How a Mediterranean coral thrives in heat that kills others

In the warming waters of the Mediterranean Sea, most corals are struggling. But one small, scrappy coral species is flipping the script on what it means to survive in a changing ocean.

Meet Oculina patagonica. It doesn’t build massive reefs. It doesn’t always rely on algae for food. And it’s not dying off like many of its tropical cousins. Instead, this coral is spreading.

What makes Mediterranean coral different

Most corals depend on tiny algae that live inside their cells. These algae make energy through photosynthesis, kind of like plants do. That energy keeps the coral alive and helps it grow. In return, the coral gives the algae a safe home.

But when the water gets too warm, things go south. The coral gets stressed and kicks the algae out. It turns pale, stops growing, and often doesn’t recover. This is what scientists call bleaching.

Oculina doesn’t follow this script. When temperatures spike past 84°F (29°C), it also bleaches – but it doesn’t die. It keeps going, even without algae. Later, when the water cools, the algae return.

That ability to bounce back – and even survive without algae – makes Oculina patagonica a bit of a rebel in the coral world.

A comeback millions of years old

This coral was first spotted near Genoa, Italy, in 1966. At first, scientists thought it was an invader from the Atlantic. But later research revealed it had been in the Mediterranean all along, living quietly in small numbers for millions of years.

Now it’s spreading fast. It’s been found along most shallow Mediterranean coasts, where temperatures swing from under 50°F in winter to over 86°F in summer.

“When first documented in Levantine waters, it was thought that O. patagonica would not survive because summer temperatures were too high,” said study author Dr. Shani Levy, who carried out the work at the Centre for Genomic Regulation (CRG) in Barcelona. “But against all predictions, it established itself and populations are growing.”

When algae disappear

Most corals can’t handle life without algae. But Oculina has a backup plan. When the algae leave, it switches gears. Its cells change their behavior. It builds up its digestive system and starts catching food directly from the water – things like plankton and tiny organic particles.

This switch from relying on algae to feeding on its own is called heterotrophy – and Oculina is remarkably good at it.

Dr. Xavier Grau-Bové, one of the researchers, explained: “Oculina’s ability to live without a photosynthetic partner allows it to settle in deeper waters where less light is abundant, or tolerate turbid waters, where light is blocked by increased sedimentation caused by navigating ships.”

“That’s a huge advantage in the human-altered Mediterranean Sea and one of the reasons we chose to study this species.”

Inside the coral’s survival system

The team wanted to understand how this coral pulls off such a flexible lifestyle. They looked at Oculina’s genome and analyzed tens of thousands of its cells. They also studied two tropical coral species that rely heavily on algae, to compare.

Researchers found that when Oculina has algae, its cells soak up fats (lipids) from them. These fats serve as building blocks and energy reserves – more stable than sugar alone.

When the algae are gone, it’s a different story. The coral fires up its immune-like cells to clear out the dying algae-hosting cells. At the same time, it builds up its digestive tissues to catch and eat food from its surroundings.

“Oculina is resilient because it does not strictly depend on photosynthetic products from the algae,” said ICREA Research Professor Arnau Sebe-Pedrós. “It can obtain photosynthetic products when the algae are present, and that’s probably optimal, but Oculina can also survive feeding only heterotrophically, eating small organic particles and plankton captured and digested in the gut.”

Ancient skills, not new tricks

When the team compared the gene activity of Oculina to other coral species, they made an unexpected discovery: the ability to feed without algae isn’t unique to Oculina. The same genes and cell types exist in other corals – they’re just not used as often.

“Oculina’s life strategy seems to be one of resilience through diversification. It didn’t need to invent a completely new lifestyle from scratch but rather dust off some old tools in its toolbox,” said Dr. Grau-Bové.

That means this survival strategy may have been hiding in other corals all along.

Lessons from the Mediterranean coral

The Mediterranean Sea is a tough place to live. It’s a semi-enclosed body of water with big swings in temperature, salt levels, and nutrients. That makes it a natural stress lab.

“It acts like a natural stress test,” said Dr. Levy. “Corals and other organisms living here are already coping with more extreme fluctuations, so the Mediterranean gives us a kind of preview of how marine life might fare under accelerated climate change.”

Oculina patagonica may offer a clue to how some corals could survive in the future. But there’s a limit. This coral isn’t a major reef-builder. It won’t replace the vast reef ecosystems that are disappearing worldwide.

“The best way to help any marine ecosystem, reefs included, to withstand this warming world will always be to prevent the warming in the first place,” said Dr. Grau-Bové.

The full study was published in the journal Nature.

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