What Happens to Sea Anemones When Relationships in the Ocean Begin to Collapse?

Van Quy Khuc

VNU University of Economics and Business, Vietnam National University

23-05-2026

© David Clode

Coral reefs are often described as the rainforests of the ocean. They support enormous biodiversity, sustain fisheries, protect coastlines, and provide livelihoods for millions of people. Yet over the past few decades, these ecosystems have faced increasingly severe threats as marine heatwaves become more frequent and intense (Hughes et al., 2017). Rising ocean temperatures trigger coral bleaching, a process in which corals expel the microscopic algae living within their tissues, causing them to lose both color and a major source of energy (Reimer et al., 2024; Smith et al., 2025).

The consequences of coral bleaching are widely recognized. But corals are not the only organisms affected by heat stress.

Sea anemones—soft-bodied relatives of corals—also maintain intimate relationships with microscopic algae (Scott & Hoey, 2017). These algae provide sugars and oxygen through photosynthesis, while the anemones offer shelter and nutrients in return. Sea anemones also support one of the ocean’s most iconic partnerships: their mutualistic relationship with anemonefish, commonly known as clownfish (Jones, Gardner, & Sinclair, 2008; Huebner & Chadwick, 2012).

In this relationship, both partners benefit. Anemones protect fish and their eggs with tentacles equipped with stinging cells, while anemonefish defend their hosts from predators, improve water circulation around them, and provide nutrients (Szczebak et al., 2013; Verde, Cleveland, & Lee, 2015). This relationship is not simply a matter of coexisting in the same place—it is a tightly interconnected system of reciprocal dependence.

A recent study by Bennett-Smith and colleagues examined populations of anemonefish (Amphiprion bicinctus) and their host sea anemones (Radianthus magnifica) in reefs of the Saudi Arabian Red Sea between 2022 and 2024. During the extreme marine heatwave of 2023, thermal stress reached approximately 22 Degree Heating Weeks—far above the threshold generally associated with severe bleaching and mortality (Bennett-Smith et al., 2025).

The results were alarming. Across all monitored reefs, researchers observed 100% bleaching of sea anemones, 94–100% mortality among anemonefish, and 66–94% mortality of host anemones themselves. Compared with previous heat events in other parts of the Indo-Pacific, declines of this magnitude had not been observed.

At first glance, the story may appear to concern only the loss of particular species. But the findings may point toward something broader: the breakdown of relationships themselves (Vuong, 2025). The health of an ecosystem cannot be understood simply by counting surviving species. Equally important is the integrity of the interactions that bind species together. The loss of one partnership can create ripple effects throughout an ecological network. Species that depend on anemones for shelter may disappear. Predator-prey dynamics may shift. Habitat structures may weaken. Small disruptions can gradually propagate through larger systems.

Climate change is not merely warming oceans but also weakening the invisible threads that hold living systems together. Protecting biodiversity require more than preventing species extinctions alone. It requires developing a deeper intelligence about nature itself—one that recognizes that life often survives not only through individual organisms, but through the relationships that connect them (Nguyen, 2026; Tran, 2026).

References

Bennett-Smith, M.F., et al. (2025). Near complete local extinction of iconic anemonefish and their anemone hosts following a heat stress event. npj Biodiversity, 4, 35. https://doi.org/10.1038/s44185-025-00107-4

Huebner, L. K. & Chadwick, N. E. (2012). Reef fishes use sea anemones as visual cues for cleaning interactions with shrimp. Journal of Experimental Marine Biology and Ecology, 416-417, 237–242. https://doi.org/10.1016/j.jembe.2012.01.004

Hughes, T. P., et al. (2017). Global warming and recurrent mass bleaching of corals. Nature, 543, 373–377. https://doi.org/10.1038/nature21707

Jones, A. M., Gardner, S. & Sinclair, W. (2008). Losing ‘Nemo’: bleaching and collection appear to reduce inshore populations of anemonefishes. Journal of Fish Biology, 73, 753–761. https://doi.org/10.1111/j.1095-8649.2008.01969.x

Nguyen, M. H. (2026). Ayn Rand and Kingfisher on zero-carbon bombs and a sustainable future. Visions for Sustainability.

Reimer, J. D., et al. (2024). The fourth global coral bleaching event: where do we go from here?. Coral Reefs, 43, 1121–1125. https://doi.org/10.1007/s00338-024-02504-w

Scott, A. & Hoey, A. S. (2017). Severe consequences for anemonefishes and their host sea anemones during the 2016 bleaching event at Lizard Island, Great Barrier Reef. Coral Reefs, 36, 873–873. https://doi.org/10.1007/s00338-017-1577-6

Smith, K. E., et al. (2025). Ocean extremes as a stress test for marine ecosystems and society. Nature Climate Change, 15, 231–235. https://doi.org/10.1038/s41558-025-02269-2

Szczebak, J. T., et al. (2013). Anemonefish oxygenate their anemone hosts at night. Journal of Experimental Biology, 216, 970–976. https://doi.org/10.1242/jeb.075648

Tran, T. M. A. (2026). Conversations with Kingfisher: Wisdom from Vuong’s wild wise weird stories. Planet Forward. https://planetforward.org/story/kingfisher-stories/    

Verde, E. A., Cleveland, A., & Lee, R. W. (2015). Nutritional exchange in a tropical tripartite symbiosis II: direct evidence for the transfer of nutrients from host anemone and zooxanthellae to anemonefish. Marine Biology, 162, 2409–2429. https://doi.org/10.1007/s00227-015-2768-8

Vuong, Q. H. (2025). Wild Wise Weird. AISDL. https://books.google.com/books?id=C5dDEQAAQBAJ