Heatwave wipes out clownfish in what was supposed to be a safe haven
The Red Sea was thought to host ecosystems resilient to warming waters, but a 2023 heatwave proves otherwise.
When healthy, anemones look like pink flowers blossoming on rocks and coral, and serve as homes for clownfish, made famous by the film Finding Nemo. But in 2023 biologists found an entire population of anemones in the Red Sea that had turned ghostly white in a bleaching event that also wiped out most clownfish residents.
In a new paper published in the journal npj biodiversity, scientists recorded a marine heatwave that caused all the anemones in three central Saudi Arabian Red Sea reefs to bleach. While anemones can recover from bleaching events, more than 66 percent of the anemones (Radianthus magnifica) and over 94 percent of clownfish (Amphiprion bicinctus) died.
“Maybe we've stumbled onto some kind of tipping point where there's a maximum [temperature] that these species just simply can't handle,” says Morgan Bennett-Smith, a doctoral candidate at Boston University and lead author of the study.
This mass-casualty event could spell trouble for reefs around the world, as this region was believed to be more resilient to warming events.
Bleached anemones
As sea surface temperatures continue to heat up, severe coral bleaching happens more frequently. But in the concern over losing fragile corals, other creatures have been overlooked. “Everyone was really worried about what happens to coral reefs,” says Bennett-Smith. “No one was really talking about the anemones.”
Like corals, anemones are animals that have a mutually beneficial relationship with a microalgae called zooxanthellae. In return for a safe home inside its host, the algae shares food it creates through photosynthesis.
Anemones bleach when waters get too hot, but they seem to survive better than coral. Scientists aren’t sure why but it might be because they’re better at feeding on particles that float past in the water, so they can last longer without the snacks provided by their algae partner.
The loss of sea anemones can also mean ruin for their famous resident: clownfish, also known as anemonefish. As we know from Finding Nemo, anemonefish find safety from predators by living inside the anemone’s stinging tentacles—the fish themselves are protected by a layer of mucus on their skin. “They can live in the same anemone in the same spot on the reef for 25 years,” says Bennett-Smith.
A thermal refuge
The Red Sea is known for its extremely warm waters, which reach around 90 degrees Fahrenheit each summer. Some experts have suggested that this makes the animals living there more resilient to heat stress.
“Everything is a little bit more used to the warmer temperatures that are natural in the Red Sea,” says Bennett-Smith. For example, individual clownfish have been recorded shrinking to cope with marine heatwaves, and corals in the Gulf of Aqaba have survived four consecutive marine heatwaves.
This mass mortality challenges this idea. “The Red Sea had been considered a thermal ‘safe haven’ for these species, but this event shows otherwise,” says Jules Kajtar, senior research scientist and physical oceanographer at the National Oceanography Centre, who wasn’t involved in the study.
Despite being used to warm waters, “those animals may already be pushing up against their heat limits,” says Tye Kindinger, a marine ecologist at National Geographic Pristine Seas, who was not involved in the study either. A tiny increase can push them over the edge, and kill them.
Microhabitats
Whether anemones recover from bleaching depends on various factors, including the length, severity, and frequency of bleaching events, as well as the presence of anemonefish.
Without their anemone homes, clownfish can’t survive. “We see the anemone bleach, then the fish disappear, and then we see the anemone die,” says Bennett-Smith.
Exactly why the little fishes vanish is yet to be confirmed. These poor swimmers might struggle to find a new home and are at higher risk of getting eaten by a predator when they leave. “Maybe the aggression between fishes increases,” he says. “Maybe they just stray too far. Maybe the heat stress alone is affecting the fish physiologically.”
Anemones don’t just provide a home for clownfish. Shrimp, crabs, and juvenile fish also shelter inside. “They provide places where animals can go to feed, breed, and even get cleaned,” says Kindinger.
If this important microhabitat is lost, the impact could be catastrophic. “When they bleach and die, it's not just like you're losing one animal or two animals,” says Bennett-Smith. “You’re potentially losing an important habit for dozens.”
Reefs in hot water
Both Kindinger and Kajtar were troubled by the bleak report of the Red Sea bleaching event, especially in the context of worsening marine heatwaves. “These findings ring alarm bells,” says Kajtar. “The prospect of increasing widespread mortalities is deeply concerning, making it all the more critical to reverse the trajectory of greenhouse gas emissions.”
“It's all very doom and gloom,” says Bennett-Smith. But he refuses to give up hope.
There are things we humans can do to help, adds Kindinger, such as tackling overfishing, pollution, and habitat destruction, and putting fully protected marine areas in place. “By removing all the damaging human activities, we can give our reefs the best chance for survival in the face of an increasingly warming climate,” she says.
