The Three Sieges on a Coral City

I. A Building Full of Tenants

Genus Platygyra, with a brain like structure.

I am a stony coral. To be precise, I belong to the genus Platygyra, some of the human also call me Brain coral. You may think I am a huge stone, with a brain like structure when you see me, but actually I am an animal. My skeleton are made of calcium carbonate, building up layer by layer over decades. But I am not the one doing the building, the real workers are the tiny creatures living inside me, they are called polyps. Each one is no bigger than a grain of rice, and each one is a separate, living individual.

Think of me as a tall and board apartment building, my skeleton is the concrete walls, the polyps are the residents. They move in, they live, they die, and their children inherit the rooms. Over time , they have generation after generation, adding new floors to the building, expanding outward and upward. That is how I became what I am now, a solid, rounded fortress, home to thousands of polyps. It takes times, we are all snow growing animals,

In the gaps between my polyps and the crevices of my skeleton live other creatures like small fish, crabs, marine worms, shrimp. I hold an entire neighborhood inside my body.

The reason my polyps can survive and build so much is because they made an ancient deal with another organisms live inside the polys. Inside each polyp lives a microscopic algae called zooxanthellae, these algae are the real secret to our success. They use sunlight to perform photosynthesis and turn it into food for us. In return, the polyp gives them a safe home and the carbon dioxide they need to grow by it's own products in respiration.

It is a perfect partnership. The algae get shelter. The polyp gets food. And I, the building they all live in, get to grow larger every year. This arrangement has worked for over 200 million years.

Hong Kong's waters used to be ideal for this partnership. It located at a subtropical region, the winters are not too cold and the summers are not too hot. The currents bring fresh plankton and carry away waste. For decades, my polyps lived comfortably, building their limestone apartments, raising their young, welcoming new tenants.

But lately, the water has started to feel wrong.

II. The First Siege: Bones Dissolving

The first change that confused me came from the chemistry of the sea.

Scientists call it ocean acidification. Humans burn fossil fuels for generate electricity, releasing carbon dioxide into the atmosphere. The ocean absorbs a large portion of that CO₂. When it dissolves into seawater, it forms carbonic acid, and the pH level of seawater drops. When the water becomes more acidic, you may not feel this change, but my skeleton can.

My polyps need to pull calcium carbonate ions from the seawater to build and repair their homes. When the water turns more acidic, those ions become fewer. Each polyp has to spend much more energy just to secrete the same amount of skeleton. It is like trying to build an apartment building when the price of concrete has tripled. You can still build, but every new floor costs you more than it used to.

Young polyps suffer the most. When a baby polyp first settles on a surface, it has to start secreting its own calcium carbonate base immediately. In acidifying water, that first step becomes extremely difficult. Many young polyps simply do not survive their first year. Along the edges of my body, I have not seen any new polyps settling for years. Only the old apartments remain.

III. The Second Siege: Evicting the Roommates

The second change came more directly and more violently. It was temperature.

My polyps have a very narrow tolerance range for heat. If the summer sea temperature rises just one or two degrees Celsius above normal and stays there for a few weeks, the zooxanthellae inside them start to malfunction. They produce toxins instead of food, and the polyps get stressed.

When the stress builds up enough, the polyps make a desperate decision, they will evict their roommates. They expel the zooxanthellae out from their bodies.

Once those algae leave, the polyps lose their color and become completely transparent. You can see right through them to the white skeleton underneath. Humans call this coral bleaching.

Bleaching is not death. My polyps are still alive, but they are starving. Without the zooxanthellae providing energy, they have to rely on catching plankton from the water with their tentacles. That is barely enough to keep them alive, certainly not enough to build new skeleton. If the water temperature returns to normal within a few weeks, the zooxanthellae can sometimes move back in the polys, and the partnership can resumes, the colors will return slowly.

But if the heat persists too long, the polyps starve to death. One by one, the apartments become empty. The building stops growing.

Hong Kong has experienced several major bleaching events in the last fifteen years. During the worst one, large stretches of reef turned white at the same time. The seabed looked like it had been covered in a white sheet if you look at the top view. Some of my neighboring buildings recovered, but some did not. I was one of the luckier ones, the water cooled down fast enough that autumn, and zooxanthellae returned and my polyps recovered. The colors came back.

But I know that if it happens again, once or twice more, many of my polyps might not survive.

IV. The Third Siege: Poison on the Dinner Table

The third change is more subtle than the first two. It is microplastics.

My polyps do not have eyes, they cannot identify what they are eating. Every night, my polys extend their tiny tentacles into the water to catch plankton. My polyps do not have eyes, they cannot identify what they are eating. Any organic particle of the right size could become dinner. This is how they supplement their energy, especially after bleaching, when the zooxanthellae are gone and every bit of food matters.

But now, the seawater contains not just plankton, but countless plastic particles. They come from broken-down plastic bottles and bags, washed synthetic clothing, scrubbing beads in toothpaste, microspheres in cosmetics. These particles are too small to see, but the polyps cannot tell the difference.

So they just eat the plastics. The particles stick to their tentacles and mouths, clogging their feeding structures. And now they cannot capture real food. What is even worse, the plastic that gets swallowed takes up space in their tiny stomachs but provides zero nutrition. They feel full, but they are actually starving.

It is a slow poisoning. Not dramatic like bleaching, which turns the whole building white overnight. Not gradual like acidification, which makes every new floor more expensive. Microplastics are a daily, ongoing drain. My polyps spend more and more energy dealing with things that should not be on their dinner table. The energy left for growth and repair gets smaller every year. New polyps struggle to establish themselves. Old polyps weaken and die.

The building is still standing. But fewer apartments are occupied. And the ones that are occupied are barely getting by.

V. An Observer at the Edge of the Reef

I am still here.

I have been growing for decades. I have survived typhoons, sediment runoff and human disturbance. My skeleton is thick. I still have thousands of polyps living on my surface and I have some resilience. But these three sieges, acidification making every new apartment more expensive to build, warming forcing my polyps to evict their roommates, microplastics poisoning my polys dinner table, each one is wearing me down.

Scientists come regularly to monitor this part of the sea. They dive down, take photos, collect samples, and record water temperature and pH. Some are working in laboratories, trying to breed zooxanthellae or polyps that tolerate heat and acid better. Others track how microplastics move through the food chain.

I do not know if their efforts will succeed. What I do know is my fate, and the fate of the other coral buildings around me, depends on whether humans choose to reduce their carbon emissions, whether they choose to use less plastic, and whether they choose to treat the ocean as something worth protecting.

I am still growing. Every year, the polyps on my surface add a thin new layer to my skeleton. It is the only thing they know how to do, keep building, keep eating, keep making more polyps and keep waiting.