Clownfish face collapse as ocean heat breaks unique anemone bond

If you’ve ever seen Finding Nemo, you know clownfish -bright orange, striped, and full of personality. In the wild, they rely on sea anemones for safety, shelter, and survival – a partnership now tested by rising ocean temperatures.

Over the last three years, extreme heat has pushed the Red Sea’s tropical waters past their usual high temperatures of 85 to 90°F. That heat hasn’t just made life uncomfortable – it’s triggered a serious breakdown of that partnership. This isn’t just bad news for Nemo. It’s a warning for us all.

The clownfish-anemone partnership

Clownfish and anemones rely on each other to survive. The fish gain a safe place to hide, lay eggs, and stay protected from predators inside the anemone’s stinging tentacles.

In return, the clownfish help keep the anemone clean and may even scare off its enemies. This kind of teamwork is called mutualism – and it’s crucial to life in the ocean.

Anemones also rely on tiny algae living in their tissues, called zooxanthellae. These algae are their main energy source. They’re the same type of algae that live in corals.

But when ocean temperatures rise too high, the algae get expelled. That’s called bleaching. Bleached anemones turn ghostly white. And without their algae, they start to starve.

The heatwave that broke the system

Between 2022 and 2024, researchers studied Red Sea clownfish (Amphiprion bicinctus) and their host anemones (Radianthus magnifica) on three reefs along the central coast of Saudi Arabia. During that time, the region experienced a major marine heatwave in 2023.

“We always hope that anemones and clownfish groups survive bleaching events, as they have over and over again in the last 10 years, but it hit a point where it was too extreme,” said Morgan Bennett-Smith (GRS’25,’27), a Ph.D. candidate in BU’s Marine Evolutionary Ecology Laboratory and lead author of the paper.

The anemones stayed bleached for about six months. As a result, 94 to 100 percent of the clownfish died. Between 66 and 94 percent of the anemones died too.

The team behind the study includes researchers from Boston University (BU) and King Abdullah University of Science and Technology. They’ve tracked this relationship for over a decade, and they say this collapse is the worst they’ve seen.

“It’s especially painful because the Red Sea is a place many researchers have been hoping and hypothesizing is a thermal refuge,” said Bennett-Smith. “The fact that even this thermal refugium is collapsing in different ways is especially horrific.”

When the home disappears

Without a healthy anemone, clownfish are in trouble. These small fish don’t swim far from home. When their anemone bleaches, they stick out against the white background. They lose their camouflage. Worse, the stinging protection they rely on weakens too.

“Historically, anemones have bleached relatively little compared to corals, said Peter Buston, an associate professor of biology at BU and a senior author on the paper. “Now we’re getting up to levels where the anemones are bleaching, and that results in a catastrophic breakdown in the mutualism with the anemonefishes that everybody knows and loves.”

“We’re seeing nearly a 100 percent die-off in a population of fish in response to a heating event. This should be a big warning for ourselves as well.”

Back in the lab at Boston University, researchers are testing how bleaching affects the fish and the anemones. They’ve already found a few problems.

“There’s a lot going on; increased conspicuousness, increased time outside of the anemones, increased conflict between the fish, and reduced protection from the anemones,” Buston said.

“These anemonefish become sitting ducks. They’re not great swimmers and don’t stray from their anemones, so they presumably get taken off by predators.”

Even if the anemones survive the bleaching and recover, they’re now without their fish partners – and that makes them easier targets for predators like butterflyfish.

Not just the Red Sea

This isn’t an isolated problem. Similar bleaching events have hit other parts of the world too. The same team is now watching what’s happening in Papua New Guinea, another region where clownfish and anemones usually thrive.

Earlier this year, the researchers found that clownfish populations in Papua New Guinea actually shrink to survive heat stress. That’s right – they physically shrink their bodies.

“I’ve studied anemonefishes for a long time, and unavoidably, I and all my students now study them in the context of climate change,” said Buston. “You get enough of these local extinction events, and it ultimately leads to complete extinction.”

Clownfish and the climate clock

Losing clownfish isn’t just about losing a cute, recognizable species. These fish play an important role in keeping reefs balanced. Their disappearance triggers more instability in already fragile ecosystems.

The researchers are now pushing for more surveys – both in other parts of the Red Sea and around the world – to better understand the risks. The goal is to help protect areas that are still hanging on and to support local conservation efforts before more of these systems collapse.

For the clownfish, time is running out. But their story is bigger than one species. It’s a clear signal that climate change isn’t coming – it’s already here. And the ocean, once thought to be resilient, is reaching its limits.

The full study was published in the journal npj Biodiversity.

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