A fundamental question in modern-day ecology is: How will populations and their interactions respond
to a rapidly changing climate? Documenting local collapses of ecologically and economically
important populations can offer insight into broader patterns of decline. Here, we monitored
anemonefish (Amphiprion bicinctus) and their host sea anemones (Radianthus magnifica) on three
central Saudi Arabian Red Sea reefs from 2022 to 2024, including a 2023 marine heatwave that peaked
at a Degree Heating Weeks (DHW) value of ~22 °C-weeks. Across all reefs, we observed a sequence of
100% anemone bleaching, 94.3–100% anemonefish mortality, and 66.4–94.1% anemone mortality.
We compare these findings to other recent Indo-Pacific heat stress events of varying intensity, where
similar declines were not observed. Our study highlights the vulnerability of mutualistic reef species to
extreme heat and suggests that such events may drive local, if not regional, extinctions of ecologically
important symbioses.