 0.17) between sponge perforation and depth, which indicates adaptation to different reef depths and that the availability of corals and their bleaching are the most important variables. This novel report of sponges colonizing key and bleached coral builders underscores the potential for opportunistic expansion of thermally resistant bioeroders under ongoing global climate change, even in reefs dominated by resilient coral species.