A fragile branch: the silent decline of neglected Brazilian milleporids amid the fourth global bleaching event

Milleporid hydrocorals are essential to South Atlantic reef ecosystems, where their unique morphology provides habitat and structural complexity. Among the four milleporid hydrocorals in South Atlantic, three are endemic to Brazil (Millepora braziliensis, M. laboreli, and M. nitida), for whom baseline data on abundance and response to thermal stress events remain scarce or non-existent, jeopardizing assessments of its population status. We monitored the bleaching incidence and living coral surface area of M. nitida and M. braziliensis species before, during, and after the fourth global bleaching event (2023–24). Living surface area and bleaching were assessed through photographic surveys and thermal stress was quantified using satellite-derived degree heating weeks data. We found that M. braziliensis populations suffered 100% bleaching and a complete loss of live cover under severe thermal stress of ~ 20.0 °C-weeks, underscoring the vulnerability of this species to marine heatwaves. For M. nitida, which faced milder thermal stress (7.6  °C-weeks), 40% of the colonies bleached, but no significant change in live cover was observed. These findings highlight the urgent need for conservation measures to protect M. braziliensis populations, which face heightened extinction risks under increasing thermal stress. This study provides critical baseline data on South Atlantic endemic hydrocorals to support conservation status assessments and efforts to safeguard these species. By documenting their responses to environmental stress, this research reinforces the importance of species-specific conservation strategies to mitigate climate change impacts on South Atlantic reef ecosystems.